Capacitor Value

[Johnny G]

Hi Everyone, I'm new and need to know what value capacitor I need to power a 12VDC coil in a DPDT Relay that has a average consumption rate of .9 watts for 1.5 to 2 seconds after the 12 volt supply is dropped to the coil. Any help would be greatly appreciated. Johnny G

 

[ronv]

If we assume thr relay will "drop out" at about 8 volts, it would take about 25,000 Ufd. to hold it on.

 

[Johnny G]

thanks ronv, Drop-out voltage threshold 10% and will be supplying 12vdc to coil.

 

[ronv]

10.8 volts will make the cap very large. It might be easier (and cheaper) to build a small circuit to produce the delay. Are you up for it?
What "picks" the relay?

 

[Johnny G]

What do you consider large? I would like to keep this as simple as possible.

 

[Johnny G]

How small of a circuit? I will certainly try.

 

[alec_t]

What do you consider large?

It will need to be about 0.1 Farad (100000uF) for a 1.5 sec hold time, or 0.15F (150000uF) for 2 secs.

 

[Johnny G]

Thanks alec_t, I will test that out ASAP I Appreciate the Help.

johnny G

 

[Johnny G]

Would a 33000uF 35v work for this?

 

[alec_t]

No. That will give you a hold time of only ~ 0.55 secs.

 

[crutschow]

A way to use a smaller capacitor is just to add a transistor with an RC delay on the input, such as the attached below. The values shown give about a 2s delay on turn-off.

This circuit does require that you maintain 12V on the relay. If that's not possible then you will need to use a large capacitor as alec_t stated.

 

[Johnny G]

I will use the .1f, Do I need to precharge this value and if so what is the simplest circuit to use?

 

[ronv]

Do I need to precharge this value and if so what is the simplest circuit to use?

That's the question...

What switches the relay? The current will be very high to charge the cap if there is nothing to limit it.