Transistor Ql and resistors Rl, R2, and R3 form a constant current source and the charge current might be adjusted
to be as low as a few nanoamperes. This current would, of course, not be sufficient to fire the UJT where IP=0.2 A, unless the peak current was supplied from another source. Field-effect transistor Q2, acting as a source follower, supplies the current flowing into the emitter lead prior to firing and diode Dl provides a low-impedance discharge path for Ce- Dl must be selected to have a leakage that is much lower than the charge current. Because IB is small, the delay time will vary linearly with R3. The voltage (E), applied across R3 and the base-emitter junction of Ql, is set by the variable resistor Rl. Time delays up to 10 hours are possible with this circuit. Resistor R4, in series with the FET drain terminal, must be large enough not to allow currents in excess of Iy to flow when the UJT is on. Otherwise, the UJT will not turn off and the circuit will latch up.