keep relay energized with Capacitor or 555 ?

[settra]

hello . i want to create a circuit for the following propuse:

i have a 12v relay, that turns on with a 12v signal. (the signal stays HIGH and keeps the relay energized for alot of time). after the signal drops to LOW, i need the relay to stay energized for about 1 minute extra...

for small periods of time, i can do this by just adding a 12v capacitor parallel to the relays solenoid inputs...

but as i mentioned, i would like to keep it alive for about a minute... so i guess i will need to use a 555 timer??? the problem is i dont have the slightest idea how it works.... anyone can give me an advice???

 

[MikeMl]

Do you have any NFETs with a drain current rating > relay coil current, with a Vth >3V?

 

[jpanhalt]

Use a mosfet to turn the relay on/off. Then power the mosfet gate with a resistor/capacitor. Since the mosfet gate draws negligible current in the on state, you can control the turn off time by using an appropriately large resistor and/or capacitor (i.e., RC) to get the delay you want.

John

 

[settra]

MikeMi. i dont udnerstand anything

my application would be this :

i want to have an arduino on my car for some reasons. i want the arduino to be powered, only when ACC is on (when the key is turned). so i was thinking, one positive wire from the battery, to power the arduino, and the flow of current on that cable would be controled by the relay (wich would be controled by the ACC wire). using the Mosfet, i suppose its prety much the same as using the 555 as i see it now... but do you think, maybe i could skip the relay, and just use a Mosfet on its place???

 

[MikeMl]

If you want to keep the 12V applied to the regulator that powers the Arduino, then you need a "high-side" switch. I will redraw the timer circuit without the relay...