Unijunction transistor

[John M0GLN]

I have never used a Unijunction transistor before but I'm now making a simple low voltage detector from the web which uses a Radio Shack RS-2029 and the leads are shown as E, B1 and B2, now I don't have any of these but I have a few 2N6027 UJT's, however the leads on these are Anode, Gate and Cathode, after comparing various circuits on the web am I correct in assuming that the Cathode will be B1 and the Anode B2? Or will it disappear in a puff of smoke? Any comments will be most welcome.

Thanks

John

[ppppenguin]

I have my doubts about using the 2N6027 in palce of an ordinary UJT. Its a programmable unijunction and the structure is more like a thyristor. See data sheet: http://www.farnell.com/datasheets/112532.pdf All you can do is try it.

[Kalee20]

Yes, the two are rather different.

A unijunction transistor (eg Texas TIS43) is exactly that - a strip of failrly high-resistance N-type material (with connections b1, b2 at the ends) and a blob of P-type material stuck on it somewhere along the length (maybe mid-position, maybe not). This forms the junction.

The two base connections - which have no relation to the base of a NPN or PNP transistor - are connected to a voltage source, the thing behaves just like a resistor. If the emitter is less positive than the strip, at the point where it joins, nothing happens, because the diode junction is reverse biased. But if it's more positive, current will flow. The useful property is that when current flows, charge carriers are injected into the N-type strip which cause it to lower its resistance, and an avalanche effect results.

A programmable unijuction transistor (eg Mullard BRY39) is a 4-layer PNPN device (like a thyristor), so actually 3 junctions. Often there's a connection to the anode gate as well as the normal cathode gate. It is possible to use a couple of external resistors to define a voltage at the gate (the so-called programming bit). If the voltage on the anode (or cathode) exceeds the gate voltage in the right direction, then thyristor action takes over and again you get an avalanche effect.

The two divices ar internally quite diffetrent, and the PUT needs a couple of extra resistors. It might be possible to use one in place of the other, but not without a few circuit tweaks.

[John M0GLN]

Thank you both for your replies, I have gained a little more knowledge today which must be good, the TIS43's seem a bit difficult to get hold of so I searched Google for an equivalent to the RS-2029 and I see that the type 2N2646 is recommended and this is the one I will order.

Thanks

John