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Topic Title: Using Capacitor To Keep On Relay
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Created On: 20 December 2011 04:11 PM
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 20 December 2011 04:11 PM
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I need to keep a relay energised for a short amount of time after the supply voltage has been removed. Have been looking at using a capacitor to hold the relay on. If using a capacitor, should I use a rectifier and resistor? Not entirely sure of length of time but I imagine no longer than 2 seconds. Relay coil is 24v, have not attended site yet so have not measured resistance of coil. Just looking for general advice on the best way to achieve the desired result! Not aware of any product available that does this out of the packet but if anyone knows of one please let me know.

Thanks in advance
 20 December 2011 08:12 PM
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how many contacts do you need and what current rating? you could always just fit a "True Delay On De-energisation timer, off the shelf from any good suppler and only about 25 quid, probably even cheaper
 21 December 2011 12:10 PM
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A delay-on-de-energisation timer is usually used as Dave 69 said.
If you want to do it yourself, you need to know the coil resistance and the voltage at which the relay will drop out. This is lower than the pull-in voltage. See manufacturer's spec sheet.
A diode must be used as other parallel loads will also discharge the capacitor. Also, take into account the tolerance on the capacitor, although choosing the next size up usually gives you plently of spare.
Make sure your diode can take the burst of current when charging the capacitor. Strictly you need to do a tolerance analysis, taking into account minimum coil reistance when cold, maximum power supply voltage, etc., although in most cases these are second order effects.
 09 January 2012 01:54 PM
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depends how you are driving the relay and the spec of the relay coil the Q = CV charge = cpacitance * voltage. you could use a BJT amplifier with negative feedback on the output stage of a 555 timer there are many designs out there on the internet but pin 3 output can be held high relative to T= 1.1 * R1 * C1 connected across pin 6 and pin 7 from supply to GND holding in reset pin 4 to pin 8 sounds elaborate but the capacitor your need to hold the charge for that period of time and load would be impractical bare in mind,
time = capacitance * resistance but then you got reactance of the capacitor and resistivity of the resistor 14uF * 640 ohm at 230v would give 20ms approx but how much current is needed by the relay coil...

hope thats of some help
 20 January 2012 11:54 AM
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Hi guys,

Thanks for the replies! 1st time posting on the forum, wasn't aware anybody had replied as I didn't recieve any sort of notification!! But thank you for the suggestions, only need the one NO contact so will just grab one off the shelf, haven't required one before so I wasn't aware they were readily available.

Thanks again

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