The Silicon Controlled Rectifier (one type of thyristor) is a four layer device, PNPN from anode (A) to cathode (here labeled "C", sometimes also labeled "K"). An SCR is like a junction transistor with a fourth layer and therefore a total of three P-N junctions. Meanwhile, a third terminal, the gate (G), makes an SCR function like an odd hybrid of transistor and diode:

SCR symbolSCR cross-section

The two outer junctions are forward biased by the voltage as shown, but the inner junction is reverse biased. As a result, a small current in the gate electrode can turn on a current between anode and cathode. SCRs are latching devices; once an SCR is turned on, or "fired," it remains on (i.e., the current will continue to flow) until the driving voltage between anode and cathode is removed. The minimum anode - cathode current required to keep the SCR on (once triggered) is called the holding current, IH.

An SCR is called a rectifier because (much like a "vanilla" diode) it conducts current in only direction. It is a unidirectional thyristor, in contrast to a triac. You can think of an SCR as being a conventional (diode) rectifier, with a gate controlling forward resistance. Additionally, an SCR can be switched on (in the absense of a gate voltage) by applying enough forward voltage to overcome its internal resistance. This is normally considered a design limitation, though, and switching is normally controlled via gate voltage.

Knowing all this, an SCR's characteristic V - I curve then looks like this:

Graphic courtesy of American Microsemiconductor

SCRs are not often used in BEAM -- they are tough to find since they're special-purpose devices in electronics (used for lighting control, motor speed control and other variable power applications). In combination with a UJT or PUT they can, though, make a mean solar engine.

In a pinch, you can build up something very like an SCR using discrete transistors wired as a complementary feedback pair:


Here, as soon as any current flows in either transistor, this becomes base current for the other transistor, and both transistors turn on hard. This means you can only build up this circuit using low-leakage transistors ('though this should be the case with any decent-quality modern transistor).

See also TLA; contrast to triacs. Meanwhile, AMS has a good SCR tutorial page here.

For an example SCR's data sheet, see the 2N5060 in the BEAM Reference Library's Datasheet Collection.

Complementary feedback transistor pairs are discussed in more detail on the 4QD website here.

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Page author: Eric Seale  
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