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Old November 25th, 2021, 05:35 PM   #1
Maxkling
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Diode DMM Question

Maybe someone can elaborate, I’ve done some quick googling and haven’t found a good answer. While troubleshooting a 24VDC contactor, I attempted to OHM out the freewheel diode and didn’t get any readings. Switched to a DMM that had a diode test and it showed the proper voltage drop reading, showing the diode as good. From what I recall, I’ve always used just a continuity test to test diodes, are there situations or certain types of diodes that won’t behave “logically” under a normal continuity check? Or was this just a “fluke”, ha.
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Old November 25th, 2021, 10:31 PM   #2
ndzied1
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This may help.

https://www.fluke.com/en-us/learn/bl...to-test-diodes
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Old November 25th, 2021, 11:16 PM   #3
jdbrandt
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Quote:
Originally Posted by Maxkling View Post
Maybe someone can elaborate, I’ve done some quick googling and haven’t found a good answer. While troubleshooting a 24VDC contactor, I attempted to OHM out the freewheel diode and didn’t get any readings. Switched to a DMM that had a diode test and it showed the proper voltage drop reading, showing the diode as good. From what I recall, I’ve always used just a continuity test to test diodes, are there situations or certain types of diodes that won’t behave “logically” under a normal continuity check? Or was this just a “fluke”, ha.
Certain selenium diodes might cause a bit of trouble, but we don't see these much anymore...except on slate panel Ward Lenord DC drive systems.
ZENER diodes, as well, depending on their rating, might not test correctly, depending on the available voltage in the DMM.
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Old November 25th, 2021, 11:54 PM   #4
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Back in the horse and buggy (and Simpson analog meter) days, before DMMs were common, I carried a 6 Volt lamp and dry cell battery in my toolkit to test SCRs in vibratory feeder controllers and soft start drives. The setup could also be used to test diodes.
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Old November 26th, 2021, 04:22 AM   #5
parky
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One problem that a meter of any kind i.e. analogue or digital regardless if it has diode test is that in some circumstances, the diode may fault under load or higher voltage, multi meters only check at low voltage, however, diodes have been known to breakdown with a higher voltage or current.
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Old November 26th, 2021, 08:17 AM   #6
Manglemender
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One problem that a meter of any kind i.e. analogue or digital regardless if it has diode test is that in some circumstances, the diode may fault under load or higher voltage, multi meters only check at low voltage, however, diodes have been known to breakdown with a higher voltage or current.
Very true! And it isn't just diodes that can break down. I remember looking at an old analogue drive for a custoemt a "few" years back where the machine would not go over about half speed. It turned out the the capacitor in the integrator curcuit was breaking down at about 5 volts but actually tested ok for capacitance so had been overlooked by another technician.

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Old November 27th, 2021, 01:59 PM   #7
bill4807
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If there was a case where you were testing lots of diodes or other component level devices.
A huntron tractor/ or curve tracer is what bench repair techs will use. Since it measures current flow, voltage drop, and phase shift.
If only checking a few, price probably isn't feasible.
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