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We have some big resistor test loads, like 8 Ω 250 W loads for simulating speakers:

enter image description here

Are there also big variable load resistors for testing power supplies? What are they called?

If not, what do people use instead?

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    \$\begingroup\$ They do exist, I have seen them with a big (4" diameter) knob to adjust the resistance. I have no clue what they are called though. \$\endgroup\$ – Kellenjb Apr 28 '11 at 15:48
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As MikeJ-UK mentioned, there are rheostats. I personally don't like them for the same reasons what Mike mentions. But there are two more options:

  1. Build a a resistor box and use switches to change the load. Or if you're fancy you can use MOSFET's and a MCU to do the job as well. Careful selection of the resistor values could give you a very wide range of resistances with fairly fine control.

  2. Buy an "electronic load". Just google the term (in quotes) and you'll come up with a lot of pages. Basically it's the opposite of a benchtop power supply. Most will work in 3 modes: Constant Current, Constant Voltage, and Constant Resistance. My employer has one that's rated for 30 KWatts! Of course they make smaller ones.

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Large open-frame wirewound potentiometers are called rheostats. You have to be careful when using these to test power supplies as it's too easy to burn them out! A fixed series padding resistor is a good idea.

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If you are going for cheap, get a large metal bucket and a bag of rock salt.

http://en.wikipedia.org/wiki/Liquid_rheostat

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    \$\begingroup\$ Strangely the Wikipedia article makes no mention of the large amounts of Hydrogen (explosive) and Chlorine (toxic) that will be produced! \$\endgroup\$ – MikeJ-UK Apr 28 '11 at 23:33
  • \$\begingroup\$ Do it outside :) \$\endgroup\$ – endolith Apr 29 '11 at 1:08
  • \$\begingroup\$ @MikeJ-UK: Is that a major problem with AC, or only with DC? \$\endgroup\$ – endolith Oct 2 '13 at 14:49
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I've used large arrays of parallel carbon plates in a vise-like structure for testing power supply overload. You close the vise tight to compress the plates and lower the resistance, and see what the power supply does.

I use active loads in my day-to-day work, mind you.

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  • \$\begingroup\$ Can you elaborate a bit on the active loads? \$\endgroup\$ – endolith Apr 28 '11 at 19:29
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    \$\begingroup\$ Well, I deal in power supplies between 12V and 54V, generally, and the company has a large number of (now discontinued) Hewlett-Packard HP6050 active load mainframes. I use between 10 and 12 channels of 60V/60A load to test the power supplies, using LabVIEW-based automation software. Agilent (HP succesor for test equipment) has a replacement (Agilent N33xx series) and Chroma has similar-styled loads. \$\endgroup\$ – Adam Lawrence Apr 29 '11 at 12:50
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    \$\begingroup\$ I used the carbon disk rheostats for discharging Ni Cad aircraft batteries in the Airforce. they can handle high wattage. \$\endgroup\$ – Jim C Apr 29 '11 at 22:37
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The limiting factor on any given rheostat is generally RMS current rather than power, though within a family the current limit is usually, up to a certain point, roughly inversely proportional to full-scale resistance; many families have a specified power rating, and the allowable RMS current will be that power divided by full-scale resistance.

If you need to generate a continuous range of resistances in the range 50-1000 ohms, you could use a 1,000-ohm rheostat, but you could probably much more cheaply use a 100-ohm rheostat plus fixed 50, 100, 200, 200, and 500-ohm resistors (many other combinations would work). Since you'd be using a 100-ohm rheostat instead of a 1K rheostat, you could use one that was rated for 1/10 the power that you'd otherwise need.

Incidentally, before the advent of triac-controlled dimmers, theatrical lighting used to use rheostats. Certainly many of those rheostats have been junked over the years; people who keep them for sale would might charge a fair bit for them, but if you asked at the right places you might be able to get them for practically nothing.

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  • \$\begingroup\$ Oh, the rheostat would be acting as a load, so it would be down near 10 Ω. \$\endgroup\$ – endolith Apr 28 '11 at 19:30
  • \$\begingroup\$ In that case, I would suggest that you might want to use series and parallel resistors together with the rheostat. If you want a load between 5 and 30 ohms, put a 20-ohm resistor in series with a 10-ohm rheostat, and add 15 100-ohm resistors that can be switched in parallel with them (assembled as parallel groups of 1, 2, 4, and 8 resistors). That will allow 16 overlapping ranges; at the lower ranges, relatively little power will go through the rheostat. \$\endgroup\$ – supercat Apr 28 '11 at 20:18
  • \$\begingroup\$ For example, with a 1-watt rheostat, 2-watt 20-ohm resistor, and half-watt 100-ohm resistors, one could handle about 6.3 volts at any scale. At 5.45 ohms (all resistors enabled--rheostat at full scale), one would be dissipating 7.2 watts, but only one watt of that would be in the rheostat. \$\endgroup\$ – supercat Apr 28 '11 at 20:24
  • \$\begingroup\$ One could increase the series resistance of the rheostat (thus reducing the amount of power it would have to handle) by adding more parallel resistors. The difference in parallel-resistance current between adjacent selections has to be less than the difference between full-scale and zero-scale rheostat current. \$\endgroup\$ – supercat Apr 28 '11 at 20:26

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