Home >> Hobbies >> Other Hobbies

How Does a Unijunction Transistor Operate?

A unijunction transistor, or UJT, is an electronic component that acts as a voltage-sensitive switch. Unlike a standard junction transistor, it does not amplify currents and, as such, is not used in amplifier circuits. Instead, its switching action makes it useful for triggering and timing applications. In 2011, electronics designers frequently prefer digital circuits over components such as the UJT, though it offers simplicity and low power consumption.

Design
- A UJT has three leads -- called base 1, base 2, and emitter -- used to connect the device to a circuit. A standard transistor has two silicon junctions, but a unijunction transistor has one. Inside the device, the emitter lead connects to a tiny bead of positively-charged silicon. The bead sits on a larger block of negatively-charged silicon that carries the two base leads. A plastic or metal case contains the silicon pieces, protecting them from damage and contamination.
Negative Resistance
- The curve of current versus voltage for a unijunction transistor exhibits an unusual property called "negative resistance." At first, voltage and current rise together, as they do for most simple electronic devices. After voltage hits a peak, however, voltage drops as current continues to rise between the emitter and base 2 terminals. The negative resistance gives the UJT its triggering behavior.
Interbase Voltage
- To make the UJT work, it needs a positive voltage, called the interbase voltage, across the base 2 and base 1 leads. The unijunction transistor has a maximum interbase voltage rating, so the circuit cannot exceed it without damaging the part. The value of the interbase voltage sets the emitter-base triggering voltage.
Relaxation Oscillator
- A circuit called a relaxation oscillator exploits the UJT's negative resistance. In the oscillator, a voltage rises, then drops in a cycle that repeats indefinitely. In addition to the transistor, the circuit needs only a capacitor and a few resistors. The circuit forms the basis of electronic timers, tone generators, and noise makers.

Other Hobbies