Diode DMM Question

Maxkling

Member
Join Date
Mar 2011
Location
Atlanta
Posts
511
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.
 
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.
 
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.
 
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.
 
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.

Nick
 
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.
 

Similar Topics

I have a SC9-LS24-3 surge protection solenoid cable from automation direct...
Replies
9
Views
1,302
I have an application where one ControlLogix needs to read data from another ControlLogix. The second PLC is part of a validated pharma process...
Replies
9
Views
2,891
Hi all, I have a couple of solenoids I'm using on a very low temperature application, and the documentation notes that they don't have any...
Replies
31
Views
7,014
Hello guys, Looking for some quick easy answers with out having to put in any sweat equity. :o: 1) I have some 1N9437-E3/54 1amp fast switching...
Replies
30
Views
10,239
Link 1:http://www.drillspot.com/products/152298/Square_D_8501RSD34V53_General_Purpose_Relay...
Replies
1
Views
2,696
Back
Top Bottom