Tecnico

Can an Ohmmeter Test
Thyristors and Triacs?
Question:
Can I use an ohmmeter to test thyristors and triacs satisfactorily? If so, can someone provide a description and a
sketch showing the properhookup? Also, what kind of meter is recommended? - M.B.L.
Answer:
When M.B.L. says thyristor, I assume he is referring to a semiconductor controlled rectifier (SCR), also called a
reverse-blockingtriode or thyristor. The SCR is a three-terminal device, being essentially an open circuit (anode-tocathode) with gate voltage absent, and a solid-state rectifier when gate voltage is applied. It maybe tested as follows:
•

Connect the ohmmeter as shown in Fig. 1A. Use the R x 1
range. The measured resistance should be high (greater
than 1 megohm). Next, reconnect the ohmmeter with
reversedleads as in B. Again, the measured resistance
should be high.

•

With the ohmmeter leads connected as in B, momentarily
connect the gate to the anode. The resistance should now
read low (lessthan 1000 ohms), even when the gate
connection has been broken. The SCR resistance instantly
goes again high in both directions when either ohmmeter
lead is lifted.

The triac is a thyristorwith three terminals. It is an open
circuit with no gate signal present, but conducts in both
directions when gate voltage is applied. It is tested as follows:
•

Connect the ohmmeter as shown inFig. 2C. The measured
resistance should be high. Momentarily connect the gate to
anode 2; the resistance should go low and remain low even
when the gate connection is broken. Reconnect the
ohmmeterwith opposite polarity as shown in Fig. 2D. The
reading should again be high until the gate-to-anode 2
connection is made, then go low as it did before. Although
you will occasionally face an SCRor triac for which the
ohmmeter can't supply sufficient testing current, and current
from the R x 1 range may damage some sensitive-gate
SCRs, the basic multimeters are effective for these tests....