bridge rectifier and cap


Old 04-18-12, 09:55 PM
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bridge rectifier and cap

I'm attempting to put together a rig that will convert ac into dc through a bridge rectifier. I'll use it to run a 90v, 2.8a, 1/4hp, dc motor and will supply it with 120v ac. I've read that I can get a smoother operation if I place a reservoir capacitor on the load side of the bridge rectifier.

Does anyone know how to calculate the size of capacitor needed for such a circuit?
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Old 04-19-12, 06:09 AM
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I'd suggest a switching power supply to convert 120ac to 90dc.
Old 04-19-12, 07:38 PM
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Thank's Justin. This really is a pretty simple device. I'm making a foot pedal for a pottery wheel. I have the diagram for the device but I've read that a reservoir capacitor will help even out the signal. I use one just like it at work and am partial to the 0-100% output the variable transformer gives. I have the variable transformer, the switch and the fuse, and the bridge rectifier is about 4 bucks at Radio Shack. I'm kinda committed at this point.
Old 04-19-12, 08:01 PM
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Old 04-20-12, 04:38 AM
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A 3300 microfarad 250-volt polarized electrolytic cap will do the job. It's a good idea to wear safety glasses and put something between you and the cap (like a chunk of plywood) when you test it. Caps can blow up with a surprising amount of force. Also, since you'll be dealing with relatively high DC voltages, the cap will be storing lethal energy. Treat it as if touching it will kill you -- because it can. After it's energized and before you work on the circuit, make sure you discharge the cap properly.
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