How do SCR’s work? Are there any simple methods for testing them?
It’s fairly easy to make an SCR using an NPN and a PNP transistor.
SCRs are the same as two transistors hooked in a strange front-to-back circuit(see links below). If the first transistor happens to be off, then it keeps the second transistor off, and the second one also keeps the first one turned off. But if either one happens to be on, then the first one keeps the second one turned on, and vice versa.
If you apply a voltage to your two-transistor SCR-like circuit, it will start off in the “off” state. But if you apply a pulse of the proper polarity to either of the base connections, the device will be triggered into the “on” state.
http://www.eteonline.com/scrs.html
http://www.tpub.com/neets/book7/26d.htm
http://www.americanmicrosemi.com/tutorials/scr.htm
AND… if you have a pair of 3-silicon-layer transistors (one NPN and one PNP,) then you’ll find that you can meld them together, creating a four-layer device which behaves the same as two cross-connected NPN/PNP transistors. NPN plus PNP equals NPNP. Real SCRs are four-layer devices, not two independant transistors.
Some SCRs can be tested with nothing but an ohmmeter. You must know polarity of the ohmmeter’s terminals (is red the positive, or the negative?) Hook the scr’s A and K to the ohmmeter with A to positive and K to negative. The meter indicates an open circuit. Then use a short bit of wire to briefly touch the SCR’s gate terminal to the A terminal (or to the K? I forget which) and the SCR will be triggered into the “on” state, and the ohmmeter will display a low reading.
Unfortunately, this doesn’t seen to work with all SCRs (or with all DVMs). If the above test shows that the SCR does function, then well and good. But if the SCR seems “bad”, it still might be OK. In that case you’ll probably have to perform a better test. Wire the SCR up to a power supply of the proper voltage with a light bulb or resistor in series, put the proper value of current-limiting resistor in series with the gate lead, then apply a brief pulse and see if the SCR is triggered into the “on” state.
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