LONG_DURATION_TIME_DELAY

Published:2009/7/11 1:07:00 Author:May | From:SeekIC

Transistor Q1 and resistors RI, R2, and R3 form a constant current source and the charge cur-rent 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 μA, 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 D1 provides a low-impedance discharge path for CE. D1 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 Q1, is set by the variable resistor R1. 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 IV to flow when the UJT is on. Otherwise, the UJT will not turn off and the circuit will latch up.



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