Current limiting with a mosfet?...

On Fri, 16 Jun 2023 17:49:22 +0100, \"Commander Kinsey\"
<CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 23:03:50 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 21:57:37 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 17:17:42 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 13:45:13 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 12:58:43 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Thu, 15 Jun 2023 08:58:58 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16klskj8mvhs6z@ryzen.home>:

On Thu, 15 Jun 2023 06:23:51 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.
You could use a switcher with filter.
So define what you find \'simple\'
?

Can I just do this?

https://electronics.stackexchange.com/questions/377246/how-do-mosfets-and-potentiometers-work-together

What would the minimum voltage drop be?

That is a source follower,
Basicaly you put a voltage on the gate, the drain goes to the power supply output,
and the load to the source.
The problem is that at 50 A and when you drop 5 V over the MOSFET,
it will dissippate 5 x 50 = 250 W when the load is 50 A, less with a lower load current.
Basically the MOSFET will melt.
If it is on a heatsink and heatsink plus thermal resistance of the MOSFET is 1.5 degrees C per Watt then at 250 W it will rise
in temperature by 1.5 * 250 = 375 degrees C.
Add the 350 to say 20 degrees C ambitient temperature and the MOSFET internal will then be at 395 degrees C.
See the problem?

But I\'m not dropping 5V. I\'m dropping about 0.5V.

Plus you need a high enough voltage at the gate to go all the way up to 12 V, a souce follower does not provide that.

Doesn\'t it try to match the voltage? If I put 12.85V form the supply into the gate, it will fully open to 12.85V through the source-drain path.

The minimum voltage drop depends on the MOSFET\'s \'on\' resistance and you can find that in the dataheets as Rds_on,
it maybe very low, some milli-ohms,

I found one with 1 milliohm.

it will not get very hot in such a case at 50 A (but still calculate it
and use a heatsink, for example 50 mOhm Rds_on at 50 A, gives P = i^2 x R so 50 * 50 * .05 still makes 125 Watt!

The 1 milliohm one at 50A will produce only 2.5W when fully on, and 25W when dropping 0.5V.

If you just want to limit to 50 A as in blowing a fuse
then you need a sensing shunt and a trigger circuit to quickly switch the MOSFET off.

As a switch you could use the MOSFET upside down like I do here for example to get enough gate drive:
https://panteltje.nl/panteltje/cb/tx_power_switch2.jpg
but even that is 30 A peak, not continous...

So maybe simpler to just use a fuse?

No, I definitely don\'t want it cutting out. Then the other supplies would be even more overloaded, they would cut out, and I\'d have no power.

There is more to it.
Else just get a few power MOSFETs and play with those in some test circuit to get the hang of it.

I have ordered some and will play around with them and some lightbulbs as a load and see what voltages I can get.

Why not use resistors or, as someone has suggested, wire?

Because I want to be able to tweak it. A variable resistor of that power output, and at fractions of an ohm, is impossible to obtain.

Measure the power suppies and pick the resistor or wire length.

I\'ve never seen milliohm huge variable resistors.

If the mosfet works, I can just turn a dial until my amp clamp says they\'re leveled.

I\'ve found a 200A 1 milliohm mosfet, that oughta do the trick. FFS I had to order it from America, couldn\'t even find stock in China.

We stock IXFH400N075T2, a 1000 watt, 1000 amp mosfet. 2.3 mohms with
10 volts on the gate. You\'ll need 10 mohms, roughly.

Too late, already paid the ludicrous postage.

How much would two of yours have cost me to get shipped to Scotland if you\'re not in Scotland. Judging by \"highland\" I\'m guessing you are.
Click to expand...

No, San Francisco.

I started my biz in the basement of my old Victorian house on Highland
Avenue. I decided to incorporate and my lawyer asked me for the
company name so I said \"Umm, Highland Technology?\"

This is the Highland Avenue Bridge, which hops over the old Bernal
Cut, originally a railroad track.

https://www.dropbox.com/scl/fi/422n7y4bkany7zwwvp7mr/Highland_Bridge.jpg?dl=0&rlkey=rujkh96w0ksjmh35rs7wha5ej

Do you want some fets? We could ship you a couple.
 
On Fri, 16 Jun 2023 17:58:05 +0100, \"Commander Kinsey\"
<CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 04:16:02 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 02:19:21 -0000 (UTC), Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Thu, 15 Jun 2023 17:19:23 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-15 01:23, Jan Panteltje wrote:
On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount?
Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.

You could use a switcher with filter.
So define what you find \'simple\'
?


Not to mention that a good many modern FETs have safe operating areas
that are hard to distinguish from the Y axis. ;)

Cheers

Phil Hobbs

On. Off. What else is there?



Gas. Some of those jokey IR datasheets give new meaning to ‘vaporware’.
Click to expand...

As in, exploded all over the inside of a box.


You mean the dpaks that run at 200 amps?

Uh oh....
I got these: https://www.vishay.com/docs/77646/sum40014m.pdf
Click to expand...

375 watts is absurd for that toy fet. Even 125 is crazy.

Phil is referring to IR posting absurd specs for their fets, like
ignoring the leads, or submerging them in boiling coolants to get the
ratings. So, everybody else had to do it to be competitive.
 
On Thursday, June 15, 2023 at 8:32:37 PM UTC-7, Phil Hobbs wrote:
John Larkin <jla...@highlandSNIPMEtechnology.com> wrote:

When we were doing NMR gradient amps, we tested lots of mosfets to
destruction. We used shrapnel shields. Then we measured voltage and
current in real time and computed junction temperature for shutdown.

Any reliability issues show up?
Click to expand...

For big SCR switches, I\'ve heard of quarter inch steel plate used for
the shields. Reliability was good, but not inspection convenience.
 
On Friday, June 16, 2023 at 9:49:31 AM UTC-7, Commander Kinsey wrote:
On Thu, 15 Jun 2023 23:03:50 +0100, John Larkin <jla...@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 21:57:37 +0100, \"Commander Kinsey\"
C...@nospam.com> wrote:

On Thu, 15 Jun 2023 17:17:42 +0100, John Larkin <jla...@highlandsnipmetechnology.com> wrote:


Why not use resistors or, as someone has suggested, wire?

Because I want to be able to tweak it. A variable resistor of that power output, and at fractions of an ohm, is impossible to obtain.

Measure the power suppies and pick the resistor or wire length.

I\'ve never seen milliohm huge variable resistors.
Click to expand...

They\'re not called resistors, rather are \'shunts\'.
<https://m.media-amazon.com/images/I/41-myRflg9L.jpg>

They\'re variable by grinding material off the edges...

.... and with a bit of bronze shim stock, or (at lower power dissipation) wire,
they\'re nearly trivial to build. Brazing is better than soldering, though, and
attaching wires is not easy; tapered plugs fitted to conical holes DOES make a
good mechanical and electrical connection for welders.
 
On Thu, 15 Jun 2023 16:15:40 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 14 Jun 2023 23:41:50 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 23:18:04 +0100, boB <boB@k7iq.com> wrote:

On Wed, 14 Jun 2023 19:37:16 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 18:31:09 +0100, boB <boB@k7iq.com> wrote:

On Wed, 14 Jun 2023 18:07:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 17:59:32 +0100, Commander Kinsey <CK1@nospam.com> wrote:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

If it\'s simpler, a potentiometer to vary the voltage drop across the mosfet would do.

I\'m trying to balance a few power supplies all running in parallel, so they do the same work each. I don\'t mind tweaking a potentiometer on each one to make the current about equal on each.


You could make a circuit that shuts the FET OFF when you hit 51
amps. If you want to limit current by lowering the output voltage
using that FET, then you are asking for blown up FET I think.

You\'d need a huge amount of dissipation to make that work and maybe
you don\'t want to reduce the output voltage.

But just shutting off could work fairly easily.

Shutting it off would worsen the problem - the other supplies would then be likely to hit their limits.

Surely the TO-247 MOSFETs can take a high current? Or is that only when turned fully on? I guess it would be complicated to make them pulse, and probably upset the SMPS it\'s adjusting.

At the moment I\'m just going to sort out the biggest imbalance - two of the supplies being 12.85V, two being 12.35V, and one adjustable. The two high voltage ones I can put big TO-220 schottkys on to drop half a volt.

Well, if you have, say, a 1 milliOhm FET which are obtainable, then at
50 amps that is 2.5 watts which can be OK with a good heat sink.

If you were to say, drop 1 volt across the FET at 49 amps, now you are
talking a out 1 X 49 = 49 watts which is NOT going to work even with
a TO-247 package.

A TO-247 on a big heatsink and fan arrangement could easily dissipate 50W.

So, you turn off the FET and latch off if you want to limit current
and then it only dissipate a few watts at most while fully on.

But I want to LIMIT the current, not fuse it.


OK then.

Use an N-channel FET with a small Ohm resistor in series with its
source for the output. 15 milli-Ohms is about right for this one.

Drain goes to your source voltage. 12V here, right ?

Then, tack in a small NPN transistor with its emitter to the output
load, its base to the source of the FET and its collector to the gate
of the FET. Choose a series R that drops around 0.7 volts at 50 amps
and then the NON will turn on, reducing the gate-source voltage and
therefore reducing the output current. That\'s about the simplest
that can be done.
Click to expand...

Interestingly, that config tends to blow out the NPN.

And the fet gate would have to be pullled up to +20ish.

Something with an opamp, and maybe a p-fet, would be more adjustable
and run from +12.

0.7 volts and 50 amps across the resistor is 35 watts.



You will need to use a resistor driving the gate so that the NPN can
easily reduce that Vgs to limit the current.

boB
Click to expand...
 
On Fri, 16 Jun 2023 10:56:05 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Friday, June 16, 2023 at 9:49:31?AM UTC-7, Commander Kinsey wrote:
On Thu, 15 Jun 2023 23:03:50 +0100, John Larkin <jla...@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 21:57:37 +0100, \"Commander Kinsey\"
C...@nospam.com> wrote:

On Thu, 15 Jun 2023 17:17:42 +0100, John Larkin <jla...@highlandsnipmetechnology.com> wrote:


Why not use resistors or, as someone has suggested, wire?

Because I want to be able to tweak it. A variable resistor of that power output, and at fractions of an ohm, is impossible to obtain.

Measure the power suppies and pick the resistor or wire length.

I\'ve never seen milliohm huge variable resistors.

They\'re not called resistors, rather are \'shunts\'.
https://m.media-amazon.com/images/I/41-myRflg9L.jpg

They\'re variable by grinding material off the edges...

... and with a bit of bronze shim stock, or (at lower power dissipation) wire,
they\'re nearly trivial to build. Brazing is better than soldering, though, and
attaching wires is not easy; tapered plugs fitted to conical holes DOES make a
good mechanical and electrical connection for welders.
Click to expand...

There are giant wirewound resistors with slider contacts. ebay may
have some.
 
On Fri, 16 Jun 2023 18:37:58 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:49:22 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 23:03:50 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 21:57:37 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 17:17:42 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 13:45:13 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 12:58:43 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Thu, 15 Jun 2023 08:58:58 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16klskj8mvhs6z@ryzen.home>:

On Thu, 15 Jun 2023 06:23:51 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.
You could use a switcher with filter.
So define what you find \'simple\'
?

Can I just do this?

https://electronics.stackexchange.com/questions/377246/how-do-mosfets-and-potentiometers-work-together

What would the minimum voltage drop be?

That is a source follower,
Basicaly you put a voltage on the gate, the drain goes to the power supply output,
and the load to the source.
The problem is that at 50 A and when you drop 5 V over the MOSFET,
it will dissippate 5 x 50 = 250 W when the load is 50 A, less with a lower load current.
Basically the MOSFET will melt.
If it is on a heatsink and heatsink plus thermal resistance of the MOSFET is 1.5 degrees C per Watt then at 250 W it will rise
in temperature by 1.5 * 250 = 375 degrees C.
Add the 350 to say 20 degrees C ambitient temperature and the MOSFET internal will then be at 395 degrees C.
See the problem?

But I\'m not dropping 5V. I\'m dropping about 0.5V.

Plus you need a high enough voltage at the gate to go all the way up to 12 V, a souce follower does not provide that.

Doesn\'t it try to match the voltage? If I put 12.85V form the supply into the gate, it will fully open to 12.85V through the source-drain path.

The minimum voltage drop depends on the MOSFET\'s \'on\' resistance and you can find that in the dataheets as Rds_on,
it maybe very low, some milli-ohms,

I found one with 1 milliohm.

it will not get very hot in such a case at 50 A (but still calculate it
and use a heatsink, for example 50 mOhm Rds_on at 50 A, gives P = i^2 x R so 50 * 50 * .05 still makes 125 Watt!

The 1 milliohm one at 50A will produce only 2.5W when fully on, and 25W when dropping 0.5V.

If you just want to limit to 50 A as in blowing a fuse
then you need a sensing shunt and a trigger circuit to quickly switch the MOSFET off.

As a switch you could use the MOSFET upside down like I do here for example to get enough gate drive:
https://panteltje.nl/panteltje/cb/tx_power_switch2.jpg
but even that is 30 A peak, not continous...

So maybe simpler to just use a fuse?

No, I definitely don\'t want it cutting out. Then the other supplies would be even more overloaded, they would cut out, and I\'d have no power.

There is more to it.
Else just get a few power MOSFETs and play with those in some test circuit to get the hang of it.

I have ordered some and will play around with them and some lightbulbs as a load and see what voltages I can get.

Why not use resistors or, as someone has suggested, wire?

Because I want to be able to tweak it. A variable resistor of that power output, and at fractions of an ohm, is impossible to obtain.

Measure the power suppies and pick the resistor or wire length.

I\'ve never seen milliohm huge variable resistors.

If the mosfet works, I can just turn a dial until my amp clamp says they\'re leveled.

I\'ve found a 200A 1 milliohm mosfet, that oughta do the trick. FFS I had to order it from America, couldn\'t even find stock in China.

We stock IXFH400N075T2, a 1000 watt, 1000 amp mosfet. 2.3 mohms with
10 volts on the gate. You\'ll need 10 mohms, roughly.

Too late, already paid the ludicrous postage.

How much would two of yours have cost me to get shipped to Scotland if you\'re not in Scotland. Judging by \"highland\" I\'m guessing you are.


No, San Francisco.

I started my biz in the basement of my old Victorian house on Highland
Avenue. I decided to incorporate and my lawyer asked me for the
company name so I said \"Umm, Highland Technology?\"

This is the Highland Avenue Bridge, which hops over the old Bernal
Cut, originally a railroad track.

https://www.dropbox.com/scl/fi/422n7y4bkany7zwwvp7mr/Highland_Bridge.jpg?dl=0&rlkey=rujkh96w0ksjmh35rs7wha5ej

Do you want some fets? We could ship you a couple.
Click to expand...

I don\'t need any now as I bought them from mouser. Might do in the future. How much would you charge to ship say 6 fets capable of this sort of thing to Scotland? Milliohmish, 200A ish. 100W+.

And what do you sell other than fets?
 
On Fri, 16 Jun 2023 18:43:59 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:58:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 04:16:02 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 02:19:21 -0000 (UTC), Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Thu, 15 Jun 2023 17:19:23 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-15 01:23, Jan Panteltje wrote:
On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount?
Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.

You could use a switcher with filter.
So define what you find \'simple\'
?


Not to mention that a good many modern FETs have safe operating areas
that are hard to distinguish from the Y axis. ;)

Cheers

Phil Hobbs

On. Off. What else is there?



Gas. Some of those jokey IR datasheets give new meaning to ‘vaporware’.

As in, exploded all over the inside of a box.
Click to expand...

I blew a fet across a room once. I didn\'t know the soldering iron was grounded.

You mean the dpaks that run at 200 amps?

Uh oh....
I got these: https://www.vishay.com/docs/77646/sum40014m.pdf

375 watts is absurd for that toy fet. Even 125 is crazy.
Click to expand...

I don\'t like the package. I like them screwed firmly to the heatsink like TO-247. Those are the size I see inside 1-3kW supplies.

Phil is referring to IR posting absurd specs for their fets, like
ignoring the leads, or submerging them in boiling coolants to get the
ratings. So, everybody else had to do it to be competitive.
Click to expand...

It does say \"at 25C\".
 
On Fri, 16 Jun 2023 11:00:24 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

On Thu, 15 Jun 2023 16:15:40 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 14 Jun 2023 23:41:50 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 23:18:04 +0100, boB <boB@k7iq.com> wrote:

On Wed, 14 Jun 2023 19:37:16 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 18:31:09 +0100, boB <boB@k7iq.com> wrote:

On Wed, 14 Jun 2023 18:07:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 17:59:32 +0100, Commander Kinsey <CK1@nospam.com> wrote:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

If it\'s simpler, a potentiometer to vary the voltage drop across the mosfet would do.

I\'m trying to balance a few power supplies all running in parallel, so they do the same work each. I don\'t mind tweaking a potentiometer on each one to make the current about equal on each.


You could make a circuit that shuts the FET OFF when you hit 51
amps. If you want to limit current by lowering the output voltage
using that FET, then you are asking for blown up FET I think.

You\'d need a huge amount of dissipation to make that work and maybe
you don\'t want to reduce the output voltage.

But just shutting off could work fairly easily.

Shutting it off would worsen the problem - the other supplies would then be likely to hit their limits.

Surely the TO-247 MOSFETs can take a high current? Or is that only when turned fully on? I guess it would be complicated to make them pulse, and probably upset the SMPS it\'s adjusting.

At the moment I\'m just going to sort out the biggest imbalance - two of the supplies being 12.85V, two being 12.35V, and one adjustable. The two high voltage ones I can put big TO-220 schottkys on to drop half a volt.

Well, if you have, say, a 1 milliOhm FET which are obtainable, then at
50 amps that is 2.5 watts which can be OK with a good heat sink.

If you were to say, drop 1 volt across the FET at 49 amps, now you are
talking a out 1 X 49 = 49 watts which is NOT going to work even with
a TO-247 package.

A TO-247 on a big heatsink and fan arrangement could easily dissipate 50W.

So, you turn off the FET and latch off if you want to limit current
and then it only dissipate a few watts at most while fully on.

But I want to LIMIT the current, not fuse it.


OK then.

Use an N-channel FET with a small Ohm resistor in series with its
source for the output. 15 milli-Ohms is about right for this one.

Drain goes to your source voltage. 12V here, right ?

Then, tack in a small NPN transistor with its emitter to the output
load, its base to the source of the FET and its collector to the gate
of the FET. Choose a series R that drops around 0.7 volts at 50 amps
and then the NON will turn on, reducing the gate-source voltage and
therefore reducing the output current. That\'s about the simplest
that can be done.

Interestingly, that config tends to blow out the NPN.

And the fet gate would have to be pullled up to +20ish.

Something with an opamp, and maybe a p-fet, would be more adjustable
and run from +12.

0.7 volts and 50 amps across the resistor is 35 watts.
Click to expand...

Yeah, that is a bit much of dissipation I agree. I was trying to show
a very cheap way to do this that has worked fine for me.

Doesn\'t hurt the NPN as long as the drive signal (your +20V) has too
low of impedance. Just use a resistor.

But you should NOT need 20V to the gate either. 12V to 16V works
fine.

Using an open collector comparator might be an OK way to go EXCEPT
that the FET might turn ON slow and burn up if a simple Vgs pull-up
resistor is used.

boB




You will need to use a resistor driving the gate so that the NPN can
easily reduce that Vgs to limit the current.

boB
Click to expand...
 
On Fri, 16 Jun 2023 21:16:29 +0100, \"Commander Kinsey\"
<CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 18:43:59 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:58:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 04:16:02 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 02:19:21 -0000 (UTC), Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Thu, 15 Jun 2023 17:19:23 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-15 01:23, Jan Panteltje wrote:
On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount?
Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.

You could use a switcher with filter.
So define what you find \'simple\'
?


Not to mention that a good many modern FETs have safe operating areas
that are hard to distinguish from the Y axis. ;)

Cheers

Phil Hobbs

On. Off. What else is there?



Gas. Some of those jokey IR datasheets give new meaning to ‘vaporware’.

As in, exploded all over the inside of a box.

I blew a fet across a room once. I didn\'t know the soldering iron was grounded.

You mean the dpaks that run at 200 amps?

Uh oh....
I got these: https://www.vishay.com/docs/77646/sum40014m.pdf

375 watts is absurd for that toy fet. Even 125 is crazy.

I don\'t like the package. I like them screwed firmly to the heatsink like TO-247. Those are the size I see inside 1-3kW supplies.

Phil is referring to IR posting absurd specs for their fets, like
ignoring the leads, or submerging them in boiling coolants to get the
ratings. So, everybody else had to do it to be competitive.

It does say \"at 25C\".
Click to expand...

That FET is perfect for this application. BUT it will still have to
just turn OFF when you exceed your 50 amp threshold.
You can not use it in a linear voltage regulating mode at these high
values of current. And you won\'t be able to keep the tab at 25
degrees C either.

You COULD turn it into a switcher though and buck converter to reduce
the voltage. Just add some switching circuitry, FET drive and an
inductor and capacitor basically.

Forget trying to dissipate hundreds of watts in that one FET.


boB
 
On Fri, 16 Jun 2023 14:15:06 -0700, boB <boB@K7IQ.com> wrote:

On Fri, 16 Jun 2023 11:00:24 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Thu, 15 Jun 2023 16:15:40 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 14 Jun 2023 23:41:50 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 23:18:04 +0100, boB <boB@k7iq.com> wrote:

On Wed, 14 Jun 2023 19:37:16 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 18:31:09 +0100, boB <boB@k7iq.com> wrote:

On Wed, 14 Jun 2023 18:07:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 17:59:32 +0100, Commander Kinsey <CK1@nospam.com> wrote:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

If it\'s simpler, a potentiometer to vary the voltage drop across the mosfet would do.

I\'m trying to balance a few power supplies all running in parallel, so they do the same work each. I don\'t mind tweaking a potentiometer on each one to make the current about equal on each.


You could make a circuit that shuts the FET OFF when you hit 51
amps. If you want to limit current by lowering the output voltage
using that FET, then you are asking for blown up FET I think.

You\'d need a huge amount of dissipation to make that work and maybe
you don\'t want to reduce the output voltage.

But just shutting off could work fairly easily.

Shutting it off would worsen the problem - the other supplies would then be likely to hit their limits.

Surely the TO-247 MOSFETs can take a high current? Or is that only when turned fully on? I guess it would be complicated to make them pulse, and probably upset the SMPS it\'s adjusting.

At the moment I\'m just going to sort out the biggest imbalance - two of the supplies being 12.85V, two being 12.35V, and one adjustable. The two high voltage ones I can put big TO-220 schottkys on to drop half a volt.

Well, if you have, say, a 1 milliOhm FET which are obtainable, then at
50 amps that is 2.5 watts which can be OK with a good heat sink.

If you were to say, drop 1 volt across the FET at 49 amps, now you are
talking a out 1 X 49 = 49 watts which is NOT going to work even with
a TO-247 package.

A TO-247 on a big heatsink and fan arrangement could easily dissipate 50W.

So, you turn off the FET and latch off if you want to limit current
and then it only dissipate a few watts at most while fully on.

But I want to LIMIT the current, not fuse it.


OK then.

Use an N-channel FET with a small Ohm resistor in series with its
source for the output. 15 milli-Ohms is about right for this one.

Drain goes to your source voltage. 12V here, right ?

Then, tack in a small NPN transistor with its emitter to the output
load, its base to the source of the FET and its collector to the gate
of the FET. Choose a series R that drops around 0.7 volts at 50 amps
and then the NON will turn on, reducing the gate-source voltage and
therefore reducing the output current. That\'s about the simplest
that can be done.

Interestingly, that config tends to blow out the NPN.

And the fet gate would have to be pullled up to +20ish.

Something with an opamp, and maybe a p-fet, would be more adjustable
and run from +12.

0.7 volts and 50 amps across the resistor is 35 watts.


Yeah, that is a bit much of dissipation I agree. I was trying to show
a very cheap way to do this that has worked fine for me.

Doesn\'t hurt the NPN as long as the drive signal (your +20V) has too
low of impedance. Just use a resistor.

But you should NOT need 20V to the gate either. 12V to 16V works
fine.
Click to expand...

If the nfet source is at +12, the gate needs to be a bunch more
positive.
 
On Fri, 16 Jun 2023 21:12:48 +0100, \"Commander Kinsey\"
<CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 18:37:58 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:49:22 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 23:03:50 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 21:57:37 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 17:17:42 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 13:45:13 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 12:58:43 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Thu, 15 Jun 2023 08:58:58 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16klskj8mvhs6z@ryzen.home>:

On Thu, 15 Jun 2023 06:23:51 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.
You could use a switcher with filter.
So define what you find \'simple\'
?

Can I just do this?

https://electronics.stackexchange.com/questions/377246/how-do-mosfets-and-potentiometers-work-together

What would the minimum voltage drop be?

That is a source follower,
Basicaly you put a voltage on the gate, the drain goes to the power supply output,
and the load to the source.
The problem is that at 50 A and when you drop 5 V over the MOSFET,
it will dissippate 5 x 50 = 250 W when the load is 50 A, less with a lower load current.
Basically the MOSFET will melt.
If it is on a heatsink and heatsink plus thermal resistance of the MOSFET is 1.5 degrees C per Watt then at 250 W it will rise
in temperature by 1.5 * 250 = 375 degrees C.
Add the 350 to say 20 degrees C ambitient temperature and the MOSFET internal will then be at 395 degrees C.
See the problem?

But I\'m not dropping 5V. I\'m dropping about 0.5V.

Plus you need a high enough voltage at the gate to go all the way up to 12 V, a souce follower does not provide that.

Doesn\'t it try to match the voltage? If I put 12.85V form the supply into the gate, it will fully open to 12.85V through the source-drain path.

The minimum voltage drop depends on the MOSFET\'s \'on\' resistance and you can find that in the dataheets as Rds_on,
it maybe very low, some milli-ohms,

I found one with 1 milliohm.

it will not get very hot in such a case at 50 A (but still calculate it
and use a heatsink, for example 50 mOhm Rds_on at 50 A, gives P = i^2 x R so 50 * 50 * .05 still makes 125 Watt!

The 1 milliohm one at 50A will produce only 2.5W when fully on, and 25W when dropping 0.5V.

If you just want to limit to 50 A as in blowing a fuse
then you need a sensing shunt and a trigger circuit to quickly switch the MOSFET off.

As a switch you could use the MOSFET upside down like I do here for example to get enough gate drive:
https://panteltje.nl/panteltje/cb/tx_power_switch2.jpg
but even that is 30 A peak, not continous...

So maybe simpler to just use a fuse?

No, I definitely don\'t want it cutting out. Then the other supplies would be even more overloaded, they would cut out, and I\'d have no power.

There is more to it.
Else just get a few power MOSFETs and play with those in some test circuit to get the hang of it.

I have ordered some and will play around with them and some lightbulbs as a load and see what voltages I can get.

Why not use resistors or, as someone has suggested, wire?

Because I want to be able to tweak it. A variable resistor of that power output, and at fractions of an ohm, is impossible to obtain.

Measure the power suppies and pick the resistor or wire length.

I\'ve never seen milliohm huge variable resistors.

If the mosfet works, I can just turn a dial until my amp clamp says they\'re leveled.

I\'ve found a 200A 1 milliohm mosfet, that oughta do the trick. FFS I had to order it from America, couldn\'t even find stock in China.

We stock IXFH400N075T2, a 1000 watt, 1000 amp mosfet. 2.3 mohms with
10 volts on the gate. You\'ll need 10 mohms, roughly.

Too late, already paid the ludicrous postage.

How much would two of yours have cost me to get shipped to Scotland if you\'re not in Scotland. Judging by \"highland\" I\'m guessing you are.


No, San Francisco.

I started my biz in the basement of my old Victorian house on Highland
Avenue. I decided to incorporate and my lawyer asked me for the
company name so I said \"Umm, Highland Technology?\"

This is the Highland Avenue Bridge, which hops over the old Bernal
Cut, originally a railroad track.

https://www.dropbox.com/scl/fi/422n7y4bkany7zwwvp7mr/Highland_Bridge.jpg?dl=0&rlkey=rujkh96w0ksjmh35rs7wha5ej

Do you want some fets? We could ship you a couple.

I don\'t need any now as I bought them from mouser. Might do in the future. How much would you charge to ship say 6 fets capable of this sort of thing to Scotland?
Click to expand...

I just give stuff to Matt and he ships it.


Milliohmish, 200A ish. 100W+.

And what do you sell other than fets?
Click to expand...

We sell equipment, not parts.

http://www.highlandtechnology.com/

We give away parts, like to customers who fried a transformer or broke
a cable or something. They seem to like that.

There\'s a problem on a box we sold to NIF, 54 units. Sometimes one of
the ADI parts has excessive leakage current and creates a small offset
that, in theory, could cost a Nobel prize or two. A small new PCB with
a few parts and SMA connectors will fix all the boxes. For free. I\'ll
hack a test board this weekend and let them try it.

I hate it when people charge insane prices for spare parts, like 20x
what they must cost to make.
 
On Fri, 16 Jun 2023 21:16:29 +0100, \"Commander Kinsey\"
<CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 18:43:59 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:58:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 04:16:02 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 02:19:21 -0000 (UTC), Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Thu, 15 Jun 2023 17:19:23 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-15 01:23, Jan Panteltje wrote:
On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount?
Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.

You could use a switcher with filter.
So define what you find \'simple\'
?


Not to mention that a good many modern FETs have safe operating areas
that are hard to distinguish from the Y axis. ;)

Cheers

Phil Hobbs

On. Off. What else is there?



Gas. Some of those jokey IR datasheets give new meaning to ‘vaporware’.

As in, exploded all over the inside of a box.

I blew a fet across a room once. I didn\'t know the soldering iron was grounded.

You mean the dpaks that run at 200 amps?

Uh oh....
I got these: https://www.vishay.com/docs/77646/sum40014m.pdf

375 watts is absurd for that toy fet. Even 125 is crazy.

I don\'t like the package. I like them screwed firmly to the heatsink like TO-247. Those are the size I see inside 1-3kW supplies.

Phil is referring to IR posting absurd specs for their fets, like
ignoring the leads, or submerging them in boiling coolants to get the
ratings. So, everybody else had to do it to be competitive.

It does say \"at 25C\".
Click to expand...

Solder it to half the universe made of silver.
 
On Fri, 16 Jun 2023 14:25:06 -0700, boB <boB@K7IQ.com> wrote:

On Fri, 16 Jun 2023 21:16:29 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 18:43:59 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:58:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 04:16:02 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 02:19:21 -0000 (UTC), Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Thu, 15 Jun 2023 17:19:23 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-15 01:23, Jan Panteltje wrote:
On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount?
Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.

You could use a switcher with filter.
So define what you find \'simple\'
?


Not to mention that a good many modern FETs have safe operating areas
that are hard to distinguish from the Y axis. ;)

Cheers

Phil Hobbs

On. Off. What else is there?



Gas. Some of those jokey IR datasheets give new meaning to ‘vaporware’.

As in, exploded all over the inside of a box.

I blew a fet across a room once. I didn\'t know the soldering iron was grounded.

You mean the dpaks that run at 200 amps?

Uh oh....
I got these: https://www.vishay.com/docs/77646/sum40014m.pdf

375 watts is absurd for that toy fet. Even 125 is crazy.

I don\'t like the package. I like them screwed firmly to the heatsink like TO-247. Those are the size I see inside 1-3kW supplies.

Phil is referring to IR posting absurd specs for their fets, like
ignoring the leads, or submerging them in boiling coolants to get the
ratings. So, everybody else had to do it to be competitive.

It does say \"at 25C\".


That FET is perfect for this application. BUT it will still have to
just turn OFF when you exceed your 50 amp threshold.
You can not use it in a linear voltage regulating mode at these high
values of current. And you won\'t be able to keep the tab at 25
degrees C either.
Click to expand...

He precisely wants to use it as a 50 amp linear regulator.


You COULD turn it into a switcher though and buck converter to reduce
the voltage. Just add some switching circuitry, FET drive and an
inductor and capacitor basically.

Forget trying to dissipate hundreds of watts in that one FET.
Click to expand...

He would dissipate 50 watts or less. One big fet on a giant heat sink
could do that, but not a surface-mount dpak sort of thing.

I was recently playing with the idea of mounting power fets on a cpu
cooler-fan assembly, as a programmable dummy load, for maybe 200
watts. Turns out that lots of fets are available in TO-247 and the
same chip in TO-220, and the thermal resistance is the same. So I may
as well use TO-220s, cause I can fit more on a cooler.

The Dynatron R25 is a beast. Lots of copper surface for mounting fets
on, like eight TO-220s maybe.
 
On Fri, 16 Jun 2023 14:32:20 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

On Fri, 16 Jun 2023 14:15:06 -0700, boB <boB@K7IQ.com> wrote:

On Fri, 16 Jun 2023 11:00:24 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Thu, 15 Jun 2023 16:15:40 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 14 Jun 2023 23:41:50 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 23:18:04 +0100, boB <boB@k7iq.com> wrote:

On Wed, 14 Jun 2023 19:37:16 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 18:31:09 +0100, boB <boB@k7iq.com> wrote:

On Wed, 14 Jun 2023 18:07:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Wed, 14 Jun 2023 17:59:32 +0100, Commander Kinsey <CK1@nospam.com> wrote:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

If it\'s simpler, a potentiometer to vary the voltage drop across the mosfet would do.

I\'m trying to balance a few power supplies all running in parallel, so they do the same work each. I don\'t mind tweaking a potentiometer on each one to make the current about equal on each.


You could make a circuit that shuts the FET OFF when you hit 51
amps. If you want to limit current by lowering the output voltage
using that FET, then you are asking for blown up FET I think.

You\'d need a huge amount of dissipation to make that work and maybe
you don\'t want to reduce the output voltage.

But just shutting off could work fairly easily.

Shutting it off would worsen the problem - the other supplies would then be likely to hit their limits.

Surely the TO-247 MOSFETs can take a high current? Or is that only when turned fully on? I guess it would be complicated to make them pulse, and probably upset the SMPS it\'s adjusting.

At the moment I\'m just going to sort out the biggest imbalance - two of the supplies being 12.85V, two being 12.35V, and one adjustable. The two high voltage ones I can put big TO-220 schottkys on to drop half a volt.

Well, if you have, say, a 1 milliOhm FET which are obtainable, then at
50 amps that is 2.5 watts which can be OK with a good heat sink.

If you were to say, drop 1 volt across the FET at 49 amps, now you are
talking a out 1 X 49 = 49 watts which is NOT going to work even with
a TO-247 package.

A TO-247 on a big heatsink and fan arrangement could easily dissipate 50W.

So, you turn off the FET and latch off if you want to limit current
and then it only dissipate a few watts at most while fully on.

But I want to LIMIT the current, not fuse it.


OK then.

Use an N-channel FET with a small Ohm resistor in series with its
source for the output. 15 milli-Ohms is about right for this one.

Drain goes to your source voltage. 12V here, right ?

Then, tack in a small NPN transistor with its emitter to the output
load, its base to the source of the FET and its collector to the gate
of the FET. Choose a series R that drops around 0.7 volts at 50 amps
and then the NON will turn on, reducing the gate-source voltage and
therefore reducing the output current. That\'s about the simplest
that can be done.

Interestingly, that config tends to blow out the NPN.

And the fet gate would have to be pullled up to +20ish.

Something with an opamp, and maybe a p-fet, would be more adjustable
and run from +12.

0.7 volts and 50 amps across the resistor is 35 watts.


Yeah, that is a bit much of dissipation I agree. I was trying to show
a very cheap way to do this that has worked fine for me.

Doesn\'t hurt the NPN as long as the drive signal (your +20V) has too
low of impedance. Just use a resistor.

But you should NOT need 20V to the gate either. 12V to 16V works
fine.

If the nfet source is at +12, the gate needs to be a bunch more
positive.
Click to expand...

OK, right. 12V plus 12V = 24V or so IF it is in the positive leg.
Could put it in the negative leg. He didn\'t say it had to be positie
leg but I assumed +12 and a follower.

We use this circuit but that FET is also a voltage regulator using a
zener on the gate and a pullup to a plentiful voltage pullup. It was
easier in that kind of circuit that has a higher input voltage
available.

Not a big deal as it limits current anyway. He wanted to choose a
method that would go linear when it started to current limit.

You have to do some homework with this kind of stuff and add whatever
is needed to make it work. Or not work in this case.

And that 35 watts will be nothing compared to what that FET would be
dissipating when it starts to limit..

boB
 
On Fri, 16 Jun 2023 14:56:14 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

On Fri, 16 Jun 2023 14:25:06 -0700, boB <boB@K7IQ.com> wrote:

On Fri, 16 Jun 2023 21:16:29 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 18:43:59 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:58:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 04:16:02 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 02:19:21 -0000 (UTC), Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Thu, 15 Jun 2023 17:19:23 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-15 01:23, Jan Panteltje wrote:
On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount?
Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.

You could use a switcher with filter.
So define what you find \'simple\'
?


Not to mention that a good many modern FETs have safe operating areas
that are hard to distinguish from the Y axis. ;)

Cheers

Phil Hobbs

On. Off. What else is there?



Gas. Some of those jokey IR datasheets give new meaning to ‘vaporware’.

As in, exploded all over the inside of a box.

I blew a fet across a room once. I didn\'t know the soldering iron was grounded.

You mean the dpaks that run at 200 amps?

Uh oh....
I got these: https://www.vishay.com/docs/77646/sum40014m.pdf

375 watts is absurd for that toy fet. Even 125 is crazy.

I don\'t like the package. I like them screwed firmly to the heatsink like TO-247. Those are the size I see inside 1-3kW supplies.

Phil is referring to IR posting absurd specs for their fets, like
ignoring the leads, or submerging them in boiling coolants to get the
ratings. So, everybody else had to do it to be competitive.

It does say \"at 25C\".


That FET is perfect for this application. BUT it will still have to
just turn OFF when you exceed your 50 amp threshold.
You can not use it in a linear voltage regulating mode at these high
values of current. And you won\'t be able to keep the tab at 25
degrees C either.

He precisely wants to use it as a 50 amp linear regulator.



You COULD turn it into a switcher though and buck converter to reduce
the voltage. Just add some switching circuitry, FET drive and an
inductor and capacitor basically.

Forget trying to dissipate hundreds of watts in that one FET.

He would dissipate 50 watts or less. One big fet on a giant heat sink
could do that, but not a surface-mount dpak sort of thing.
Click to expand...

That particular FET is about the best junction to case resistance you
are going to get for the price. Even a TO-247 is not going to be much
better, if at all. These D-Squared-pak FETs are FULL of silicon.

But they can\'t \"easily\" dissipate 50 watts I would say. Maybe with a
lot of air blowing. How much does he want to spend on this ? I guess
that is really the question.

However, he could parallel FETs and make it a bit easier but then you
know how that goes for a linear stage of FETs.

I was recently playing with the idea of mounting power fets on a cpu
cooler-fan assembly, as a programmable dummy load, for maybe 200
watts. Turns out that lots of fets are available in TO-247 and the
same chip in TO-220, and the thermal resistance is the same. So I may
as well use TO-220s, cause I can fit more on a cooler.

The Dynatron R25 is a beast. Lots of copper surface for mounting fets
on, like eight TO-220s maybe.
Click to expand...

We use TO-220s on big heat sinks too. We also make D2PAK FETs work
well through PCB heat transfer. Much easier to work with than TO-220s
but a bit less good of sinking of course.

boB
 
On Fri, 16 Jun 2023 22:43:42 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 21:12:48 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 18:37:58 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:49:22 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 23:03:50 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 21:57:37 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 17:17:42 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 13:45:13 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 12:58:43 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Thu, 15 Jun 2023 08:58:58 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16klskj8mvhs6z@ryzen.home>:

On Thu, 15 Jun 2023 06:23:51 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.
You could use a switcher with filter.
So define what you find \'simple\'
?

Can I just do this?

https://electronics.stackexchange.com/questions/377246/how-do-mosfets-and-potentiometers-work-together

What would the minimum voltage drop be?

That is a source follower,
Basicaly you put a voltage on the gate, the drain goes to the power supply output,
and the load to the source.
The problem is that at 50 A and when you drop 5 V over the MOSFET,
it will dissippate 5 x 50 = 250 W when the load is 50 A, less with a lower load current.
Basically the MOSFET will melt.
If it is on a heatsink and heatsink plus thermal resistance of the MOSFET is 1.5 degrees C per Watt then at 250 W it will rise
in temperature by 1.5 * 250 = 375 degrees C.
Add the 350 to say 20 degrees C ambitient temperature and the MOSFET internal will then be at 395 degrees C.
See the problem?

But I\'m not dropping 5V. I\'m dropping about 0.5V.

Plus you need a high enough voltage at the gate to go all the way up to 12 V, a souce follower does not provide that.

Doesn\'t it try to match the voltage? If I put 12.85V form the supply into the gate, it will fully open to 12.85V through the source-drain path.

The minimum voltage drop depends on the MOSFET\'s \'on\' resistance and you can find that in the dataheets as Rds_on,
it maybe very low, some milli-ohms,

I found one with 1 milliohm.

it will not get very hot in such a case at 50 A (but still calculate it
and use a heatsink, for example 50 mOhm Rds_on at 50 A, gives P = i^2 x R so 50 * 50 * .05 still makes 125 Watt!

The 1 milliohm one at 50A will produce only 2.5W when fully on, and 25W when dropping 0.5V.

If you just want to limit to 50 A as in blowing a fuse
then you need a sensing shunt and a trigger circuit to quickly switch the MOSFET off.

As a switch you could use the MOSFET upside down like I do here for example to get enough gate drive:
https://panteltje.nl/panteltje/cb/tx_power_switch2.jpg
but even that is 30 A peak, not continous...

So maybe simpler to just use a fuse?

No, I definitely don\'t want it cutting out. Then the other supplies would be even more overloaded, they would cut out, and I\'d have no power.

There is more to it.
Else just get a few power MOSFETs and play with those in some test circuit to get the hang of it.

I have ordered some and will play around with them and some lightbulbs as a load and see what voltages I can get.

Why not use resistors or, as someone has suggested, wire?

Because I want to be able to tweak it. A variable resistor of that power output, and at fractions of an ohm, is impossible to obtain.

Measure the power suppies and pick the resistor or wire length.

I\'ve never seen milliohm huge variable resistors.

If the mosfet works, I can just turn a dial until my amp clamp says they\'re leveled.

I\'ve found a 200A 1 milliohm mosfet, that oughta do the trick. FFS I had to order it from America, couldn\'t even find stock in China.

We stock IXFH400N075T2, a 1000 watt, 1000 amp mosfet. 2.3 mohms with
10 volts on the gate. You\'ll need 10 mohms, roughly.

Too late, already paid the ludicrous postage.

How much would two of yours have cost me to get shipped to Scotland if you\'re not in Scotland. Judging by \"highland\" I\'m guessing you are.


No, San Francisco.

I started my biz in the basement of my old Victorian house on Highland
Avenue. I decided to incorporate and my lawyer asked me for the
company name so I said \"Umm, Highland Technology?\"

This is the Highland Avenue Bridge, which hops over the old Bernal
Cut, originally a railroad track.

https://www.dropbox.com/scl/fi/422n7y4bkany7zwwvp7mr/Highland_Bridge..jpg?dl=0&rlkey=rujkh96w0ksjmh35rs7wha5ej

Do you want some fets? We could ship you a couple.

I don\'t need any now as I bought them from mouser. Might do in the future. How much would you charge to ship say 6 fets capable of this sort of thing to Scotland?

I just give stuff to Matt and he ships it.
Click to expand...

Ah, so I\'d get fets from you for free :)

Milliohmish, 200A ish. 100W+.

And what do you sell other than fets?

We sell equipment, not parts.

http://www.highlandtechnology.com/

We give away parts, like to customers who fried a transformer or broke
a cable or something. They seem to like that.

There\'s a problem on a box we sold to NIF, 54 units. Sometimes one of
the ADI parts has excessive leakage current and creates a small offset
that, in theory, could cost a Nobel prize or two. A small new PCB with
a few parts and SMA connectors will fix all the boxes. For free. I\'ll
hack a test board this weekend and let them try it.

I hate it when people charge insane prices for spare parts, like 20x
what they must cost to make.
Click to expand...

Mouser charge £20 shipping. Fedex priority only. I didn\'t need that.
 
On Fri, 16 Jun 2023 22:45:39 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 21:16:29 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 18:43:59 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:58:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 04:16:02 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

You mean the dpaks that run at 200 amps?

Uh oh....
I got these: https://www.vishay.com/docs/77646/sum40014m.pdf

375 watts is absurd for that toy fet. Even 125 is crazy.

I don\'t like the package. I like them screwed firmly to the heatsink like TO-247. Those are the size I see inside 1-3kW supplies.

Phil is referring to IR posting absurd specs for their fets, like
ignoring the leads, or submerging them in boiling coolants to get the
ratings. So, everybody else had to do it to be competitive.

It does say \"at 25C\".

Solder it to half the universe made of silver.
Click to expand...

I could use a liquid nitrogen setup like some crazy folk do with their CPUs to gain a few GHz. Or fill the whole computer with oil.
 
On Fri, 16 Jun 2023 23:17:31 +0100, boB <boB@k7iq.com> wrote:

On Fri, 16 Jun 2023 14:56:14 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Fri, 16 Jun 2023 14:25:06 -0700, boB <boB@K7IQ.com> wrote:

On Fri, 16 Jun 2023 21:16:29 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 18:43:59 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:58:05 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 04:16:02 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 02:19:21 -0000 (UTC), Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Thu, 15 Jun 2023 17:19:23 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-15 01:23, Jan Panteltje wrote:
On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount?
Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.

You could use a switcher with filter.
So define what you find \'simple\'
?


Not to mention that a good many modern FETs have safe operating areas
that are hard to distinguish from the Y axis. ;)

Cheers

Phil Hobbs

On. Off. What else is there?



Gas. Some of those jokey IR datasheets give new meaning to ‘vaporware’.

As in, exploded all over the inside of a box.

I blew a fet across a room once. I didn\'t know the soldering iron was grounded.

You mean the dpaks that run at 200 amps?

Uh oh....
I got these: https://www.vishay.com/docs/77646/sum40014m.pdf

375 watts is absurd for that toy fet. Even 125 is crazy.

I don\'t like the package. I like them screwed firmly to the heatsink like TO-247. Those are the size I see inside 1-3kW supplies.

Phil is referring to IR posting absurd specs for their fets, like
ignoring the leads, or submerging them in boiling coolants to get the
ratings. So, everybody else had to do it to be competitive.

It does say \"at 25C\".


That FET is perfect for this application. BUT it will still have to
just turn OFF when you exceed your 50 amp threshold.
You can not use it in a linear voltage regulating mode at these high
values of current. And you won\'t be able to keep the tab at 25
degrees C either.

He precisely wants to use it as a 50 amp linear regulator.



You COULD turn it into a switcher though and buck converter to reduce
the voltage. Just add some switching circuitry, FET drive and an
inductor and capacitor basically.

Forget trying to dissipate hundreds of watts in that one FET.

He would dissipate 50 watts or less. One big fet on a giant heat sink
could do that, but not a surface-mount dpak sort of thing.

That particular FET is about the best junction to case resistance you
are going to get for the price. Even a TO-247 is not going to be much
better, if at all. These D-Squared-pak FETs are FULL of silicon.
Click to expand...

I\'m usually going to be running 50 amps with a 0.5V drop across the fet. So 25W. I\'m sure I can get that off with a sink.

But they can\'t \"easily\" dissipate 50 watts I would say. Maybe with a
lot of air blowing. How much does he want to spend on this ? I guess
that is really the question.
Click to expand...

Not much.
 
On Sat, 17 Jun 2023 00:15:01 +0100, \"Commander Kinsey\"
<CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 22:43:42 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 21:12:48 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Fri, 16 Jun 2023 18:37:58 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Fri, 16 Jun 2023 17:49:22 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 23:03:50 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 21:57:37 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 17:17:42 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Thu, 15 Jun 2023 13:45:13 +0100, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Thu, 15 Jun 2023 12:58:43 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Thu, 15 Jun 2023 08:58:58 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16klskj8mvhs6z@ryzen.home>:

On Thu, 15 Jun 2023 06:23:51 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Wed, 14 Jun 2023 17:59:32 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16jf5imhmvhs6z@ryzen.home>:

Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

Dissipation is your issue, using a MOSFET in series
50 A with a voltage drop over it will bake it
volts multiplied by voltage dropped makes watts.
You could use a switcher with filter.
So define what you find \'simple\'
?

Can I just do this?

https://electronics.stackexchange.com/questions/377246/how-do-mosfets-and-potentiometers-work-together

What would the minimum voltage drop be?

That is a source follower,
Basicaly you put a voltage on the gate, the drain goes to the power supply output,
and the load to the source.
The problem is that at 50 A and when you drop 5 V over the MOSFET,
it will dissippate 5 x 50 = 250 W when the load is 50 A, less with a lower load current.
Basically the MOSFET will melt.
If it is on a heatsink and heatsink plus thermal resistance of the MOSFET is 1.5 degrees C per Watt then at 250 W it will rise
in temperature by 1.5 * 250 = 375 degrees C.
Add the 350 to say 20 degrees C ambitient temperature and the MOSFET internal will then be at 395 degrees C.
See the problem?

But I\'m not dropping 5V. I\'m dropping about 0.5V.

Plus you need a high enough voltage at the gate to go all the way up to 12 V, a souce follower does not provide that.

Doesn\'t it try to match the voltage? If I put 12.85V form the supply into the gate, it will fully open to 12.85V through the source-drain path.

The minimum voltage drop depends on the MOSFET\'s \'on\' resistance and you can find that in the dataheets as Rds_on,
it maybe very low, some milli-ohms,

I found one with 1 milliohm.

it will not get very hot in such a case at 50 A (but still calculate it
and use a heatsink, for example 50 mOhm Rds_on at 50 A, gives P = i^2 x R so 50 * 50 * .05 still makes 125 Watt!

The 1 milliohm one at 50A will produce only 2.5W when fully on, and 25W when dropping 0.5V.

If you just want to limit to 50 A as in blowing a fuse
then you need a sensing shunt and a trigger circuit to quickly switch the MOSFET off.

As a switch you could use the MOSFET upside down like I do here for example to get enough gate drive:
https://panteltje.nl/panteltje/cb/tx_power_switch2.jpg
but even that is 30 A peak, not continous...

So maybe simpler to just use a fuse?

No, I definitely don\'t want it cutting out. Then the other supplies would be even more overloaded, they would cut out, and I\'d have no power.

There is more to it.
Else just get a few power MOSFETs and play with those in some test circuit to get the hang of it.

I have ordered some and will play around with them and some lightbulbs as a load and see what voltages I can get.

Why not use resistors or, as someone has suggested, wire?

Because I want to be able to tweak it. A variable resistor of that power output, and at fractions of an ohm, is impossible to obtain.

Measure the power suppies and pick the resistor or wire length.

I\'ve never seen milliohm huge variable resistors.

If the mosfet works, I can just turn a dial until my amp clamp says they\'re leveled.

I\'ve found a 200A 1 milliohm mosfet, that oughta do the trick. FFS I had to order it from America, couldn\'t even find stock in China.

We stock IXFH400N075T2, a 1000 watt, 1000 amp mosfet. 2.3 mohms with
10 volts on the gate. You\'ll need 10 mohms, roughly.

Too late, already paid the ludicrous postage.

How much would two of yours have cost me to get shipped to Scotland if you\'re not in Scotland. Judging by \"highland\" I\'m guessing you are.


No, San Francisco.

I started my biz in the basement of my old Victorian house on Highland
Avenue. I decided to incorporate and my lawyer asked me for the
company name so I said \"Umm, Highland Technology?\"

This is the Highland Avenue Bridge, which hops over the old Bernal
Cut, originally a railroad track.

https://www.dropbox.com/scl/fi/422n7y4bkany7zwwvp7mr/Highland_Bridge.jpg?dl=0&rlkey=rujkh96w0ksjmh35rs7wha5ej

Do you want some fets? We could ship you a couple.

I don\'t need any now as I bought them from mouser. Might do in the future. How much would you charge to ship say 6 fets capable of this sort of thing to Scotland?

I just give stuff to Matt and he ships it.

Ah, so I\'d get fets from you for free :)
Click to expand...

Since you are always so nice, yes.
 
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