We have all had difficulty testing diodes in-circuit. Most DMM’s have a diode Vf function that measures forward drop, but what is the normal voltage drop? Analog VOM’s attempt to measure resistance of diodes, but measure a different value on each ohmmeter range. Unfortunately, the interpretation of the measurements is subjective because of differences in diodes, non-linearity in diodes as well as differences in test equipment. This little exercise will help you to understand the issues and learn more about your VOM or multimeter. You may then generate your own “Handy Reference Sticker” to place on your test instrument for future reference. I hope you learn as much and have as much fun as I did.
This exercise requires two meters—one the DUT (device under test) and another to measure current. On the Vf function and/or ohmmeter function, determine how much current is delivered by the meter. Measure this via the 2nd meter set for measuring mA. Record this measurement on each range or function and insert the data on the Excel spreadsheet—replace my data.
Open circuit voltage
Do the same for open-circuit voltage and record the data. Also note the polarity of the voltage—while 99% of meters use the red lead for positive & black for negative, a few do not, and this can be a real confusion factor. I believe (correct me if I am wrong) that some old Triplett meters have reverse polarity on the ohmmeter function.
Why measure the voltage? The open circuit voltage tells you immediately, if this function can be used for testing higher voltage drop devices such as LEDs which require as much as 3V to turn on. Also, this DC voltage can be used at times for signal injection for experimenting or testing.
Gather together a few miscellaneous diodes and rectifiers.
I chose the following:
Measure the forward voltage drop and/or the resistance in each ohmmeter range. Resistance measurements with a digital multimeter may not produce coherent results on all ranges. Meters that have auto-ranging often cannot resolve the non-linear resistance of a diode and thus generate strange readings. Record data on the spreadsheet.
About the spreadsheet
This spreadsheet is for graphical display of data rather than calculations—this will give you experience with formatting Excel data. The tables may be formatted and printed as desired to produce an informative little “Handy Reference Sticker.” Not all information lends itself easily to a table, so be imaginative in working out the best layout—not all info has to be used. After printing, the little table may be cut out and taped to the instrument via transparent box sealing tape. All the better if you have adhesive backed stickers at hand.
I did three meters—Radio Shack DMM, Simpson 260 and Inexpensive MU113. I gave up attempting to get a useful matrix on the MU113 as you can see on the convoluted results.
Armed with this troubleshooting tool, you will now know how to interpret your in-circuit Vf or resistance measurements. Is Vf or forward resistance reasonably close to what is expected? Then reverse the test probes to see if there is substantial difference thus indicating rectifier action.
Testing germanium diodes
If you have an analog VOM, you may be surprised how high the short circuit current is on the Rx1 range. Mine runs 120mA. Note that this is sufficient current to destroy a sensitive germanium diode—avoid using this range in this case.
Reverse leakage testing
Reverse leakage may be measured only after removing the component from the circuit board because shunt resistances abound in almost all circuits. Good silicon rectifiers and diodes should have, for all practical purposes, infinite resistance. On analog VOMs, any movement on the Rx10K range indicates abnormal leakage. With germanium and schottky diodes, leakage is to be expected. In this case, leakage current must be checked against manufacturer’s specifications rather than depending upon resistance readings.
Generally failures are short circuit and are easy to check. Leaky diodes and rectifiers take more patience. Open signal diodes are common, but silicon rectifiers are generally shorted. One exception is bridge rectifiers where poor quality internal interconnects are often subject to open circuit failure.