The term SCR stands for Silicon Controlled Rectifier. It can be thought of as a Shockley Diode (not to be confused with a Schottkey diode) with an extra terminal that allows a voltage to be applied in order to trigger the device. It has three terminals (or leads) named Gate, Anode and Cathode. Power SCR’s (high current devices) also have an inbuilt resistor that connects to the Gate and Cathode terminals. The resistor normally is in the range of 18 to 26 ohms. This resistor reduces the likelihood of false triggering due to spurious voltage spikes, which occur in the form of power supply transients, noise, or even ESD (Electro-Static-Discharge) events. Having this resistor in place also increases the trigger current that is demanded from the Gate driver circuit. For example with only a 5VDC trigger pulse applied between Gate and Cathode the current requirement can be higher than 250mA. This is generally not an issue for test purposes but can become a challenge when low power battery operated test Equipment is used.
SCR’s without a Gate resistor are sometimes referred to as Sensitive Gate SCR’s, and can easily be overstressed due to limited device specifications, and poor operating conditions. On the other hand, Power SCR’s are designed to perform fast switching of high currents at high voltages. However, due to these more difficult requirements and environmental effects such as high operating temperatures, EOS (Electrical-Over-Stress), and ESD events they eventually fail as well. This leads to the task of failure analysis, and product repair.
Failure analysis or Functional Test of the SCR should always be performed when the device is out of circuit. This means that the component should not be electrically connected to anything other than the Test Equipment that is used to evaluate the devices functionality.
At a Repair Center, SCR’s are tested utilizing either a DVM, or SCR Tester. The use of a DVM for this purpose is described below:
This method is purely a static test. The device is not made to function in any fashion. It involves the use of a DVM (Digital-Volt-Meter). Set the DVM to the Resistance scale (
David Willcocks is an Electronics professional, and Technical writer. For more information on Automated Test Equipment please visit http://www.imagineATE.com
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