Delay Off Circuit

[bigTomHanks]

I have an application where I need to power a small automotive relay for 15-30 seconds after cutting off electrical power(12 volts). I thought there was a 555 circuit that could perform this function but cannot find it. If someone knows the name or can provide a link to the circuit that I am trying to make that would be very helpful.

 

[MacGyverS2000]

A Google search of 555 delay circuit will turn up millions of hits...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[MikeHalloran]

Millions, indeed, and that's the problem.

If you search for 'time delay on power off', you still get a bunch, most of which seem to assume the presence of an 'always on' battery connection. ... which makes the task straightforward using a 555.

If you want to package your circuit in a black box that doesn't require a battery connection, then the black box needs to store some energy. That could be as simple as a big capacitor across the relay coil, both fed by a diode to turn the relay on fast when the power comes up. The timing is then determined by the capacitor charge, the resistance of the relay coil, and the dropout voltage of the relay. If you want better precision, then you need for the capacitor to supply power to the resistor and your timing circuit. I couldn't find a circuit like that with a bit of searching, but you may be able to work it out for yourself.



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Look for a pneumatic timing relay, or a pneumatic timing head for a standard contactor. They're the old-school way of providing a true delayed-off contact. Most are good for about 5A-10A or so, above that you'll need an external relay or contactor. Agastat is probably the best known brand, although finding a 12V Agastat might be a challenge.

Curious: if your load has a supply which is being switched by the contacts of your relay, why can't you derive power from this? Something is missing from this picture.

 

[VEBill]

"Turbo Timers" are available on eBay. Cheap. Free shipping.

Turn off the car key, and the engine continues to idle for an adjustable time... ...to cool down the turbo. Does exactly what you've described.

 

[bigTomHanks]

Thanks to all for the suggestions. I was able to put together a "Power-Off Time Delay Relay" circuit.

http://www.bowdenshobbycircuits.info/page2.htm

Another solution that would work for my application would be to wire the relay I'm trying to power into the dome light circuit since it stays on for about 30 sec. after I close the door.

I actually had a turbo timer that I tried first but it didn't work. (I think it was defective from the box and I couldn't exchange it because I got it off ebay.)

I also found an off the shelf item if anyone else is happens on this thread for the same reason. This is the easier way and it's fairly cheap and adjustable out of the box.

http://www.delcity.net/store/Time-D...t_1.r_IF1003?gclid=CIHVid7yyr4CFcxZ7AodGjkApw