How do you measure a thyristor?

Answer 1

Many users of thyristors and diodes lack the proper equipment to make measurements of semiconductor parameters. The readily available battery operated multimeter is

often used to try to determine the difference between

acceptable and non-acceptable devices using a resistance

reading. A reading of this type can lead to incorrect conclusions.

MULTIMETER MEASUREMENTS

The multimeter is generally used to measure the DC resistance between anode and cathode of thyristors and diodes

and also the gate to cathode on thyristors. These measurements are of the "off state" or blocking voltage of the device.

The only valid readings are "open circuit" and "short circuit".

The anode to cathode or gate to cathode measurement

must register a short circuit in both directions ( forward and

reverse polarity) for the device to be declared short and

infinite resistance for an open circuit.

The measurement of resistance with a multimeter is an

inappropriate measurement technique for separating good

devices from bad. When a resistance measurement is taken

with an multimeter, the internal battery voltage is typically in

the range 1.5V to 15V and the leakage current of the device

at this voltage will determine the measured resistance. A

semiconductor has a non-linear blocking voltage/leakage

current characteristic and hence a non-linear resistance

curve. The blocking voltage of a thyristor is defined as the

voltage at which it reaches a specified leakage current at the

defined temperature. Therefore devices can have a variety

of leakage current characteristics and still be within specification.

PRECAUTIONS

1. Ensure that the resistance reading is only being taken

across the device and not across something in parallel with

it.

2. If a capsule type device is being measured, make sure

that it is under sufficient load to ensure that the internal

components are pressed together and high resistance

readings are avoided.

SUMMARY

A multimeter resistance measurement is not recommended

for determining acceptable semiconductor devices. As a

quick check for devices in a circuit, a multimeter will allow

you to determine if a device has failed catastrophically. The

device with the lowest leakage current at 3V is not necessarily the one with the lowest leakage current a high voltage