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Substituting multiple lower voltage resistors for a high voltage resistor.

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Certain high voltage resistors are fairly hard to get, such as the HVR37 series from Vishay. I'm looking at redesigning a board that produces 1000V via a transformer, but I want to use parts that are more readily available.

Is it reasonable and safe to replace, say, a 1MOhm axial resistor rated for 2000V with four 250KOhm surface mount resistors rated for 500V? What might I need to watch out for?

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Yes, this is safe, but you do have to consider the layout carefully.

You have to make sure that nothing conductive gets between the pads of each resistor. For this high voltage safety purpose, you can't consider the solder mask as a reliable insulator. If a trace goes between the pads of a resistor on the same layer, then it subtracts from the clearance distance.

You also have to make sure the end to end spacing is met. For example, don't arrange the resistors in a ring so that the beginning and end of the ring are close together. If you string out the resistors in a line and make sure nothing else is near, then it is valid to add the maximum voltage ratings of the resistors to get that of the overall resistor.

Consider your pad dimensions too. Make sure there isn't too much extra pad underneath each resistor so that the clearance distance becomes too small for the full voltage the resistor can take.

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