Holding a relay on for a predetermined time? How?

Each

Simplest/cheapest would be to use a 555 timer chip. Not necessarily easiest as "tuning" a 10-minute R/C circuit is time consuming.

Get a 240Volt time delay relay like they use for downstairs lighting that can be switched on for a few minutes. Use your small 12 operated relays 240 volt contacts to trigger the time delay relay and have it operate your load.

Available at most combined electrical and lighting stores.

Hope this helps, Peter

What you are describing sounds like a classic "monostable" application. There are many IC's that will do this but not so may that will operate from

12V. Try the 555 but note that 10 mins is rather a long time constant for a monostable of this type (read the applications notes for the 555).

What do you want to happen if the 12 V goes high while the realay is still on? Do you want....

a) the relay to switch off at the end anyway or b) extend the time period or c) Go off and then come back on again?

If b) look for a "retriggerable monostable".

On the other hand I'm sure relays of the type you are looking for must exist somewhere - try a web search?

Each

I would build a little circuit and use a small 8 pin pic for the timer. You need to do a voltage level translation or use a 5V relay mosfet or magnetic. You can also buy a time delay relay but the choice is yours. A 555 can be used but long delays are not easy and I find the PIC solution costs less in parts count and is much more flexible. They also make mechanically latched relays and you can then use a supercap to power things wnen the 12VDC goes away. Larry

"Pete" wrote:

Larry G. Nelson Sr. mailto: snipped-for-privacy@ieee.org

All you need is a transistor, a cap and diode. See the drawing here:

-Bill