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Diode forward/reverse resistance measurement

Posted: Fri Oct 11, 2013 9:30 pm
by vinod
See the diagrams attached herewith.

Diode Diode arrangement

DFR- Diode Forward Resistance
DRR- Diode Reverse Resistance

Both are identical diodes and the multimeter is set in auto range(resistance mode).

MM shows 43M Ohm instead of OL! ( If I connect a multimeter positive terminal at A & negative terminal at B )

Diode Resistor arrangement

Here connecting the MM in forward mode reads 6KOhm against expected value of 1K+DFR

In reverse mode shows OPEN as expected.

Re: Diode forward/reverse resistance measurement

Posted: Sat Oct 12, 2013 5:56 am
by Externet
For every different multimeter, different readings in the resistance scale will be obtained when probing a diode, because diodes resistance is not a characteristic nor a specification, but an apparent equivalent depending on the voltage supplied by the multimeter leads and other factors as current, temperature, internal resistance of the meter...

So forget about measuring resistance in a diode. The resistance scale is for resistors, the diode check in a multimeter shows the voltage drop of the junction. And not all meters will work/show the value as for higher Vf as in LEDs.

Re: Diode forward/reverse resistance measurement

Posted: Tue Oct 15, 2013 1:31 pm
by haklesup
Yes, since a diode is non linear, the value of equivalent resistance is not very meaningful except as a qualitative comparison. The value you get is very much dependent on the drive voltage and ultimately the current that induces, tiny changes in V can have large changes in I at various points on the characteristic. The meter may assume a linear equivalent between 0V, 0A and the operating point and the operating point changes with range, generally use the lowest range.

Diode check mode on a DMM is more useful telling you the forward voltage.

Re: Diode forward/reverse resistance measurement

Posted: Wed Jan 15, 2014 7:12 am
by Rodney
The only good way I have found to check the impedance of a diode is to use an oscilloscope. Use the X and Y axis. put the current through the diode on one axis and the voltage across it on the other axis and power it with an AC voltage. The slope of the trace on the scope will be an indications of the impedance at any specific current which you can easily measure.

Re: Diode forward/reverse resistance measurement

Posted: Wed Jan 15, 2014 11:26 am
by haklesup
Rodney, What you have described is using a oscilloscope like a curve tracer. There are a variety of curve tracer circuits that will allow one to better select ranges when doing it like that and are easy to build. Of course, if you know someone with a curve Tracer, then you can just do it the formal way.

You can also manually construct a curve trace using graph paper and measuring the current and voltage as you incrementally increase one or the other.

Re: Diode forward/reverse resistance measurement

Posted: Tue Apr 02, 2019 2:48 pm
by RichMarkley
Some scopes, like the Rohde & Schwarz RTC1000, have a built-in component test function that can make this measurement. In addition to diodes, you can use this feature to test capacitors, resistors, transistors, thyristors, coils, and resulting circuits such as rectifiers. This can really come in handy when troubleshooting electronics circuits, especially when you don't have a schematic.

Re: Diode forward/reverse resistance measurement

Posted: Wed Apr 03, 2019 2:10 pm
by Lenp
I'm not sure if this discussion is about component specifications or troubleshooting. Curve tracers and component testers are needed for specification evaluation, but a good front to back ratio usually tells the story for open/short for troubleshooting.

Re: Diode forward/reverse resistance measurement

Posted: Wed Apr 03, 2019 5:18 pm
by Externet
[ Thread is from 2013 ]

Re: Diode forward/reverse resistance measurement

Posted: Thu Apr 04, 2019 12:42 pm
by haklesup
Ya, its old. Component testers produce a curve that resembles a curve tracer but is generally uncalibrated and had less control over settings. They also use a faster scan rate 50 or 200 hz in this case which produces the looping effect caused by inductors and capacitors. A curve tracer for characterization purposes would be more flexible, more accurate and sweep slower to avoid the looping and just get the measurement from the semiconductor. In this case, the 50Hz rate should produce curves similar to the old Tektronics curve tracer sweep rate which was derived from a built in variac but with less control over settings.