Accurate Way On How to Test Semiconductor Diode Using Multimeter
Testing diodes is different from testing a resistor because you
need a skill to do it.If you do not know or wrongly test a diode you will be unable to repair the equipment.A
bad diode you think it is good. This will definitely waste your precious time.
A rectifier diode can fail in one of the four ways. It can
- Breakdown when in full operating
An analog multimeter or digital multimeter can be used to check
for all the first three conditions except the last one where
the diode breakdown in full operating voltage. From my of
experienced in the electronic repairing line, i found that checking
diode using analog multimeter is more accurate than using a
digital multimeter. I could explain to you why i preferred analog
meter. I do not know about you because i really came across
quite a number of diodes where it checked ok using digital multimeter but failed when test with analog
The first step on how to test a diode is to remove one of the
diode lead. You can't always be certain if a diode is good or
bad if you perform in-circuit test, because of back circuits
through other components. To be absolutely sure, you will
need to lift, or disconnect, one diode lead from the circuit to
avoid back circuits. Unless you are very sure about the
board you are checking. Sometimes i do found bad diodes when
checking it on board. Your experienced will tell you when to
check a diode on-board or off-board. If you are new to
electronic repair, i highly recommend that you check a diode
with a lead removed from the board.
I will set my analog meter to x1 ohms to check for current
diode leakage reverse and forward testing. Connecting the black probe of your meter to the cathode and red probe
to the anode, the diode is reverse biased and should look like an open reading. Connecting the red probe of your
meter to the cathode and black probe to the anode, the diode is forward biased and the meter should read some
value of resistance. If you have two readings then most probably the diode is shorted or leaky and you should
replace it. If you don't get any reading either forward or reverse bias, the diode is considered open. The real
problem when checking a diode using the diode test function of a digital meter is that an open or leaky diode,
the meter sometimes reads ok(0.6). This is due to digital meter diode test output voltage (which you can measure
the output test probe using another meter) is around 500mv to 2v. An analog meter set to x1 ohms have output
about 3V(remember the two 1.5V battery you installed in the meter!). The 3V voltage is enough to show you the
accurate reading of a diode when under test.
Even if you have a good reading at x1 ohms doesn't mean that
the diode is good . You now have to set your meter to x10K to test the diode again. The output voltage of x10k
ohms is about 12V(remember the 9v battery in your meter-1.5v+1.5v+9v=12v). Again the diode under test should
show only one reading. This is exception to Schottky diode where it have two readings but not shorted reading.
If the meter showed one reading then the diode under test is good. If it has two readings then most probably the
diode is either shorted or leaky. The digital meter can't test it because the output from the meter is only
500mv to 2V.
If a diode breakdown when under full operating voltage, there
is no way to testing diode (unless you have a very expensive diode checker which specially designed to locate
this type of problem). Substituting with a known good diode is often the only way to prove that an intermittent
diode is causing a particular problem. Sometimes an intermittent diode could be locate using a coolant
Caution: Be certain that power is removed from any circuit
before performing any of the following diode checks, otherwise meter or circuit damage could result.
Conclusion-In order to correctly test diode function you need
to set the analog meter to x1 ohms and x10K ohms range.
Author By : Jestine Yong