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I own an inexpensive lab power supply. I do not know whether the supply is a single quadrant or dual quadrant.

What would be the best way (without reverse engineering or extensively disassembling) to test if the supply is one or two quadrant?

I have pulled the cover to look inside the supply and there are two transistors attached to the heat sink. I'm trying to figure out whether they are in parallel to give the supply more capacity (this would make it single quadrant). Or if one will source current and the other sinks current (this would make it dual quadrant).

The dual quadrant supply can both sink and source power.

schematic

simulate this circuit – Schematic created using CircuitLab

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Try connecting a battery to the output (via a 100 ohm resistor to safely limit current). Measure the voltage coming out of the power supply when the power supply voltage is set lower than the battery voltage. If it doesn't look like it is regulating to the lower voltage then it isn't a push-pull output.

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