capacitance value to add delay in circuit

[bobdxcool]

I am using a relay to turn on/off a 12V motor. I wanted to add a delay in the circuit by adding a capacitor across the relay. I wanted to know what value of capacitor should I use to introduce a delay of 1second in the circuit. What is the relationship between the capacitor value and delay value ?


This is the relay I am using, amazon.com/SainSmart-4-CH-4-Channel-Relay-Module/dp/B0057OC5O8 I am activating the relay using a raspberry pi. The raspberry pi is in turn connected to a ULN2803 Darlington array. This is the schematic, i40.tinypic.com/xknc53.png

 

[Gryd3]

I am using a relay to turn on/off a 12V motor. I wanted to add a delay in the circuit by adding a capacitor across the relay. I wanted to know what value of capacitor should I use to introduce a delay of 1second in the circuit. What is the relationship between the capacitor value and delay value ?


This is the relay I am using, amazon.com/SainSmart-4-CH-4-Channel-Relay-Module/dp/B0057OC5O8 I am activating the relay using a raspberry pi. The raspberry pi is in turn connected to a ULN2803 Darlington array. This is the schematic, i40.tinypic.com/xknc53.png

The capacitor is only one part of the circuit required to make a delay.
Do a quick google search on RC Time Constants and you'll get a better idea... there is one problem though...
They gradually gain/loose charge... so the current required to turn on the relay will gradually build-up, it won't simply be a 1 second pause and a 'click' of the relay.
This would cause some variance, and the delay may not be stable.

 

[KrisBlueNZ]

Do you want to delay the turn-ON of the relay or the turn-OFF?

You might be able to delay the turn-OFF by adding a diode and a big capacitor, especially if the relay coil doesn't draw much current.

For delaying the turn-ON, adding a capacitor isn't practical. You would need something like an SCR with an R-C delay on its gate.

Tell us the rated voltage and power consumption of the relay coil and we can suggest something.

 

[hevans1944]

Gee, you are using a raspberry pi to activate a relay? Surely you can program the delay you need instead of using old school analog technology?