A Load Box
for Testing Power Supplies.


I have done my best to make this device intuitive to operate. There are warning lights and meters but it isn't fool proof by a long way. I can't stand beside you and hold your hand to be sure you don't make a mistake and burn out either something in the box or in the power supply you are testing. You're the one who will plug in the banana plugs and turn the switches. I can't be held responsible if you mess it up.

Connecting the Load Box to the Power Supply.

The built in milliammeter has not been designed to operate at a high potential with respect to the chassis of the load box. For your own safety always connect it in the common, or ground, lead of the circuit. See the diagram below.

The load box has been designed to work with negative ground and positive ground power supplies. There is no need to operate it with the chassis of the box at a different potential from the power supply under test.

Here is how to connect the box for negative and positive ground.

 Schematic diagram

For a verbal description click here.

Using the Load Box.

Caution: It is not recommended that you change any of the switches from or to the open position while power is turned on. You should turn off the power before removing a switch from the load or adding one to the load.

The applied voltage limits where you may set the switches. For example, if you are applying 300 volts, any switch may not be set below position 3. Position 3 corresponds to 3 k ohms on the 1 k switches, 9 k ohms on the 3 k switches, and 30 k ohms on the 10 k switches. You may set them to any higher position. If you are applying 500 volts then the lower limits are 5 k ohms, 15 k ohms, and 50 k ohms respectively.

The upper left switch is labeled "FAN POWER" and it is what makes the fan run. It should always be kept in the lowest position possible. The exception to this is if you want to draw less than 100 mA from the test power supply. If you are drawing small currents the fan is not necessary.

AC Operation.

You may use the box to load the secondary of a transformer to find out how it will respond to a given load applied for a long time. The fan runs as well on Ac as DC. The overload lights give an early warning. You should monitor the voltage and adjust the switches accordingly ignoring the overload indicator lights. AC power is the same as DC power. A voltage of 100 volts per resistor is still 10 watts which is the maximum they will take without burning out.


This page last updated May 22, 2005.