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High Voltage Diode Vf Testing

Here at VMI, by virtue of working in the niche high voltage area, we get all manner of customer questions about issues that are somewhat more complicated by virtue of our unique devices.

Recently, a customer asked how they could confirm:
1)  The working ability of a high voltage diode
2)  Double-check the polarity of the device.

Background
High voltage diodes have a higher VF than the ‘ideal’ model of 0.7V, so that many of the traditional ways of checking to make sure a diode is not failed shorted, or to verify the polarity of the device, will not work with high voltage diodes.

Using a Fluke handheld multi-meter that has a diode check function, for instance, probably won’t be able to overcome the forward voltage of a diode, even at low currents. A multi-meter is providing perhaps 2.4V and some of our diodes have a rated VF of over 30V!

VMI Vf and Polarity Test Methods
VMI measures the VF of our diodes using precision, current controlled power supplies capable of providing 50V or more. Though not necessarily readily available to all users, the easiest way to test the polarity and functionality of a high voltage diode is a similar method.

Current Limiting
​Current limiting a power supply to a low level of perhaps 100µA – 1mA and connecting it up to the diode will do the trick without much effort. You need a power supply that can provide more voltage than the rating of the diode, but make sure the power supply is limited so that it cannot provide more voltage than the breakdown level of the diode.

See the simplified block diagram for an idea on how to connect the power supply up to the diode. Depending on your power supply, you might need to use a current limiting resistor and an external current meter.

How To Tell the High Voltage Diode is Good
Once you have your setup, the diode should conduct your current (100µA – 1mA, depending on how you limited the supply) in the anode-cathode direction.  Reversing the current flow by flipping the diode to cathode-anode should block the current down to the leakage levels of the diode (less than 1µA). If the diode conducts in both directions, then you have a failed (shorted) diode. If current does not flow in either direction, you have a failed (open) diode.

Questions?  We would be happy to help, so give us a call – 559.651.1402.