Transistor Or Resistor

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

Reply to
Chris W
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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
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Reply to
Tim Wescott

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.

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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

Reply to
Chris W

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

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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

Reply to
John Larkin

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.

Reply to
default

With an infinite heat sink, perhaps.

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

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

Sure, enough water could probably put the fire out.

Reply to
krw

a
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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.

Reply to
Ron M.

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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.

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Tim Wescott
Control system and signal processing consulting
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Reply to
Tim Wescott

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

Reply to
Chris W

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....)

automotive lamps are another option....

50A on 14V is 700W

25A on 5V is 125W

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

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Reply to
Jasen Betts

Chris W Inscribed thus:

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. :-)

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Best Regards:
                Baron.
Reply to
baron

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.

Reply to
default

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.
Reply to
gearhead

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.

Reply to
Bob Eld

100 Amp 6 Volt/12 Volt Battery Load Tester

Item # 90636 at Harbor Freight

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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

Reply to
ehsjr

"Bob Eld"

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

Go look up the specs of each sometime - d*****ad.

One has a big tempco, the other almost none.

** Ordinary power resistors will have no such problems at all.
** You need to look up what "second breakdown " actually refers to.
** You could start there.
** For once - a vaguely sensible idea.

** 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

Reply to
Phil Allison

gearhead Inscribed thus:

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.

--
Best Regards:
                     Baron.
Reply to
Baron

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.

Reply to
whit3rd

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:

Reply to
Chris W

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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

Reply to
David Eather

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