Power on delay for relay !

[yusuf]

Hi guys I want 2 to 5 sec delay on my relay as I power the relay !...

please help...
I want to use only RC circuit because the delay is very small... 2 to 5 second...

thanks.. : )

 

[alec_t]

I want to use only RC circuit because the delay is very small... 2 to 5 second.

That is not very small for a purely RC circuit. It would be more practical to add an active device (e.g. a FET or IC).

 

[4pyros]

Use a 555 timer or a relay with a built in delay.

 

[k7elp60]

Another way is to use a DC relay coil with a large electrolytic capacitor in parallel and a resistor in series. For example with a 12V supply use a 5 or 6V relay a 2200uF capacitor and a about a 1K resistor the time will be about 2.2 seconds. Use a DP relay and use one set of contacts to connect a proper resistor in parallel with the 1K to keep the relay energized.

 

[alec_t]

Another way is to use a DC relay coil with a large electrolytic capacitor in parallel and a resistor in series. For example with a 12V supply use a 5 or 6V relay a 2200uF capacitor and a about a 1K resistor

Afraid not. The maximum current through the relay coil would be less than ~ 6mA. Not enough to energise it.

 

[4pyros]

So what is the relay voltage and what is it switching?

 

[elexboi]

Its better to use timing circuit just like 555timer. if you have problems with the current to energize it you can use transistor for current amplification.

 

[yusuf]

I have 12v , 200 ohms relay... it need 12v and 60 ma current to work properly...

 

[alec_t]

This will give ~ 4sec delay, adjustable by varying R1 and/or C1.

Edit: A reverse-biased diode should be connected across the relay coil to protect the FET from the back-emf generated when the FET switches off.

 

[yusuf]

Thanks alec... it worked...

 

[k7elp60]

Afraid not. The maximum current through the relay coil would be less than ~ 6mA. Not enough to energise it.

You are right about the 1K resistor, but it isn't the 6mA the prevented the relay from energizing. The 1k resistor in series with the relay coil(400 ohms) prevented the capacitor from charging up to 9V(75% of nominal coil voltage) When I made my first post about using a resistor and a capacitor, I was thinking of some of the specifications of DC relays. That is that they normallly energize at 75% of nominal coil voltage and they normally release at 10% of nominal coil voltage. I change the 1K resistor to 470 with a 12V 400 ohm coil relay and the coil did energize with about a second delay. I used a 4700uf capacitor and the delay increased to about 2 seconds. On a similar note slow release relays used to be available. They had a large copper slug around the armature that caused the relays to have a slow release feature. Then one step further AC relays have a copper slug on the armature that prevents a relay buzzing on AC. If you look at a DC relay and compare it to a AC relay you will see the copper slug on the AC relay. So how does a copper slug that has no magnetic properties hold magnetism?....

 

[alec_t]

So how does a copper slug that has no magnetic properties hold magnetism?....

Currents induced in the copper create a magnetic field.

 

[alec_t]

Thanks alec... it worked...

Glad it did. But note the edit in post #9 if you want it to continue working!