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# Question on a thyristor

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#### cjlennon623

##### New Member
Good Day Folks! Thank you for reading my question. I have 4 thyristors (BT152X-400R) that I have taken out of a battery charger that is not working. When I put my 1.5v ohm meter across the anode and cathode, I get no reading. When I then charge the gate positive, I get continuity. But, if I reverse the leads on the anode and cathode making the anode negative and the cathode positive and then charge the gate, I also get continuity between the anode and cathode. Is a thyristor suppose to allow current both ways or are these 4 thyristors burned out? Also, both times I charge the gate by jumping what ever is on the anode to the gate. So, the first instance above, the gate is charged positive while the second instance (negative lead on the anode), the gate was charged negative. Hopefully this makes sense. Should a thyristor allow current to pass both ways? Thanks again for any advice you can give me!

Chris

An SCR will conduct when + given to anode and - given to cathode and the gate is supplied at +ve(suppose shorted to anode).

Hey Transistor495, appreciate the reply but will an SCR conduct the other way with the anode set negative and the cathode set positive? That is what I am struggling with in order to determine if these SCRs are any good. Thanks.

Chris

Hey Transistor495, appreciate the reply but will an SCR conduct the other way with the anode set negative and the cathode set positive? That is what I am struggling with in order to determine if these SCRs are any good. Thanks.
Chris
I've prepared some notes on SCR testing. Set the ohmmeter at a low value scale.

*) Connect the ohmmeter's negative lead to the SCR's cathode and its positive lead to the SCR's anode. The ohmmeter should read infinity@high resistance.

*) Short circuit the gate to the anode. The ohmmeter should read almost 0 ohms. Switch off gate and the ohmmeter should still read zero ohms.

*) Reconnect the ohmmeters leads, positive lead to the SCR's cathode and its negative lead to the SCR's anode. Should read infinity@high resistance.

*) Short circuit the gate to the anode. The resistance reading should remain high because the SCR is reverse-biased and, therefore, it can't conduct.

*) Switch off gate. The resistance should remain high because the SCR is reversed-biased and has no gate current.

So what I'm getting is that they're out of function!

SCR will conduct in ONE direction (as above), not the other. Just like a diode.
TRIAC is (2) SCRs in inverse shunt. It would conduct in BOTH directions.

Triac bridges are common in heavy duty current regulating applications.

Triacs have two diodes drops. A typical DVM ohm meter may not show forward conduction unless voltage goes above the two diode drops.

I've prepared some notes on SCR testing. Set the ohmmeter at a low value scale.

*) Connect the ohmmeter's negative lead to the SCR's cathode and its positive lead to the SCR's anode. The ohmmeter should read infinity@high resistance.

*) Short circuit the gate to the anode. The ohmmeter should read almost 0 ohms. Switch off gate and the ohmmeter should still read zero ohms. not on any of my meters

*) Reconnect the ohmmeters leads, positive lead to the SCR's cathode and its negative lead to the SCR's anode. Should read infinity@high resistance.

*) Short circuit the gate to the anode. The resistance reading should remain high because the SCR is reverse-biased and, therefore, it can't conduct.

*) Switch off gate. The resistance should remain high because the SCR is reversed-biased and has no gate current.

So what I'm getting is that they're out of function!
Have you actually tried this? All of my DVMs and analog meters read a diode drop on low ohms which is not anywhere near 0 ohms. And even on the low ohms scale the DVMs supply only 1.5mA which is not enough holding current for most thyristors. One analog meter supplies 60µA and the other supplies 60mA.

Have you actually tried this? All of my DVMs and analog meters read a diode drop on low ohms which is not anywhere near 0 ohms. And even on the low ohms scale the DVMs supply only 1.5mA which is not enough holding current for most thyristors. One analog meter supplies 60µA and the other supplies 60mA.
To be honest, NO.
But I'm pretty sure that it'll work out if the meter capable of providing enough holding current say say around 50mA would suffice on almost cases?(Correct me if I'm wrong!).
This may be more adequate way of testing those: