Author Topic: RC relay delay circuit question  (Read 824 times)

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Offline Chris Wilson

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RC relay delay circuit question
« on: September 18, 2012, 09:29:34 PM »
24V DC relay with coil resistance of 445 Ohms. Coil energising circuit is 110V AC via a 6A10 diode, a 1K Ohm ceramic wire wound 12 watt resistor, and 2200 uF electrolytic. Coil delay is just over 1 second. Measured voltage across relay coil is about 21 volts DC. All works well except the 12 Watt ceramic wire wound runs a bit hot, I may use 3 off 3k Ohm in parallel. Problem is relay is slow to disengage when power to the coil is removed, due to retained voltage in the electrolytic. It's meant as a safety shut off relay (interlock) for a power supply. Is there a way to make it release instantly, or near as damn it instantly? Thanks.
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                 Chris Wilson.

Offline David_AVD

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Re: RC relay delay circuit question
« Reply #1 on: September 18, 2012, 10:11:10 PM »
Perhaps a second relay run from the raw source that has it's N/O contact in series with the first relay's coil (or the output contact)?

Offline vk6zgo

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Re: RC relay delay circuit question
« Reply #2 on: September 19, 2012, 01:19:41 PM »
If you are using it as an interlock relay,why do you have an operate time delay?
Is it also being used for the slow start function you asked about before,or is it just that you need the electro to make the DC relay work on the halfwave rectified  pulsating DC?

If it is supposed to cut off the  HT to a valve Amplifier,you could drive it off a bridge rectifier from the filaments instead of your current setup,& filter the supply properly.
The interlock switch could then just break a clean DC feed to your relay,& you could ditch the Electrolytic.

If you need the operate delay,you may be able to put a diode in line with the electro,where it can charge to give the delay,but not discharge through the relay coil,so the relay will drop straight out.
As the cap will have to discharge eventually,you will need to have a higher value resistor across the coil all the time.
I'm not sure this will work with the pulsating nature of the halfwave rectified AC.

Offline Simon

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Re: RC relay delay circuit question
« Reply #3 on: September 19, 2012, 07:54:32 PM »
You need to explain your application more. Do you need the delay ? or are you using the capacitor to smooth out the DC ? if so remove it, you can manage without. Preferably use a full wave rectifier, but I would think you can get away with reducing to 48V (or a bit less) on a single diode (50% duty) which will also reduce the heat in your resistor.

At present you need a 2.25W resistor


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