This is simple circuit that illustrates the function of the programmable unijunction transistor. It may be quickly wired on a proto-board.
PUT Flasher Specifications
How it works
The programmable unijunction transistor remains dormant until the voltage across C1 exceeds the gate voltage of Q1 by one diode drop (0.6V) or in this circuit about 6.8V in reference to circuit common. At this point, current starts to flow into the anode of Q1. When the current exceeds the “peak current” threshold (about 1.25uA), the transistor triggers and shorts all three terminals together until the anode current drops below the “valley current” (about 100uA in this case), and the transistor resets itself. When the transistor triggers, it dumps capacitor C1 across the LED. Peak current is limited by the LED internal resistance.
By varying (programming) the gate voltage, the voltage at which the transistor fires varies—in this case, it varies the flash rate.
This cute little device was developed by General Electric about 50years ago. The original GE part numbers were D13K1 and D13K2 (now 2N6027 and 2N6028). The 2N6028 has tighter specifications for more precision applications. On Semiconductor is now the sole manufacturer, but NTE has a relatively expensive equivalent device.
There are many, many applications for this device, but it is mainly useful for triggering thyristors. Other than for thyristor triggering, the ubiquitous 555 timer has taken over.
For the Future
Use of this device in a thyristor switch circuit
Bill of Materials