Electronics > Projects, Designs, and Technical Stuff
Auto-Standby circuit doesnt work as expected
Ian.M:
The timing network appears to be R71 2Meg charging C78 220uF. Its time constant is approximately 7 minutes. Its decreased a bit by the effective load of Q3 base, which can be approximated as a 30Meg resistor to ground (emitter_load*hFE) so it will only charge to 94% of the supply, less approx 0.7V Additionally you are comparing it with half the supply voltage so it *should* only delay for approx 5 3/4 minutes.
The fact you observed a 15 minute delay means either the leakage current of C78 is a large proportion of its charging current, or the reset transistor Q2 is leaky or is getting partially turned on intermittently.
The whole approach of an extremely large time constant RC network with an electrolytic cap is irretrievably flawed due to the unstable and temperature dependant leakage current. IMHO you need to redesign it with a digital timer, either an oscillator and divider chain, or a MCU.
Consider using a CD4060 configured for a low frequency RC oscillator, with a diode from the desired output to the RS input to inhibit its oscillator at the end of the delay. Drive its MR (master reset) with your activity signal.
Michaelaudio:
But the circuit works?It just doesnt work when I soldered the ICs on the PCB cause on Pin 5 the voltage drobs as soon as its higher than on Pin 6 on IC9b. I know they r smarter ways to do it but I want to solve the problem not re design it. So the time delay thing is strange yea, but anyway it works (almost)
What I found out so far:
-when the voltage goes up and down (7.7-7.6V at Pin5 IC9B (measured to virtual ground) the voltage on the Base of Q3 doesnt change, so that means c78 is full charged, so it has something to do with the comparator itself?
-when I remove R78 it works
-when I remove D16/17 it works
Im sure im close but still cant see where the problem is
Ian.M:
What do you expect to happen when IC9B acting as a comparator causes the MOSFET Q4 to switch off the Vdd supply to the timing circuit feeding its own inputs? The circuit is fundamentally flawed and if it operates at all its by pure luck.
Michaelaudio:
Oh my fault! I forgot to switch the Power Symbol, the circuit is always powered ofc.
tooki:
VDD is the unswitched always-on power, and VCC is the switched power, correct?
(And no, I don’t know why it’s not working, sorry! But I’d listen to Ian.M.)
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