UJT (Unijunction Transistor)
Transistor
A three-terminal negative-resistance device that snaps on when its emitter voltage crosses a fixed fraction of the supply — the textbook relaxation oscillator and the classic trigger for SCRs. Obsolete in industry, immortal in exams.
Schematic symbol
Bipolar transistor (NPN)
How ujt (unijunction transistor) appears in a circuit diagram.
Types & variants
Key specs
Intrinsic standoff ratio (η)
—The defining spec: the fraction of VBB at which the emitter fires. Typically 0.5–0.8; peak voltage Vp ≈ ηVBB + 0.7 V.
Interbase resistance (RBB)
kΩResistance base-1 to base-2 with emitter open — typically 4–10 kΩ.
Peak/valley current
µA / mAIp (needed to fire) and Iv (below which it turns off) bound the usable timing-resistor range.
Max emitter current
A (pulse)Pulse capability that lets the discharge spike trigger an SCR gate directly.
Markings
TO-18/TO-92 packages with standard part numbers: 2N2646, 2N2647; PUTs: 2N6027/6028. Pinout (2N2646 TO-18): tab nearest to base-2; emitter is the middle-offset lead — check the datasheet.
Standard values
2N2646: η = 0.56–0.75, RBB ≈ 7 kΩ. Oscillator timing: f ≈ 1 / (RC × ln(1/(1−η))) — with η ≈ 0.63 this neatly reduces to f ≈ 1/RC.
How to choose
You mostly don't — you choose it because the syllabus or a vintage schematic does. For new SCR-trigger designs, use a PUT (η set by two external resistors), a 555, or a DIAC. If buying, 2N2646 remains available from hobby vendors.
Pinout & package
Emitter, Base-1, Base-2 (NOT collector/base/emitter — it's a single junction into a resistive silicon bar). Output pulses are usually taken across a small resistor in the Base-1 leg.
Example circuits
- Relaxation oscillator: R charges C at the emitter until Vp, UJT fires, C dumps through B1 — sawtooth at the cap, spikes at B1
- SCR phase-control trigger: UJT spikes synchronised to mains half-cycles
- Simple sawtooth sweep generator for a scope experiment
- Long-interval timer using the µA-level peak current requirement
Common failures
Emitter junction degradation from over-current spikes, cracked vintage packages, and being confused with a BJT during testing/replacement (they are not interchangeable).
How to test
Ohmmeter: B1–B2 reads RBB (~5–10 kΩ) both ways; emitter to either base reads like a diode (conducts one way) with the resistive bar in series. Definitive test: wire the standard oscillator; a good UJT oscillates immediately.
Substitutes
PUT (2N6027) with programming resistors replicates any η; a 555 replaces it in timing/oscillator roles; a DIAC replaces it in TRIAC/SCR triggering. Direct UJT-for-UJT: match η and Ip/Iv ranges.