My scr-triac test setup.
Fig. 1 My scr-triac test setup.

Simple Triac-SCR Testing Circuits

by Lewis Loflin

This page is related to three You Tube Videos on simple test circuits for SCRs-thyristers and Triacs. Most of the explanations are in the videos.

The first part of a three-part lab on testing SCRs and Triacs.

This problem came up when my Kuman and MK-168 transistor checkers would not test most SCRs unless they had very sensitive gates, and no triacs would test at all. At least those I had shown to the right in Fig. 1.

The test assembly used a ZIF socket connected to the home built proto-board in the upper center of Fig. 1. A 25.2 volt A.C. transformer is outside the photo.

A 24-volt lamp is used as a load and measurements are made with a Cen-Tech DVM. A white socket in the center of the board allows one to plug in a TO-220 style SCR or Triac or an external adaptor cable with alligator clips for oddball parts that won't fit it or the ZIF socket.

Schematic test setup for Traic-SCR lab.
Fig. 2

Fig. 2 is the electrical schematic for the test board. Power is supplied by the transformer and gate current for either device is controlled by R1 a 5K potentiometer. A diode is used for SCR tests and is jumpered for triac tests.

Schematic test setup for Traic-SCR lab with test SCR connected.
Fig. 3

Testing SCRs

Fig 3 illustrates the connection for testing SCRs S1 is open. When the SCR is fully turned on with R1 is acts as a half-wave rectifier and the DVM will read ~12.4VDC based on the 28VAC from my particular power transformer.

Notice that the gate turn voltage varies from one SCR to the next. Two were very sensitive while some required much greater turn on current for full output voltage to the DVM.

SCR acting as a half-wave rectifier.
Fig. 4

Fig 4 illustrates an SCR being used as a half-wave rectifier with voltage calculations.

SCR acting as a half-wave rectifier with potentiometer.
Fig. 5

Fig 5 uses a potentiometer to vary the trigger point on the half-wave. The formula in the slide only works if the SCR is fully on.

Schematic test setup for Traic-SCR lab with test triac connected.
Fig. 6

Fig. 6 illustrates a triac connected to the test setup. When the triac is fully turned on by R1 the DVM reads 0 volts DC and when on the AC setting reads 27VAC. (One volt is across the test triac.)

Be sure S1 is closed!

As the attached video noted when R1 was turned up one side of the triac switched on and the device acted like an SCR producing a DC voltage. As I continued to adjust R1 the other side also turned on. The lamp became bright, no DC voltage just AC across the lamp.

Illustrates how a triac act like two back-to-back SCRs
Fig. 7

The answer to the problem is Fig. 7. A triac acts like two back-to-back SCRs with the gates connected in common. Each "SCR" has a different cut on voltage which is why one turned on and acted like an SCR until the 2nd SCR turned on.

Schematic test setup for Traic-SCR lab with test triac and diac connected.
Fig. 8

Fig. 8 our solution to the turn on problem. Let's argue (Fig. 7) Q1 turns on at 22 volts and Q2 at 28 volts. The diac that was inserted into the gate circuit fires at about 30 volts dumping sufficient current into both sides at once.

That switched on Q1 and Q2 together regardless of differing gate turn on voltages.

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