SCR Testing: Understanding Test and Measurement Parameters

This article explores the various test and measurement parameters used to evaluate Silicon Controlled Rectifiers (SCRs), along with the test setups employed for different measurements.

What is an SCR?

Introduction

• An SCR, or Silicon Controlled Rectifier, is a three-terminal, four-layer (P-N-P-N) semiconductor device.
• It’s constructed using N-P semiconductor materials. The Anode is located near the P-type material (on the left), while the Cathode is near the N-type material (on the right).
• SCRs conduct current in only one direction, making them suitable for use as switches.
• Figure 1 shows the physical appearance and symbol of an SCR.
• SCRs are also known as thyristors.
• They have a wide range of applications in electronics, primarily for power control. A single SCR can control half of an AC cycle.

Applications of SCRs (Thyristors)

• AC power control in lighting, motors, etc.
• Over-voltage protection in power supplies.
• AC power switching.
• Control elements in phase-angle triggered controllers.
• Used in photographic flashlights.

SCR Measurements

The following parameters of an SCR are typically verified during testing:

• Maximum rise of ON-state current (dI/dt): The maximum rate at which the current can increase when the SCR turns on.
• Peak Gate Current (IGM): The maximum allowable current that can be applied to the gate terminal.
• Gate trigger current (IGT): The minimum current required at the gate to switch the SCR into its conducting state.
• Over current protection (I^2^t): The SCR’s ability to withstand a surge of current for a short duration.
• Average ON-state current (IT(AV)): The average current the SCR can handle in its conducting state.

SCR Testing Video

For a better understanding of SCR operation, pin designations, and how to determine if an SCR is functioning correctly, consider watching a relevant YouTube video.

Test Setup for SCR Testing

Multimeters are commonly used for SCR testing. SCRs behave similarly to diodes, but they can handle much higher currents. Unlike regular diodes, SCRs need to be triggered into their conducting state. This test aims to determine if an SCR is working correctly. Always refer to the datasheet for the specific SCR part number to identify pin designations.

Testing Procedure

1. Set your multimeter to diode test mode.
2. Simply connecting the Anode and Cathode of the SCR to the positive and negative terminals of the multimeter will not produce any readings unless the SCR is turned ON.
3. To turn the SCR ON, apply a positive voltage to the Gate terminal.
4. Once the SCR is triggered, connecting the positive and negative multimeter leads to the Anode and Cathode will provide a reading of approximately 0.7 volts, representing the forward voltage of the SCR.
5. Similarly, a load (such as a small lamp) can be connected in series with the SCR to verify its switching action when a positive voltage is applied to the Gate.