TECH: How to test the HV diode

Recently, I have had to test HV diodes. I didn't find anything in the BW vector
faq, so I found this elsewhere on the net. Perhaps this could be added to the
faq.

This should work off of a 12vdc which is easily had off any switching power
supply. The reason why it needs to be tested this way is that most ohmmeters
will not push enough voltage across the diode to open it up, so they will read
open both directions.

JB

-----------------------------

The resistance measured across the leads of the HV diode should be greater
than 10 M ohm in at least one direction when disconnected from the circuit.
However, the HV diode is composed of multiple silicon diodes in series to
get the voltage rating. Its forward voltage drop will therefore be too great
(6 V or more) for a DMM to produce a definitive answer as to whether it
actually works as a rectifier.

The HV diode can be tested with a DC power supply (even a wall adapter of
at least 12 or 15 V output), series resistor (to limit current), and your
multimeter. This will determine proper behavior, at least at low voltages.

The following is the schematic of a simple HV diode tester:

                 240 ohms, 1 W
+ o-----------/\/\/\--------+------------o +
                                        |
                                    __|__ HV Good: 6 to 10 V
15 VDC _\_/_ diode Shorted: 0 to 2 V
                                        | Open or reversed: 15 V
                                        |
 - o-----------------------+------------o -

The voltage drop in the forward direction should be at least 6 V with a few
mA of current but may be somewhat higher (8 V or more) with a few hundred mA.
If your DMM or VOM has a resistance scale operated off a battery of at least
6 V, you may get a reading in one direction (but only one) without the need
for an external power supply.

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Received on Thu Jul 11 23:58:35 2002