# setting up an electronic load

I have an electronic load that I want to use to help me with some ATX switch mode power supply repairs. The instructions that came with the load were rather sparse and I haven't found much for my particular situation on the net. The electronic load is a CSI3710A sold by Circuit Specialist. It is a rather inexpensive model and may not have lots of features but I may not need much. I just don't want to damage it or add more damage to the power supplies. I am confused about the Constant Current, Constant Voltage and Constant Resistance Modes and which one to use. When working with a PS that has the +12,+5 and -5 should I tap the +5 or does it matter? Does it matter if the PS is not putting out any voltage or a low voltage would that damage the load? Anything else to watch out for? Thanks

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I'm not sure what you mean by tapping the +5V line. If I were testing power supplies, I would first test each source separately, then have the maximum rated load (for each source) on each one at the same time if there were no faults identified during individual testing. –  Jesse Jul 14 '10 at 12:27

For most testing constant current is the easiest mode to use. You can set the current to the appropriate current level and verify the output voltage specification.

• Constant Current -- the load will try to sink the programmed current
• Constant Resistance -- the load will try to sink a current to maintain a programmed resistance. This is done my multipliers in the control circuit.
• Constant Voltage -- the load will will increase current to maintain a programmed voltage. This is useful when trying to find the current at which the power supply folds back (knee current). You set the load to the foldback voltage and then measure the current. It is quicker than stepping the current but does require a more complex control circuit.

Since most loads are current sinks you shouldn't be able to damage it when the supply is putting out low voltages. You could damage the load with too high a voltage but for an ATX power supply this is unlikely. The output stage of most loads consists of power MOSFETs and these typically have breakdown voltages of 20V and above.

Some power supplies have minimum load requirements for the individual outputs. A comprehensive test would load all the outputs at various combinations of min and max loads but you could probably attach the load to one output at a time. Make sure that each of the other outputs has the min load.

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+1 for "Make sure that each of the other outputs has the min load". Min load is a royal pain when debugging ATX supplies. Better invest in a supply of high-power resistors. –  Kevin Vermeer Feb 15 '11 at 20:41