Thyristor / Diode Measurement With A Multimeter

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 nonacceptable 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 an not across something in parallel with it.


2. 
   
   If a disc 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.