The Programmable Unijunction Transistor behaves much like a unijunction transistor (UJT), but is "programmable" via external resistors (that is, you can use two resistors to set a PUT's peak voltage). Note that the name is a bit of a misnomer -- as a thyristor, it is a four layer device, unlike a true unijunction transistor which has but two layers.

Like other thyristors, a PUT looks much 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 a PUT function like a hybrid of transistor and diode:

PUT symbolPUT cross-section

PUTs are not often used in BEAM; they're essentially special-purpose devices in electronics, used for lighting control, motor speed control and other variable power applications. In combination with an SCR they can, though, make a mean solar engine.

In a pinch, you can build up something much like a PUT from discrete transistors wired as a complementary feedback pair:


Here, as soon as any current flows in either transistor, this current 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 ).

As part of a larger circuit, the pseudo-UJT would look like this (including its "programming" resistors):


Compare to SCR and UJT; see also thyristor and TLA.

For an example PUT's data sheet, see the 2N6027/8 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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