How to determine the voltage output polarity of a power supply

I have a high voltage power supply that can go up to 35000 volt.

1. How do I test to see if the unit is outputting positive polarity voltage or negative polarity voltage ?

2. What is the difference between a high power supply that put out positive polarity voltage and negative polarity voltage ?

• positive/negative wrt what ? Commented Dec 16, 2021 at 6:42
• Does it have a make/model information or manual? Commented Dec 16, 2021 at 6:44
• To Justme. Model Name: HV350CC. Made by information Unlimited. Web site for HV350CC is at amazing1.com/hv-dc-power-supplies.html Commented Dec 16, 2021 at 6:46
• To tobalt. What is wrt means ? Commented Dec 16, 2021 at 6:49
• The label at the bottom of the unit said positive but the invoice said it is a negative unit. The folks that sold the product do make mistake. Now, back to the original question. What test I need to do to determine if the output has positive or negative polarity ? Commented Dec 16, 2021 at 8:52

The instructions supplied by the manufacturer show that the model number should be on the top of the enclosure. The parenthetical suffix gives the polarity: the HV350CC(+) has positive output while the HV350CC(-) has negative. That document also says where the high-voltage supply, high-voltage return, and earth (ground) wires can be found.

The device's polarity (+ or -) is the polarity of the high-voltage supply line compared to ("with respect to") the high-voltage return line. The high-voltage return line is the virtual ground (not the actual earth ground). Using a multimeter1 you can measure the device's output voltage. If the voltage on the supply line is higher than that of the return line then the device's polarity is +, if it is lower then the polarity is -. Likewise, using the meter in current mode with with its positive lead connected to the supply wire and its negative lead connected to the return wire will show a positive current if the device is the + model and a negative current if the device is the - model.

1 Although the current is fairly modest make sure the meter you use for testing is suitably rated for the voltage and that appropriate safety precautions are taken whilst testing.

• Thank you Alex Hajnal for your answer. Is it true that the high-voltage supply line is positive (ex: Red Wire) and the high-voltage return line is negative (ex: Black Wire) ? You mentioned about testing for polarity. I have a 40KVolt meter. 1) I take a black wire on this probe and clamp it on the high-voltage return line of the power supply. Now I take the tip of the probe and I place it onto the high voltage supply line of the power supply and I got a voltage. So then how do I know which side where the voltage is higher than which side ? Commented Dec 17, 2021 at 9:34
• If the voltage you read is positive then the device's polarity is positive. If the reading is negative then the device's polarity is negative. Commented Dec 17, 2021 at 9:37

The following method may not be safe depending on the ability of the HV supply to deliver short-circuit power. Risk mitigation is added at the end, but be aware, that depending on the size of the HV supply's output capacitors, the operator could receive a lethal strike, when doing these measurements carelessly.

Connect your multimeter in current mode 1 between output side A and GND.

If it measures 0, this side is GND or isolated. If it measures 1 mA, this side is positive with respect to GND. If it measures -1 mA, this side is negative with respect to GND.

If side A was reading 0, repeat for side B.

If both sides show 0, then the output is isolated, which means that you can choose if it is negative or positive by connecting one side to GND.

1 The moment when you connect the multimeter to a differential voltage of 30 kV, the output caps of the HV suppl discharge through the multimeter in much the same way as an ESD strike. However, the total strike energy can be higher than for ESD and damage the meter. To avoid this, it would be safer to first short-circuit the measurement path with a wire, then connect the meter, and then remove the short-circuit.

• This sounds very dangerous to me. Even if the current is small, applying 35kV to the average hobbyist multimeter is likely to be a bad idea. Commented Dec 16, 2021 at 11:36
• @ElliotAlderson I agree that it sounds dangerous, but think about it. Are you afraid of killing your multimeter with ESD? Anyway, I will add some risk mitigation to the answer. Commented Dec 16, 2021 at 13:06