Testing Diode-The 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
become: -Open -Shorted -Leaky -Breakdown when in full operating
voltage 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 meter.
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 spray.
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.