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Electrical Engineering Question
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ghporter
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Dec 8, 2014, 09:42 PM
 
I have a 12 VDC, 20 A "LED Power Supply" that I'm using to run some electronics - small video monitor, LED lights, a video camera, etc. It has a grounded, Line/Neutral/Ground AC input, but the output is +12V and "common." Common is isolated from (not in any way connected to) frame/chasis ground. So far, so good, right? Now it gets interesting: there's a 2-3 VAC, 60 Hz ripple on the DC common output (referenced to ground).

First, is this a problem? I haven't actually touched the common lugs with the power supply on, but that sounds like enough to feel. Is that enough to worry about?

Second, if it is a real problem, what value and type of capacitor should I put between common and ground?

I'm taking the 12v output into a small aluminum box I've put a couple of 9VDC regulator circuits in, and of course their outputs will have the same "common" reference point, but I can't find a reasonable way to isolate connectors for the output from the box the regulators are in, and the box will be uninsulated on the outside. Obviously, if my "common" is now ON the aluminum box, I want to to whatever I need to in order to avoid problems.

Thanks for your expertise.

Glenn -----OTR/L, MOT, Tx
     
reader50
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Dec 9, 2014, 12:59 AM
 
2-3 VAC isn't enough to feel. You'd need amazingly low skin resistance to get enough milliamps to flow with such little voltage. Like pressing an open wound into the voltage source, and another open wound against ground.

First off, see if it's a real or phantom AC voltage. Modern multitesters can measure a voltage using amazingly low current values. Get a decent sized cap, attach one lead to the DC common, then place an AC ammeter between the other cap and the chassis. If you want to be really cautious, throw in an inline resistor for safety. You may find no measurable current flowing. If so, it's a voltage that's technically there, but with a very high internal resistance. Probably not able to muck anything up, but easily killed with almost any capacitor.
     
ghporter  (op)
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Dec 9, 2014, 06:45 PM
 
Cool. I pretty much thought that with that low a ripple it was most likely nothing to worry about, but I've been out of the hardware business long enough to want confirmation.

Glenn -----OTR/L, MOT, Tx
     
ghporter  (op)
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Dec 14, 2014, 11:20 PM
 
I finally got the chance to check the ripple current. There is NO measurable ripple current between "common" and ground. I'm not going to worry about it, since I couldn't measure any current on the lowest AC range my meter goes.

I finished up the regulator box - it has 12v come in, go through a switch, then branch to the inputs of the two 9v regulators (7809s) and to output jacks for 12v. The two regulators' outputs go to individual 9v output jacks. I've run the system, and it gives me great video with the flick of a switch. My LED light strips give me plenty of light for the camera, and the only thing left may be to put a small shade on the monitor so lights around it don't wash it out.

Thanks for the help.

Glenn -----OTR/L, MOT, Tx
     
   
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