High-Current Load Bank for testing transformers

How do you test a transformer or power controller to see that it works properly over the full range of currents and voltages?  You need something that will cover the range of 0-20 volts and 0-10 amps, or even higher currents for testing overcurrent protection circuits or high-current devices such as TPCs.  Furthermore, the load should be capable of sustained operation so that you can check for long term overheating in the device being tested, even though the load bank may be dissipating around 200 watts.

This is not a trivial problem.  There is no readily-available inexpensive giant 10-ohm rheostat that can dissipate 200 watts when set to only 10% of its maximum value.  (Rheostats are rated for wattage and current.  The wattage rating assumes that the entire arc of the rheostat is evenly dissipating heat.  As you reduce the resistance, the heat is concentrated in only a portion of the arc, reducing the maximum usable power.  At 10% of full value, the maximum power may drop to 15 or 20% of the rated power value.  For our case, a steady 200 watts at 10% would require a rating of at least 1000 watts!  Also, the current must not exceed the rated maximum value, which may be determined by the wiper arm and sliding contact interface.)

A common solution is to use a bank of power resistors that can be switched to various series and parallel connections to achieve the desired range of resistances.  Here are a few of the problems along the way:

First, you would need a reasonably accurate AC meter.  Many handheld meters do not have a range that goes up to 10 amps for AC.

Second, you need a bank of resistors that will allow different resistances from about 20 ohms to less than 1.5 ohms.  When talking about resistances like 1.5 ohms, you need to consider the resistance of the wiring, switches, attachment clips (large alligator clips maybe?).  You also need to use large wire that will carry 15 amps without any problems. #10 gauge wire is 1 milliohm/foot.

Third, since you will be generating 180 watts of heat, you need a way to cool the resistors.  You would like to keep the temperature reasonable so that the resistances don’t increase too much due to temperature.  The “quickie” way is to dunk the power resistors in a container of water.  (I used this method for many years!)  A better way is a heatsink with forced air cooling.

My Solution

I now use a bank of 25-watt power resistors mounted to a large aluminum heatsink.  I have switches for arranging serial and parallel connections.  The heatsink is mounted on a shoebox that has an internal AC-powered cooling fan blowing air across the heatsink.  (The fan is sucking air into the box and expelling it upward over the heatsink fins through a cutout under the heatsink.)

Resistor box top

Resistor box back

I don’t know of a simple solution to this problem, but I am certainly willing to listen to your ideas!!

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