# Using capacitors to supply peak current

I hope this thread is allowed. Based upon my previous topic, which turned out to be the exact opposite of what I needed to accomplish, I have a few questions.

I have a 240W 24V Supply: https://www.us.tdk-lambda.com/ftp/Specs/dpp120-240.pdf

I have a coil with a 750W turn on rating and a 5W hold in rating: http://www.kvc.com.my/EnterpriseChannel/SharedResources/Datasheet/0/?ProductId=1000066755&Filename=SCHNEIDER-LX4-FH024.pdf

The device will also be used for some sensitive equipment, so I want to be safe and not allow too much voltage fluctuation.

50ms (closing time) * 30A= 1.5C (Thanks @DaveTweed) To be safe, 1.5C/4V= 375,000uF

What voltage rating? I've read to derate 2-3x.. for a .4F 50V cap and above, I'm looking at \$1-200 per capacitor. Am I playing it way too safe with these numbers, or is this really what I need?

Ok, so if my above power supply uses a forward folding method of current limiting, do I need to use a resistor/thermistor to ensure the capacitor itself doesn't overdraw my supply after it's emptied? I will probably have at minimum a minute of time for the cap to recharge.

As far as wiring, since there are other items in my circuit, I want the capacitor in parallel with my coil rather than directly off my supply, correct? If I were to need a thermistor/resistor would that be in series with the + side of the coil, prior to the capacitor?

• You won't need to derate that much : 25V would be dangerously close but 35V would have enough margin. Alternatively, and for a good deal less money, consider trickle charging a couple of SLA (lead acid) batteries off 24V and powering the contactors from them.
– user16324
Aug 27, 2015 at 10:34
• So at first I was going to put the capacitor straight across the contactor, which I realize doesn't make any sense as the capacitor will not charge when the control relay is off, which is when it should be charging. i.imgur.com/SRv9YPb.png So I'd put the return side straight to ground so the contactor is released but the capacitor can charge. i.imgur.com/LQRPc7L.png Will this also eliminate the potential for reverse current induced by the contactor releasing, or can that still potentially be an issue? Aug 27, 2015 at 21:11