Continuous to Momentary Circuit needed

[verno]

I have a 12v DC (continuous) source that holds an exhaust brake on a diesel pick-up closed. The new exhaust brake requires a momentary 12v DC pulse. Is there a simple way to convert a continuous source to momentary?

Thanks.

 

[kchriste]

A capacitor in series with a relay coil would work. That's about as simple as you could get. How long does the momentary pulse need to be? How much current does the exhaust brake draw?

 

[verno]

Thats what I figured, just wanted a confirmation.
The draw is only the coil on a 12v automotive solenoid. It needs to pulse about a 1/2 second.
12v pos. to coil, 12v neg to capacitor then ground?

 

[kchriste]

Yup. The size of the capacitor will depend on the relay. With a typical 150hm: automotive relay coil, try 4700uf or so. You may need a diode across the coil to prevent the relay from clicking again when the voltage disappears.

 

[verno]

What voltage rating should the capacitor be?

12v ?

 

[kchriste]

You should always have some margin for error. Choose a 16V one or higher because the charging system on the truck typically will float the battery up around 14.2V when the engine is running.

 

[Hero999]

You can' get timer relays for this sort of thing.