AN13: Delay timers, Power-on sequencing using TL431 in comparator mode
(c) klausmobile 2001
TL431 without feedback can be considered as an NPN or N-MOS device with high transconductance and precise 2.5V turn-on voltage. Tube heads beware: the upper (+supply) pin on TL431 is cathode, lower (-supply) - anode... it's another world.
Heavy load (over 100 mA) can be powered through fully decoupled relays or optocouples, or with an PNP (P-MOS) switching transistor. Use special PNP types developed for regulators - these have low voltage drop and don't need a lot of base current.
To add hysteresis (positive feedback) - add another PNP invertor. RS-DS regulator decouples input network from power supply ripple.
Basic timer configuration. Set starting current through RT to 100uA (well above 2uA drawn by base input). Turn-on delay T1 = 2.5V/U1 * (RT*СТ). Power-off delay is usually set by load current, but you must consider effects of power supply capacitanca and leakage (C2, R2).
Working example: Two consecutive power-on delays (45s, 60s) in a tube amp. Zener regulator decouples timing capacitor from power supply (it can motorboat without it - relay current is quite high, 100mA).
If two consecutive delays have a relatively small time spread - use a single timing capacitor:
