Transistor Or Resistor

[Chris W]

I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much as
14V and from about 1 to 25 amps on 5V and 3.3V. Can someone recommend a
specific transistor that would work good? I am hoping I can do it with
fewer transistors. I do plan on using a large heat sink and fan to keep
this cool.

Thanks,
Chris W

 

[Tim Wescott]

Chris W wrote:

I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much as
14V and from about 1 to 25 amps on 5V and 3.3V. Can someone recommend a
specific transistor that would work good? I am hoping I can do it with
fewer transistors. I do plan on using a large heat sink and fan to keep
this cool.

2N3055, if they're still around. You'll spend lots on the transistors
and heatsinks, though.

Car headlights work well for this, at least for 12 volts, with car tail
lights to trim the current. They're bulkier than resistors, but loads
cheaper -- and they'll brighten your day .

Or get a space heater that uses resistance wire, chop it up into ten
equal sections -- viola! ten power resistors.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

 

[Chris W]

Tim Wescott wrote:

Chris W wrote:
I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much
as 14V and from about 1 to 25 amps on 5V and 3.3V. Can someone
recommend a specific transistor that would work good? I am hoping I
can do it with fewer transistors. I do plan on using a large heat
sink and fan to keep this cool.

2N3055, if they're still around. You'll spend lots on the transistors
and heatsinks, though.

Car headlights work well for this, at least for 12 volts, with car tail
lights to trim the current. They're bulkier than resistors, but loads
cheaper -- and they'll brighten your day .

Or get a space heater that uses resistance wire, chop it up into ten
equal sections -- viola! ten power resistors.

The data sheet says it will dissipate 115 watts. And they cost $1.35 at
mouser. That will total to a lot less than the 50 watt resistors. The
cost isn't as big of an issue as the complexity of wiring up a bunch of
resistors and having to switch them in one at a time so I have to use
lots of switches too.

On a side note can this thing.
http://mouser.com/ProductDetail/Fairchild-Semiconductor/FDL100N50F/?qs=GBxGW0xXju923CYRyhG5QQ%3d%3d

really disapate 2,500 watts? If so I assume you need a very good heat
sink. Is really possible to realistically have enough heat sink to
dissipate enough heat to put anywhere near that much wattage through
that thing for more than a very short time?

Chris W

 

[John Larkin]

On Wed, 28 Apr 2010 17:36:26 -0500, Chris W <1qazse4@cox.net> wrote:

I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much as
14V

Which is 700 watts. No single transistor is going to do that. A
reasonable number would be 10 big power transistors on a huge
fan-cooled heat sink. It becomes non-trivial to drive them properly.

There are commercial transistor-based load boxes

http://www.teknetelectronics.com/Search.asp?p_ID=18842&pDo=DETAIL&Kikusui_PLZ700W

but as you can see they are a pretty big deal.

Resistors are a lot simpler. These are neat:

ftp://jjlarkin.lmi.net/Welwyn.JPG

but the best heat sink is no heat sink, like just letting some
nichrome wire get red hot.

John

 

[default]

On Wed, 28 Apr 2010 18:14:12 -0500, Chris W <1qazse4@cox.net> wrote:

Tim Wescott wrote:
Chris W wrote:
I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much
as 14V and from about 1 to 25 amps on 5V and 3.3V. Can someone
recommend a specific transistor that would work good? I am hoping I
can do it with fewer transistors. I do plan on using a large heat
sink and fan to keep this cool.

2N3055, if they're still around. You'll spend lots on the transistors
and heatsinks, though.

Car headlights work well for this, at least for 12 volts, with car tail
lights to trim the current. They're bulkier than resistors, but loads
cheaper -- and they'll brighten your day .

Or get a space heater that uses resistance wire, chop it up into ten
equal sections -- viola! ten power resistors.


The data sheet says it will dissipate 115 watts. And they cost $1.35 at
mouser. That will total to a lot less than the 50 watt resistors. The
cost isn't as big of an issue as the complexity of wiring up a bunch of
resistors and having to switch them in one at a time so I have to use
lots of switches too.

On a side note can this thing.
http://mouser.com/ProductDetail/Fairchild-Semiconductor/FDL100N50F/?qs=GBxGW0xXju923CYRyhG5QQ%3d%3d

really disapate 2,500 watts? If so I assume you need a very good heat
sink. Is really possible to realistically have enough heat sink to
dissipate enough heat to put anywhere near that much wattage through
that thing for more than a very short time?

Chris W

You can also use a combination of transistor and resistor(s) to give
you control and limit the maximum current (most of the heat in the
resistor not the transistor)

In a simple circuit with a transistor and potentiometer to set the
current - you will quickly learn all about thermal runaway. As the
transistor heats, the gain goes up, so the current goes up. Not a
problem if you are sitting there watching and correcting for it, but a
problem for long term unsupervised testing. The converse also works -
you get the current where you want it after multiple adjustments and
the ambient temperature drops a little and the current drops too.

An adjustable constant current circuit is a better safer bet.
--

 

[krw@att.bizzzzzzzzzzzz]

On Wed, 28 Apr 2010 18:14:12 -0500, Chris W <1qazse4@cox.net> wrote:

Tim Wescott wrote:
Chris W wrote:
I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much
as 14V and from about 1 to 25 amps on 5V and 3.3V. Can someone
recommend a specific transistor that would work good? I am hoping I
can do it with fewer transistors. I do plan on using a large heat
sink and fan to keep this cool.

2N3055, if they're still around. You'll spend lots on the transistors
and heatsinks, though.

Car headlights work well for this, at least for 12 volts, with car tail
lights to trim the current. They're bulkier than resistors, but loads
cheaper -- and they'll brighten your day .

Or get a space heater that uses resistance wire, chop it up into ten
equal sections -- viola! ten power resistors.


The data sheet says it will dissipate 115 watts.

With an infinite heat sink, perhaps.

And they cost $1.35 at mouser. That will total to a lot less than the
50 watt resistors.

What's the cost of your infinite heat sink, though.

The cost isn't as big of an issue as the complexity
of wiring up a bunch of resistors and having to switch them in one at a
time so I have to use lots of switches too.

You could use the transistors as switches (let resistors get hot - that's what
they're for).

On a side note can this thing.
http://mouser.com/ProductDetail/Fairchild-Semiconductor/FDL100N50F/?qs=GBxGW0xXju923CYRyhG5QQ%3d%3d

really disapate 2,500 watts? If so I assume you need a very good heat
sink. Is really possible to realistically have enough heat sink to
dissipate enough heat to put anywhere near that much wattage through
that thing for more than a very short time?

Sure, enough water could probably put the fire out.

 

[Tim Wescott]

Chris W wrote:

Tim Wescott wrote:
Chris W wrote:
I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I
will need 15 of them to get the current drain I want. I would also
like to load 5V and 3.3V lines and of course that would require
different resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much
as 14V and from about 1 to 25 amps on 5V and 3.3V. Can someone
recommend a specific transistor that would work good? I am hoping I
can do it with fewer transistors. I do plan on using a large heat
sink and fan to keep this cool.

2N3055, if they're still around. You'll spend lots on the transistors
and heatsinks, though.

Car headlights work well for this, at least for 12 volts, with car
tail lights to trim the current. They're bulkier than resistors, but
loads cheaper -- and they'll brighten your day .

Or get a space heater that uses resistance wire, chop it up into ten
equal sections -- viola! ten power resistors.


The data sheet says it will dissipate 115 watts. And they cost $1.35 at
mouser. That will total to a lot less than the 50 watt resistors. The
cost isn't as big of an issue as the complexity of wiring up a bunch of
resistors and having to switch them in one at a time so I have to use
lots of switches too.

On a side note can this thing.
http://mouser.com/ProductDetail/Fairchild-Semiconductor/FDL100N50F/?qs=GBxGW0xXju923CYRyhG5QQ%3d%3d


really disapate 2,500 watts? If so I assume you need a very good heat
sink. Is really possible to realistically have enough heat sink to
dissipate enough heat to put anywhere near that much wattage through
that thing for more than a very short time?

2500 watts if you maintain the case at 25C, based on a thermal
resistance of 0.05 C/W junction to case, and a maximum junction
temperature of 150C.

Of course, they claim a _minimum_ of 0.1 C/W case to sink, so that drops
you down to 833 watts, and that's _if_ you maintain the heat sink
temperature at 25C. You'll spend a lot more time with the thermal
engineering of the heat sink than you would wiring up a few switches and
resistors, that's for sure.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

 

[Ron M.]

On Apr 28, 6:36 pm, Chris W <1qaz...@cox.net> wrote:

I want to make a load center to test power supplies and batteries.  I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want.  I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor.  The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much as
14V and from about 1 to 25 amps on 5V and 3.3V.  Can someone recommend a
specific transistor that would work good?  I am hoping I can do it with
fewer transistors.  I do plan on using a large heat sink and fan to keep
this cool.

Thanks,
Chris W

You can buy the replacement nichrome coils in varying wattage levels
at most electrical supply houses. I used an old hair dryer for an
audio dummy load for long term amp testing (BURN IN). Works great and
can be easily mod'd for your ap. Handles 1300 watts too. Get's rather
toasty though.

 

[Chris W]

Tim Wescott wrote:

2500 watts if you maintain the case at 25C, based on a thermal
resistance of 0.05 C/W junction to case, and a maximum junction
temperature of 150C.

Of course, they claim a _minimum_ of 0.1 C/W case to sink, so that drops
you down to 833 watts, and that's _if_ you maintain the heat sink
temperature at 25C. You'll spend a lot more time with the thermal
engineering of the heat sink than you would wiring up a few switches and
resistors, that's for sure.

So how many watts could it realistically dissipate? Even if it is only
200 that is still 4 times more than a 50 watt resistor. Then with some
kind of circuit to regulate the current and maybe 5 or 10 of those I
could use that at any of the voltages I want to test at. Couldn't I?


Thanks again for the help

Chris W

 

[Jasen Betts]

On 2010-04-28, Chris W <1qazse4@cox.net> wrote:

I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

Or you can make your own from nichrome wire (which is free if you have an
old toaster, or hair dryer (etc...) handy for parts....)

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

automotive lamps are another option....

The goal is to have a variable load of about 3 to 50 amps on as much as
14V

50A on 14V is 700W

25A on 5V is 125W

Can someone recommend a
specific transistor that would work good?

I do plan on using a large heat sink and fan to keep
this cool.

I'd go with nichrome wire and a fan, build a low voltage hair dryer.


--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

 

[baron]

Chris W Inscribed thus:

Tim Wescott wrote:

2500 watts if you maintain the case at 25C, based on a thermal
resistance of 0.05 C/W junction to case, and a maximum junction
temperature of 150C.

Of course, they claim a _minimum_ of 0.1 C/W case to sink, so that
drops you down to 833 watts, and that's _if_ you maintain the heat
sink
temperature at 25C. You'll spend a lot more time with the thermal
engineering of the heat sink than you would wiring up a few switches
and resistors, that's for sure.


So how many watts could it realistically dissipate? Even if it is only
200 that is still 4 times more than a 50 watt resistor. Then with
some kind of circuit to regulate the current and maybe 5 or 10 of
those I
could use that at any of the voltages I want to test at. Couldn't I?


Thanks again for the help

Chris W

Which ever way you go you are going to have to get rid of around 700
watts of heat. Water cooling is starting to look good.

--
Best Regards:
Baron.

 

[default]

On Thu, 29 Apr 2010 13:10:17 +0100, baron
<baron.nospam@linuxmaniac.nospam.net> wrote:

Which ever way you go you are going to have to get rid of around 700
watts of heat. Water cooling is starting to look good.

You are on to something there. Use resistors to heat water?

I used to work at a PS manufacturer - we used transistor resistor
loads, carbon piles, and some old theater lamp dimmer rheostats, to
test and set the over current circuits. For burn-in, fixed resistors
are hard to beat for fiddle free operation, cost and reliability.
--

 

[Bob Eld]

"Chris W" <1qazse4@cox.net> wrote in message
news:ML2Cn.162708$EE6.87845@newsfe23.iad...

I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much as
14V and from about 1 to 25 amps on 5V and 3.3V. Can someone recommend a
specific transistor that would work good? I am hoping I can do it with
fewer transistors. I do plan on using a large heat sink and fan to keep
this cool.

Thanks,
Chris W

Lamps and nichrome wire have been proposed but any element that gets hot
enough will have a significant change in resistance making the load
non-linear which might be a problem. Power transistors will have this
problem unless in an active circuit is used to keep resistance, current or
load constant or controlled. Be ware of second breakdown if using power
transistors. This occurs when both voltage and current are present at the
same time even though neither individual rating is exceeded. Check the data
sheets.

One thing I've done for high power loads is to put power resistors under
water in a can or bowl. This keeps them cool, the resistance constant and
allows them to dissipate many times their rated power. A single 4 ohm 50
watt wire wound resistor will probably dissipate 500 watts when under water
without a problem. An array of resistors can be used to make an adjustable
load.

Of course water is corrosive over time so the resistors are removed from the
water and dried off when not in use.

 

[ehsjr]

Chris W wrote:

I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much as
14V and from about 1 to 25 amps on 5V and 3.3V. Can someone recommend a
specific transistor that would work good? I am hoping I can do it with
fewer transistors. I do plan on using a large heat sink and fan to keep
this cool.

Thanks,
Chris W

100 Amp 6 Volt/12 Volt Battery Load Tester

Item # 90636 at Harbor Freight
http://www.harborfreight.com/100-amp-6-volt-12-volt-battery-load-tester-90636.html

On sale now for $19.99

Use as is, or use the element as a load resistor in whatever
circuit you design. Using it as is will save you $$, burned
out power transistors, large heat sinks etc - and the need
for Joerg to provide sound effects for circuit demise.

Ed

 

[Phil Allison]

"Bob Eld"

Lamps and nichrome wire have been proposed but any element that gets hot
enough will have a significant change in resistance making the load
non-linear which might be a problem.

** Tungsten lamps and Ni-Chrome wire are very different animals.

Go look up the specs of each sometime - dickhead.

One has a big tempco, the other almost none.

Power transistors will have this
problem unless in an active circuit is used to keep resistance, current or
load constant or controlled.

** Ordinary power resistors will have no such problems at all.

Be ware of second breakdown if using power
transistors. This occurs when both voltage and current are present at the
same time even though neither individual rating is exceeded.

** You need to look up what "second breakdown " actually refers to.

Check the data sheets.

** You could start there.

One thing I've done for high power loads is to put power resistors under
water in a can or bowl. This keeps them cool, the resistance constant and
allows them to dissipate many times their rated power. A single 4 ohm 50
watt wire wound resistor will probably dissipate 500 watts when under
water
without a problem. An array of resistors can be used to make an
adjustable
load.

** For once - a vaguely sensible idea.

Of course water is corrosive over time so the resistors are removed from
the
water and dried off when not in use.

** The corrosion problem is worse when DC voltages are applied to
esistors - but only IF bare metal conductors are left exposed to the
water.

Easy enough to coat or over them with insulation materials.


.... Phil

 

[gearhead]

On Apr 29, 8:10 am, baron <baron.nos...@linuxmaniac.nospam.net> wrote:

Chris W Inscribed thus:



Tim Wescott wrote:

2500 watts if you maintain the case at 25C, based on a thermal
resistance of 0.05 C/W junction to case, and a maximum junction
temperature of 150C.

Of course, they claim a _minimum_ of 0.1 C/W case to sink, so that
drops you down to 833 watts, and that's _if_ you maintain the heat
sink
temperature at 25C.  You'll spend a lot more time with the thermal
engineering of the heat sink than you would wiring up a few switches
and resistors, that's for sure.

So how many watts could it realistically dissipate? Even if it is only
200 that is still 4 times more than a 50 watt resistor.  Then with
some kind of circuit to regulate the current and maybe 5 or 10 of
those I
could use that at any of the voltages I want to test at.  Couldn't I?

Thanks again for the help

Chris W

Which ever way you go you are going to have to get rid of around 700
watts of heat.  Water cooling is starting to look good.

--
Best Regards:
                Baron.

Absotively posolutely.
I used to have a dummy load for tuning the output tubes on my old Swan
350.
It was a fat-ass resistor in a paint can that you fill with oil.
OP can kluge up his own liquid-cooled load, but speaking from
experience the water gets nasty.
Which is probably why people prefer oil, even though thermally it
probably doesn't quite have the performance of water. Plus oil
doesn't evaporate. But for something just to bang together for a
quick test, I just dunk a power resistor in a yogurt container or
something.
And as others have pointed out, there are plenty of other options,
like incandescent lamps, hair dryers and such.
OP should be thinking about how to get rid of heat, not how to hook up
a lot of transistors. In this situation, the only sensible use of
transistors is as switches in full saturation (for bjt's) or triode
mode (mosfets). He doesn't even know how to interpret the thermal
info on a datasheet properly.

 

[Baron]

gearhead Inscribed thus:

On Apr 29, 8:10 am, baron <baron.nos...@linuxmaniac.nospam.net> wrote:

Which ever way you go you are going to have to get rid of around 700
watts of heat.  Water cooling is starting to look good.



Absotively posolutely.
I used to have a dummy load for tuning the output tubes on my old Swan
350.
It was a fat-ass resistor in a paint can that you fill with oil.
OP can kluge up his own liquid-cooled load, but speaking from
experience the water gets nasty.

I agree it does if its static. Actually I was thinking along the lines
of flowing water through a tank or tube containing the resistors.

Which is probably why people prefer oil, even though thermally it
probably doesn't quite have the performance of water. Plus oil
doesn't evaporate. But for something just to bang together for a
quick test, I just dunk a power resistor in a yogurt container or
something.
And as others have pointed out, there are plenty of other options,
like incandescent lamps, hair dryers and such.
OP should be thinking about how to get rid of heat, not how to hook up
a lot of transistors. In this situation, the only sensible use of
transistors is as switches in full saturation (for bjt's) or triode
mode (mosfets). He doesn't even know how to interpret the thermal
info on a datasheet properly.

--
Best Regards:
Baron.

 

[Chris W]

Thanks for all the replies. I still have a few questions.
First it seams that using only transistors is not a good idea. The main
reason I was hoping to get away from using all the resistors is the
cumbersome way of adjusting the load by switching in various numbers of
resistors and the fact that the resistors are only going to be able to
be used to dissipate the maximum amount of energy at one voltage.

Some one suggested using transistors as switches to the resistors. This
could make it a bit easier because I could then use a single small
switch to add several resistors to the load. However that doesn't
really do much to make the interface to adjust the load any more elegant.

Using that method the best idea I have come up with to adjust the load
is to configure it so my first switch added 1 resistor to the load, the
second switch added 2, the third, 4 and so on. Then I would treat the
row of switches like a binary number to increment the load.

The ideal situation would be to have a single pot that I could use to
adjust the load. Alternatively having 4 or 5 pots where I would use the
first one to turn the load up to it's max then the second one to add in
that load, etc. How hard would it be to use transistors to control the
current the resistors are sinking? That seems like it might be the best
solution. If I'm not mistaken the transistors wouldn't have to sink
very many watts at all.

As for dissipating 700 watts, it is closer 1000 watts
14V * 50A + 5V * 25A + 3.3V * 25A = 907.5 watts
That is the worst case, and will likely rarely see that high of a load
except for a very short time (30 second or less). The most it will ever
see for an extend time will be 600 watts. I have a 100 watt RF dummy
load. A heat sink 10 times the size of that dummy load would be big,
but still manageable. A few good fans would make it even more manageable.

Any thoughts?


Chris W wrote:

I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want. I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much as
14V and from about 1 to 25 amps on 5V and 3.3V. Can someone recommend a
specific transistor that would work good? I am hoping I can do it with
fewer transistors. I do plan on using a large heat sink and fan to keep
this cool.

Thanks,
Chris W

 

[David Eather]

On 29/04/2010 9:14 AM, Chris W wrote:

Tim Wescott wrote:
Chris W wrote:
I want to make a load center to test power supplies and batteries. I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I
will need 15 of them to get the current drain I want. I would also
like to load 5V and 3.3V lines and of course that would require
different resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor. The 50 Watt resistors are going to cost a little over $3
each and I will probably need 30 of them to get the loads I want.

The goal is to have a variable load of about 3 to 50 amps on as much
as 14V and from about 1 to 25 amps on 5V and 3.3V. Can someone
recommend a specific transistor that would work good? I am hoping I
can do it with fewer transistors. I do plan on using a large heat
sink and fan to keep this cool.

2N3055, if they're still around. You'll spend lots on the transistors
and heatsinks, though.

Car headlights work well for this, at least for 12 volts, with car
tail lights to trim the current. They're bulkier than resistors, but
loads cheaper -- and they'll brighten your day .

Or get a space heater that uses resistance wire, chop it up into ten
equal sections -- viola! ten power resistors.


The data sheet says it will dissipate 115 watts. And they cost $1.35 at
mouser. That will total to a lot less than the 50 watt resistors. The
cost isn't as big of an issue as the complexity of wiring up a bunch of
resistors and having to switch them in one at a time so I have to use
lots of switches too.

On a side note can this thing.
http://mouser.com/ProductDetail/Fairchild-Semiconductor/FDL100N50F/?qs=GBxGW0xXju923CYRyhG5QQ%3d%3d


really disapate 2,500 watts? If so I assume you need a very good heat
sink. Is really possible to realistically have enough heat sink to
dissipate enough heat to put anywhere near that much wattage through
that thing for more than a very short time?

Chris W

At 15 volts a 2n3055 will safely pass only 7.5 amps. Unless you use a
heatsink to control temperature it will only safely dissipate about 3.5
watts

 

[whit3rd]

On Apr 28, 3:36 pm, Chris W <1qaz...@cox.net> wrote:

I want to make a load center to test power supplies and batteries.  I
was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will
need 15 of them to get the current drain I want.  I would also like to
load 5V and 3.3V lines and of course that would require different
resistors.

I was wondering if this wouldn't be a lot easier with a power
transistor.

The water heater in your home has two 5kW resistors, something
in the range of 10 ohms; these are cheap enough as replacement
items (and if your municipality recycles steel water heaters,
used ones might be easily available, too). If you have long tests
in mind, it might pay to circulate water and/or use a radiator
core and airflow.

Immersible design and cooling water are the key elements.
Jumpering for parallel or series connection is tedious, but
flexible enough for LOTS of load values.

I've seen (for laboratory magnets) arrays of dozens of 2N3055
transistors, on copper plate heatsink with a soldered-on tube
for cooling water... NOT pretty.