1. Open circuit in both directions.
2. Low resistance in both directions.
3. Leaky.
4. Breakdown under load.
TESTING A DIODE ON AN ANALOGUE METER
Testing a diode with an Analogue Multimeter can be done on any of the resistance
ranges. [The high resistance range is best - it sometimes has a high voltage battery
for this range but this does not affect our testing]
There are two things you must remember.
1. When the diode is measured in one direction, the needle will not move at all. The
technical term for this is the diode is reverse biased. It will not allow any current to
flow. Thus the needle will not move.
When the diode is connected around the other way, the needle will swing to the right
(move up scale) to about 80% of the scale. This position represents the voltage drop
across the junction of the diode and is NOT a resistance value. If you change the
resistance range, the needle will move to a slightly different position due to the
resistances inside the meter. The technical term for this is the diode is forward
biased. This indicates the diode is not faulty.
The needle will swing to a slightly different position for a "normal diode" compared to a
Schottky diode. This is due to the different junction voltage drops.
However we are only testing the diode at very low voltage and it may break-down
when fitted to a circuit due to a higher voltage being present or due to a high current
flowing.
2. The leads of an Analogue Multimeter have the positive of the battery connected
to the black probe and the readings of a "good diode" are shown in the following two
diagrams:
The diode is REVERSE BIASED in the
diagram above and diodes not conduct.
The diode is FORWARD BIASED in the
diagram above and it conducts
TESTING A DIODE ON A DIGITAL METER
Testing a diode with a Digital Meter must be done on the "DIODE" setting as a digital
meter does not deliver a current through the probes on some of the resistance settings
and will not produce an accurate reading.
The best thing to do with a "suspect" diode is to replace it. This is because a diode has
a number of characteristics that cannot be tested with simple equipment. Some diodes
have a fast recovery for use in high frequency circuits. They conduct very quickly and
turn off very quickly so the waveform is processed accurately and efficiently.
If the diode is replaced with an ordinary diode, it will heat up as does not have the
high-speed characteristic.
Other diodes have a low drop across them and if an ordinary is used, it will heat up.
Most diodes fail by going: SHORT-CIRCUIT. This can be detected by a low resistance
(x1 or x10 Ohms range) in both directions.
A diode can also go OPEN CIRCUIT. To locate this fault, place an identical diode across
the diode being tested.
A leaky diode can be detected by a low reading in one direction and a slight reading
the other direction.
However this type of fault can only be detected when the circuit is working. The output
of the circuit will be low and sometimes the diode heats up (more than normal).
A diode can go open under full load conditions and perform intermittently.
Diodes come in pairs in surface-mount packages and 4 diodes can be found in a
bridge.
They are also available in pairs that look like a 3-leaded transistor.
The line on the end of the body of a diode indicates the cathode and you cannot say
"this is the positive lead." The correct way to describe the leads is to say the "cathode
lead." The other lead is the anode. The cathode is defined as the electrode (or lead)
through which an electric current flows out of a device.
The following diagrams show different types of diodes:
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