2N2646 UJT

2N2646 UJT designed as a silicon PN Unijunction Transistor explicitly for general purpose industrial applications. This UJT is prominent as a triggering device and is not perefct for driving power loads. 2N2646 widely used in applications involving designing firing circuits (for Silicon Controlled Rectifiers) and not used for amplifier circuits like transistors.This UJT is compatible for certain applications where minimum pulse amplitude is mandatory. And also where a low emitter leakage current and a low peak point emitter current (trigger current) is necessary.

Under the condition, when power turned ON, UJT switches to high resistance state and total supply voltage appears across it. So during the starting stage  UJT voltage dropwill be equal to VCC. With the flow of time the capacitor gets charged as the current flows through R3 to reach the capacitor. As soon as the capacitor charges to threshold voltage at a certain time T1, it will discharge by forcing the current through emitter of the UJT. With this current flow at the emitter the UJT switches from high resistance to low resistance state and thereby acting as a short circuit.

As soon as the capacitor discharges, the UJT returns to normal OFF state and remains in that state until the capacitor discharges again. This charging and discharging of capacitor form a triggering cycle for the UJT to generate voltage pulses at the output. Thus UJT function as an oscillator providing voltage pulses at the output and in a similar way we can use the device in other application circuits.

Applications

• General purpose industrial applications
• SCR firing circuits
• Hobby projects
• Phase control circuit
• Saw tooth wave generator
• Timing circuit
• Voltage detector
• Relaxation oscillator