Closing a circuit with a capacitor

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SUMMARY

The discussion centers on activating an electromagnet for a precise duration using a capacitor and resistor in series. While the initial suggestion involves using a capacitor to cut off the circuit after charging, it is concluded that this method results in a gradual decrease in current, which is not ideal for maintaining full power. The recommended solution is to utilize a normally-closed, timed-open relay for consistent performance. Proper discharge methods for the capacitor are also emphasized as necessary for safety.

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daniel_i_l
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I have an electromagnet that I want to be activated for two seconds as soon as I turn the power on. I think that this can be done by connecting the electromagnet to the power source in a series with a capacitor and a resistor. As soon as the power is turned on the EM would work until the capacitor gets charged up and cuts off the circuit. Is that true? If so, how do I calculate the sizes of the resistor and capacitor that I need?
Thanks.
 
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I don't think the capacitor idea is a good solution to your problem because the current will gradually decrease as the capacitor charges. I'm assuming you would prefer to have the electromagnet have it's full power for the duration of the time interval. Also, you would need to have a safe way to discharge the capacitor after power is removed. A better solution would be to use a normally-closed, timed-open, relay.
http://www.allaboutcircuits.com/vol_4/chpt_5/3.html
 

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