With old diodes that may no longer meet their rated PIV rating, to new diodes that are down labeled and are actually capable of a far higher PIV than their part number would indicate. And finally to diodes that have no label at all.

Sometimes the only way to determine the true actual PIV is to physically apply the voltage to them and see where they start exhibiting the telltale signs of breakdown.

To do that, you need a highly current limited source that will not result in a catastrophic avalanche when the diode reaches that voltage.

I will describe how to make one with readily available parts.

Keep in mind, the PIV voltages determined here are absolute PIV rating. The device should not be subjected to that voltage under normal operation. A de-rating figure should be applied to the voltage determined with this test to translate it into normal usable PIV in an actual circuit. A simple figure is 150%. A diode should be capable of more than 150% of the actual PIV it will be subjected to in the circuit.

Example.
If you are going to be subjecting it to 500V PIV, then it should be nominally capable of 750V PIV or more.

Take care, and stay safe.

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