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Need a simple power on delay circuit

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5065AGuru:
Hi,

I have a device that is powered by +20VDC at about 125-150mA. I need to add a circuit that once it senses the +20V has been turned on, delays the delivery of the +20V to the device for 2 Seconds or so. I don't want to use a relay. Is there a simple MOSFET high side switch circuit I could adapt?

Cheers,

Corby

TomS_:
Quick and hacky circuit attached. Not particularly elegant, and you might want to make sure that it doesnt output some kind of spike immediately after turn on.

Q1's gate is held high via R1 while R2-3/C1 forms an RC circuit which holds Q2's gate low until C1 charges sufficiently, thus turning Q2 on which pulls Q1's gate low allowing voltage to flow.

Otherwise there are things like processor supervisors which might do a lot better since they have voltage comparators and output delays built in. Some of these wont give 2 seconds of delay, but there are some out there which can use an external RC circuit to provide any delay you want (e.g. TL7705A). You'd just need to use something like a resistor divider to power it from a more acceptable voltage level.

Perhaps there are other better voltage monitor circuits out there that someone may come up with.

ajb:
You can skip the n-channel FET and flip the RC circuit over to save a couple of components if you're okay with a slow rise time.  You'll want a diode in parallel with R2 (cathode to V+) to ensure that C1 is discharged quickly on power off, otherwise you won't get the right delay after a short power blip.

OM222O:
is the circuit always outputting 20v or you need to sense that 20v is present, then delay it?
RC circuit can't actively monitor the voltage  ???

I know it's very common with USB-C powered devices that they have to monitor it because the voltage can be either 5 or 20 volts, depending on what device is connected.

5065AGuru:
Thanks for the ideas!

I've some parts coming and will breadboard the circuit and play with it.

Cheers,

Corby

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