Diode Testing

Diode Testing

A Diode has two resistance values, forward and reverse,
These can be measured with a multimeter as shown in the
diagram below. 

An arrow printed on the diode body identifies the positive side of a diode.

The forward resistance is being measured in Fig. A with the
positive meter lead connected to the forward side of the diode.

In Figure 'B' the meter leads have been reversed, and the reverse resistance is being measured.

An electronic digital instrument will read true electron flow, hence the resistance polarity readings will be reverse to conventional current flow, i.e. forward and reverse readings will be reversed.

A Digital Multimeter usually has a semiconductor test scale on the selector switch, marked as shown :-

This measures true electron flow, and will give a forward,
(indication reading only), or reverse (no reading) indication.
Using an analogue meter on resistance scale, the forward
resistance varies considerably, depending on the internal
impedance of the Multimeter, and the diode type.

A typical reading would be between 20 and 100 ohms.
The reverse resistance must be very much higher, usually in
excess of 100k ohms, (100,000 ohms ).

A faulty diode will give a reading in both forward and reverse
directions (short circuit), or no reading in either direction, (open circuit).