i have search and read a bunch of posts and for what i want to do seems to be pretty simple compared to other posts.

i have a relay that is 12vdc and want to keep it energized for about 1.5 seconds after power cut off.

it can be energized for longer like 2 seconds or even 5 seconds..

relay that i would like to use is HK19F
http://grobotronics.com/images/datasheets/HK19F-12V.pdf


i just don't know how to calculate the size of the capacitor to use to give me the desired result.

power ON>relay active>power OFF>relay active for 2 seconds then off

 

From the data sheet get the resistance of the coil in the relay, and the minimum voltage that it will stay latched. Using the resistance and the capacitance, graph the voltage of the capacitor as it discharges through the relay. Play with the capacitance value until you get your goal time to reach the unlatch voltage from fully charged. The coil in the relay will have some inductance so you've really got an RLC circuit, but for this case I believe you can ignore the inductance portion since your time doesn't have to be precise, and the effects of the inductance will be minimal anyway. Then build one and see how close you got.

This might help:

http://www.electronics-tutorials.ws/rc/rc_2.html

 

Not really enough information, but I can guess and you can adjust as necessary.
This relay seems to be 720 ohms and the guaranteed dropout voltage is 1.2V
Vo = -dV e^(-time/RC)
10.8V = -12V e^(-1.5 seconds/720Capacitance)
0.9V = e^(-Time /720C)
2.46 = 1.5/720C
1771C = 1.5
C = 1.5/1771
C = 847 uf

This is only a ballpark number because relay dropout voltage isn't really very predictable and I guessed about the resistance. So, what do you have in the 1,000 uf range?

 

You can also just purchase a "delay on break" relay..
But what you want is called "delay on break" and the opposite is "delay on make"

 

..........................
Absurd size? There is a way to amplify the effect of the capacitance with a transistor.

That only works if the power is still available after you turn off the relay.

 

That only works if the power is still available after you turn off the relay.

Oops.
Caught me not paying attention.

So I edited out the false part of my post.

 

WOW! this is awesome! was not expecting so much help! but i am glad that i found this little corner of the internet...

so, seems like what i want to do is create a "Delay on break" relay.. i searched online for it, and the ones i did find would be so over kill for what i need.... but i will try out different capacitance and see what happens... i'll post back results...