What Is a UJT (Unijunction Transistor)?
A UJT (Unijunction Transistor), unlike an ordinary transistor, is a single-junction switching element used not for amplification but for triggering and oscillator circuits.
How Does It Work?
It has one emitter and two base terminals. When the emitter voltage exceeds a certain threshold, the UJT suddenly starts conducting and enters a negative resistance region. This sudden transition is ideal for producing a sharp pulse.
Negative Resistance and Oscillator
A UJT together with a capacitor forms a relaxation oscillator: the capacitor charges slowly, at the threshold voltage the UJT discharges it, and the cycle repeats. So regular trigger pulses are produced.
| Terminal | Function |
|---|---|
| Emitter (E) | Trigger input |
| Base 1 (B1) | Output pulse |
| Base 2 (B2) | Supply |
Where Is It Used?
SCR and TRIAC triggering circuits, timing, relaxation oscillators and pulse generation. Today it has mostly given way to timer ICs (the 555) in most places.
Intrinsic Standoff Ratio (η)
The main parameter that sets the UJT's trigger threshold is the "intrinsic standoff ratio" (η); its typical value is between 0.5 and 0.8. The trigger voltage is approximately equal to this ratio times the supply voltage, so the η value is taken from the datasheet when calculating oscillator timing.
Common Mistakes
- Mistaking it for a transistor: a UJT doesn't amplify; it only switches/triggers.
- Threshold calculation: if the trigger voltage (η ratio) is miscalculated, the timing drifts.
- Old design: in new designs a 555 or MCU is more practical.
Frequently Asked Questions
Is a UJT like a normal transistor?+
What is a relaxation oscillator?+
Search parts related to "What Is a UJT (Unijunction Transistor)?"
Find the right part by comparing price, stock and distributors.