The unijunction transistor has two bases and one emitter. It is considered to be a digital device because it is either completely on or completely off. They are manufactured by combining three layers of semi-conductor materials.
This type of transistor has two paths for the current to flow through. One of the paths is from base 1 (Bl) to base 2 (B2). The other path is through Bl and the emitter when it operating normally there is no current flowing through either path until the applied voltage reaches approximately 10 volts higher than the voltage applied to B 1. The UJT will turn on when the voltage applied to the emitter reaches a point of 10 volts positive higher than the voltage applied to terminal Bl. The current then flows through the path from Bl to B2 and from the emitter through Bl. The current will continue to flow the UJT until the voltage applied to the emitter drops to approximately 3 volts higher than the voltage applied to Bl, the UJT will turn off. It will remain off until the applied voltage to the emitter again reaches approximately 10 volts positive higher than that applied to terminal Bl.
UJT transistors are generally used in circuits such as capacitor discharge systems. In operation, the variable resistor controls the amount of time for charging the capacitor. When the capacitor charge reaches approximately 10 volts, the UJT will turn on and allow current to flow through it and discharge the capacitor by the circuit through the emitter to Bl. When the capacitor charge reaches approximately 3 volts, the UJT will turn off, interrupting the circuit and the capacitor begins charging again. When the resistance that is connected in series with the capacitor is changed the charge time of the capacitor is also changed. The opening and closing rate of the UJT can be controlled by varying this resistance.
The UJT can be used to furnish a large output voltage because the output is produced by discharging the capacitor into the circuit. The output of the UJT is generally used to trigger an SCR gate.
The rate that the UJT will cycle on and off is determined by the amount of resistance and capacitance connected to the emitter of the UJT. Therefore, the amount of capacitance that can be connected to the UJT has limitations. The maximum capacitance that is connected to a UJT should be limited to about l0 μ. When a capacitor that is too large is connected to the UJT, the spike voltage caused by the capacitor discharging could possibly damage the UJT.