Current limiting with a mosfet?...

[Commander Kinsey]

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?

 

[Commander Kinsey]

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.

 

[boB]

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.

 

[Fred Bloggs]

On Wednesday, June 14, 2023 at 12:59:41 PM UTC-4, Commander Kinsey 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?

It\'s not that easy because the current limit works by increasing its voltage drop. At large currents that means large power dissipation in MOSFET.

 

[John Larkin]

On Wed, 14 Jun 2023 10:31:09 -0700, 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.

Yes, series fets could easily fry at 50 amps. Open loop with gate pots
would be barbaric.

If the supplies have remote sense, that could be used to balance them.

 

[Commander Kinsey]

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.

 

[Commander Kinsey]

On Wed, 14 Jun 2023 19:07:23 +0100, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Wed, 14 Jun 2023 10:31:09 -0700, 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.

Yes, series fets could easily fry at 50 amps. Open loop with gate pots
would be barbaric.

If the supplies have remote sense, that could be used to balance them.

Unfortunately they don\'t (AFAIK). They\'re old server power supplies. Two of 2600W and two of 1300W HP. There are many pins, and I\'ve heard some can even be used to talk to another supply to balance, but probably not between differing models. If a pin is a remote voltage sense, that would really help, a voltage divider could be used easily to fool it into raising the voltage very slightly. At the moment I\'ve only connected the 0V and 12V power lines, and two of the little pins to power it up (in the same way as you do with an ATX).

 

[Commander Kinsey]

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.

 

[Commander Kinsey]

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.

 

[whit3rd]

On Wednesday, June 14, 2023 at 2:37:10 PM UTC-4, Commander Kinsey wrote:

On Wed, 14 Jun 2023 19:07:23 +0100, John Larkin <jla...@highlandsnipmetechnology.com> wrote:

On Wed, 14 Jun 2023 10:31:09 -0700, boB <b...@K7IQ.com> wrote:

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

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

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

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.

Sounds iffy; if the potentiometer setting drifts, what happens?

If the supplies have remote sense, that could be used to balance them.

Unfortunately they don\'t (AFAIK).

If you really need to gang a 50A+ output, use a design that\'s meant for paralleling
outputs; there\'s switchmode converters that have provision for multiple
paralleled units, by coupling the control sections\' duty cycles.
Vicor is one source.

 

[boB]

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.

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.

boB

 

[Tabby]

On Wednesday, 14 June 2023 at 19:07:49 UTC+1, John Larkin wrote:

On Wed, 14 Jun 2023 10:31:09 -0700, boB <b...@K7IQ.com> wrote:

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

On Wed, 14 Jun 2023 17:59:32 +0100, Commander Kinsey <C...@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.


Yes, series fets could easily fry at 50 amps. Open loop with gate pots
would be barbaric.

If the supplies have remote sense, that could be used to balance them.

Barbaric, but doable for a 1-off. High current fets are free aplenty these days in waste psus, pcs etc
OTOH the whole project sounds ill conceived, but what\'s new.

 

[Commander Kinsey]

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.

 

[Jan Panteltje]

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\'
?

 

[Commander Kinsey]

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.

I\'d be dropping maybe 0.1V. 5W isn\'t a problem with a heatsink.

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

5-10 components.

If a mosfet is supplied with a certain voltage, will it drop a certain voltage? I was thinking just adjusting a voltage with a pot to feed the mosfet would adjust how much it lets through. It doesn\'t have to self regulate, I\'d just tweak it whenever loading changed so all supplies fed about the same amount.

I wanted to make the basic supplies like the variable one, where I can adjust the voltage with its trim pot. With that one, if it\'s doing more or less work than the others, I just turn the pot a bit.

 

[Jan Panteltje]

On a sunny day (Thu, 15 Jun 2023 07:37:09 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16khz7crmvhs6z@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.

I\'d be dropping maybe 0.1V. 5W isn\'t a problem with a heatsink.

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

5-10 components.

If a mosfet is supplied with a certain voltage, will it drop a certain voltage? I was thinking just adjusting a voltage with a
pot to feed the mosfet would adjust how much it lets through. It doesn\'t have to self regulate, I\'d just tweak it whenever
loading changed so all supplies fed about the same amount.

I wanted to make the basic supplies like the variable one, where I can adjust the voltage with its trim pot. With that one, if
it\'s doing more or less work than the others, I just turn the pot a bit.

I do remember we had a system that, when too high a current was experienced in one of 2 parallel supplies,
somehow it pushed the reference voltage down a bit on the highest one until both did read the same
No serial stuff needed.
But that of course requires access to the electronics of the supplies.
There are many solutions, each with their pro and cons , google is your friend, just found this
https://iopscience.iop.org/article/10.1088/1742-6596/2417/1/012009/pdf

 

[Commander Kinsey]

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?

 

[Commander Kinsey]

On Thu, 15 Jun 2023 08:02:48 +0100, Jan Panteltje <alien@comet.invalid> wrote:

On a sunny day (Thu, 15 Jun 2023 07:37:09 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16khz7crmvhs6z@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.

I\'d be dropping maybe 0.1V. 5W isn\'t a problem with a heatsink.

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

5-10 components.

If a mosfet is supplied with a certain voltage, will it drop a certain voltage? I was thinking just adjusting a voltage with a
pot to feed the mosfet would adjust how much it lets through. It doesn\'t have to self regulate, I\'d just tweak it whenever
loading changed so all supplies fed about the same amount.

I wanted to make the basic supplies like the variable one, where I can adjust the voltage with its trim pot. With that one, if
it\'s doing more or less work than the others, I just turn the pot a bit.

I do remember we had a system that, when too high a current was experienced in one of 2 parallel supplies,
somehow it pushed the reference voltage down a bit on the highest one until both did read the same
No serial stuff needed.
But that of course requires access to the electronics of the supplies.
There are many solutions, each with their pro and cons , google is your friend, just found this
https://iopscience.iop.org/article/10.1088/1742-6596/2417/1/012009/pdf

I did say *simple* circuit

I\'m gonna try resistor + potentiometer in series across the supply, feeding the gate of a mosfet (a 1 milliohm 200A 40V variety), they were only a few dollars each from mouser. Should let me drop the voltage in whatever range I choose from the resistance values. In this case I\'ll be dropping 12.85V to 12.35V to match the lower supplies, with tiny adjustments to take care of voltage drops around the bus, so I\'ll pick the pot and resistor so I can vary from a 0 to 1V drop.

 

[Jan Panteltje]

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?
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.

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, 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!


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?

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.

 

[Jan Panteltje]

On a sunny day (Thu, 15 Jun 2023 12:30:11 +0100) it happened \"Commander
Kinsey\" <CK1@nospam.com> wrote in <op.16kvklq6mvhs6z@ryzen.home>:

I\'m gonna try resistor + potentiometer in series across the supply, feeding the gate of a mosfet (a 1 milliohm 200A 40V
variety), they were only a few dollars each from mouser. Should let me drop the voltage in whatever range I choose from the resistance
values. In this case I\'ll be dropping 12.85V to 12.35V to match the lower supplies, with tiny adjustments to take care of
voltage drops around the bus, so I\'ll pick the pot and resistor so I can vary from a 0 to 1V drop.

That, source follower, does not work, not enough gate drive, see my other reply.
Look up Vgs on, or Vgs versus Ids.