Capacitor Value

J

Johnny G

New Member

• Aug 26, 2011

• #1

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

Well-Known Member

Most Helpful Member

• Aug 26, 2011

• #2

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

 

J

Johnny G

New Member

• Aug 26, 2011

• #3

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

 

ronv

Well-Known Member

Most Helpful Member

• Aug 26, 2011

• #4

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?

 

J

Johnny G

New Member

• Aug 27, 2011

• #5

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

 

J

Johnny G

New Member

• Aug 27, 2011

• #6

How small of a circuit? I will certainly try.

 

alec_t

Well-Known Member

Most Helpful Member

• Aug 27, 2011

• #7

What do you consider large?

Click to expand...

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

 

J

Johnny G

New Member

• Aug 27, 2011

• #8

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

johnny G

 

J

Johnny G

New Member

• Aug 27, 2011

• #9

Would a 33000uF 35v work for this?

 

alec_t

Well-Known Member

Most Helpful Member

• Aug 27, 2011

• #10

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

 

crutschow

Well-Known Member

Most Helpful Member

• Aug 27, 2011

• #11

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.

 

J

Johnny G

New Member

• Aug 29, 2011

• #12

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

 

ronv

Well-Known Member

Most Helpful Member

• Aug 29, 2011

• #13

Johnny G said:

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

Click to expand...

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.

Click to expand...