Welcome to G-TV! Today I’m going to show you how to test a diode using a standard digital
multi-meter. Safe and accurate testing of a diode requires the device to be out of circuit.
The type of diode I am testing today is commonly called a hockey puck diode. A diode is like
an electronic switch. It can be turned on when rated voltage is applied, commonly around
0.6 Volts for a silicon diode, and it allows current to flow in one direction and blocks
current from flowing in the opposite direction. When checking the condition of a diode or
transistor junction, a multi-meter is the preferred instrument versus an analog Volt-Ohm
Meter or VOM. The VOM can give widely varying readings and can drive undesirable currents
levels of up to 50 milliamps through the junction. Many brands of digital multi-meters have models
that include a diode test function. Today I will be using a Fluke 233. I will turn on
the multi meter by setting the dial to the diode test function indicated by the yellow
diode symbol here. On the Fluke 233 you will notice the white symbol here is for capacitance
and not diode testing. Since the symbol is yellow, I will push the yellow button here
to change the setting of the multi-meter from capacitance to diode testing. This diode test
function will send a current through the semiconductor junction and then measure the voltage drop
across the junction. A good silicon junction drops between 0.5 Volts and 0.8 Volts. For
this hockey puck style diode the polarity is determined by the diode symbol on the diode
itself. Other hockey puck style diodes may have a flange end. In this case, the cathode
is on the side of the puck with the flanged end. A hockey puck diode will often require
clamping or compression in order to conduct this test. This is because the actual silicon
device is sandwiched in-between the metal discs that make up the conductor surfaces,
or pole faces of the diode. There is no mechanical bond connecting the silicon device to the
discs. The compression force required for the device when installed in the actual application
is usually stated in the manufacturers’ specifications. For the purpose of doing this
static test, a minimal amount of clamping force will usually suffice. My clamping rig
consists of a hold down toggle clamp, a couple flat metal, and fiberglass plates. The flat
metal plates are being used to keep the force of the clamp even across the pole faces and
prevent the clamp from creating undesirable marring or denting the pole face surfaces.
Do not compress the disc more than the compression limits of the diode. The fiberglass plates
will insure that the poles of the diode are not shorted by some conductive path in the
test jig. Now that my diode is ready I will plug in my leads. The red probe should be
plugged into the appropriate positive test terminal on the multi-meter and the black
probe should be plugged into the common terminal. Now I will place the positive probe on the
anode and the negative probe on the cathode. In this case, I'm looking for a voltage drop
of about point 4, and I am listening for a short beep. This reading indicates that the
diode has passed this portion of the test. Now if I were to see an open or short circuit,
then this would indicate a bad diode. An open diode appears on the multi-meter with the
same indication you would have with the probes not touching anything. The O L on the display
indicates Open Leads or no continuity. A shorted diode appears on the multi-meter with the
same indication you would have with the probes contacting each other directly. The point
zero zero zero on the display and long continuous beep indicates a zero voltage drop. A shorted
or open diode clearly indicates that the diode is bad. For the second test, I’m going to
reverse my leads. I will place the positive probe on the cathode and the negative probe
on the anode. I should get an open reading indicated by an O L on the display, like this.
This reading indicates that the diode has passed the second part of the test. Any other
reading, indicating a voltage drop or short, would reveal that the diode is bad. Static
testing of a diode using a DMM is limited and the only certain test results are open
or short, which indicate the device is certainly bad. Passing results may not accurately determine
that a diode is actually good. The DMM is most likely not able to simulate the application
conditions, such as current load or voltage levels. A diode that fails in the actual application
environment may very well pass these limited DMM static tests. Regardless of these limitations,
static tests like these are very helpful during the process of troubleshooting electronic
circuits. Diodes and multi-meters such as the Fluke 233 along with thousands of other
products and services are available at galco.com