# delay a relay

Discussion in 'The Projects Forum' started by matelot, Apr 15, 2013.

1. ### matelot Thread Starter New Member

Apr 15, 2013
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0
Is it possible to put a capacitor across the coil of a relay to delay the relay switching on for about 2 seconds?
I have a circuit that needs to switch on a relay if two other relays are switched on, the problem is relay 1 switches off, I then press a button if I want my relay to come on, after a set period relay 1 switches on and relay 2 switches off (both could be on for a fraction of a second) when relay 2 switches back on I want my relay to also go on.
If I could delay my relays switch on for, say, 2 seconds this would solve the problem of seeing both 1 and 2 on for a moment.
Is it possible to do this with a capacitor and resistor?
I have a choice of 24v relay with a coil resistance of 660 ohms or a 12v coil with a resistance of 160 ohms both would have a resistor in series with the coil so that the relay could be turned on/off at points a and b - see attachment.
The power supply is 24volts.
Thanks,
Bob.

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2. ### Mitch conrad New Member

Apr 10, 2013
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Last edited: Apr 15, 2013
#12 likes this.
3. ### matelot Thread Starter New Member

Apr 15, 2013
24
0
thanks for that.
That was one option I had considered but I know if I introduce the delay my fellow workers will consider it being used on other relays, so I was looking for an option that would be easier, quicker and cheaper. I thought an rc circuit charged up with a dc voltage and could be used in some way to delay a relay until it had charged enough to energise the relay. My other option is to use an rc circuit and a zener that will breakdown at about 2/3 vcc and drop that across the relay.
I am not sure about the values and thought I could get some pointers here.
If the 555 is the only way I will have to go that route thanks for the help.

4. ### #12 Expert

Nov 30, 2010
16,298
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Point in fact: It can be done but the price is dreadful.
You will have over \$20 in parts and shipping to do it the "easy" way.
You can do a 555 timer for about \$2 if you're good at shopping.

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5. ### wayneh Expert

Sep 9, 2010
12,127
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You could use an RC tank on the base of a transistor which in turn is switching current to the relay coil. Cheap and simple. It will take some experimenting to get your delay right, and it won't be precise.

6. ### #12 Expert

Nov 30, 2010
16,298
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Something like this?

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7. ### Mitch conrad New Member

Apr 10, 2013
7
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I only skimmed through your post I thought you meant a cap series with the relay. A 555 is cheaper than what you want to do, and doesn't require a large capacitor as #12 pointed out. For the price of a few resistors a capacitor and 555 it should cost you below \$10 if not even lower.

8. ### wayneh Expert

Sep 9, 2010
12,127
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Yup, that's even better with the zener to sharpen the on/off transition. Depending on the component values, I think you may not need either of the 2 righthand resistors (replace base resistor by wire, omit resistor to ground). It's good to show them at this point though.

Sep 3, 2010
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10. ### vk6zgo Active Member

Jul 21, 2012
677
85
A capacitor in parallel with the relay coil will give you a delay,but it won't be a very long one,as it is dependent on the internal resistance of your supply.

A series resistor,then a cap in parallel will do better,but then you may not have a high enough voltage to consistently operate the relay.

Another downside,is that you will then have a (usually longer) delay on release of the relay.

11. ### wayneh Expert

Sep 9, 2010
12,127
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Using a transistor will allow using a much smaller capacitor and doesn't require a resistor in the power path.

12. ### #12 Expert

Nov 30, 2010
16,298
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Something like this?
(See post #6)

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13. ### wayneh Expert

Sep 9, 2010
12,127
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Yes again. I'm not sure why the OP doesn't seem interested in this simple approach.

I'd start with just the RC on the left, and the transistor, and see if it worked. I suspect it would although finding the right values might take some experimentation.