Electronic – Constant resistance mode in electronic load

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.

It might work, but I would suggest another approach that is open loop and has 8 bit accuracy.

You can implement a discrete version or buy the chip and R/3R precision component.