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Mark245 5th December 2009 10:07 PM

Safety Question about Capacitors
 
Hey everyone,

I have a general question about capacitors concerning safety issues. I have been warned to make sure that I discharge capacitors before touching their terminals. Genearlly speaking, at what voltage is this an issue?

There is something that is confusing about this. A capacitor is pretty much like a battery right? There is a voltage formed accross the two terminals. It seems like since there is no return path for the current to flow that one could not be shocked by touching just one terminal of the capacitor. A person would have to touch both terminals to be shocked. Is this true? The reason I ask is that I was talking to someone repairing a TV. He said that the flyback transformer must be discharged before taking it out. This is becuase the TV tube is acting like a capacitor and can store a huge charge at like 20kV. Touching the high voltage line could kill a person. So what path is the current taking in this case? Am I right in assuming that the electrons are going from the high voltage line through a person to ground?

This almost goes into how a capacitor works I guess. Charge is only built up on one side of the capacitor right? So I guess touching one of the terminals will cause no harm but touching the charged side could. Can some please clear this up?

Thanks,

Mark

SY 5th December 2009 10:17 PM

If one plate of a cap is charged, they're both charged. Potential difference is potential difference, regardless of where you arbitrarily set a reference.

In theory, you could touch one side of a charged cap and live. In reality, your body is probably grounded as is (most often) one side of a cap.

Strong advice: use bleeders and wait for the voltage across any of the caps to drop below 10-20V before poking your hands around. Much lower if you're wearing a ring or are introducing anything conductive in the area- even 10V from a big cap will produce an impressive arc.

Conrad Hoffman 5th December 2009 10:31 PM

I believe regulatory bodies require extra precautions above 40V. It's the current that does you harm. Given a large area of wet skin, it might not take much voltage to cause a problem. The old lick-the-9V-battery trick. When working with high voltage the best practice is to work with one hand behind your back or in a pocket- don't complete a circuit with your body. Capacitors can deliver a lot of current at whatever voltage they're charged up to, so caution is always in order. Use bleeders where needed.

Speedskater 6th December 2009 12:05 AM

Also TV picture tubes (CRT) act as high voltage capacitors. Both the glass tube's inner and outer surfaces are coated with conductive material with the glass between acting as the insulator. It's not a big capacitor but with maybe 20 thousand volts it doesn't take much. Also many high voltage capacitors have memory. After being charged for a period of time, you can dis-charge them and they will recharge with no power connections.

Mark245 6th December 2009 06:36 PM

"Also many high voltage capacitors have memory. After being charged for a period of time, you can dis-charge them and they will recharge with no power connections."

Really? How can this happen? I work with some high voltage stuff. Should I leave a short on the terminals of HV caps that I have sitting around just in case?

Also, I am wondering how is it possible that I can connect a terminal of a battery to earth ground and it will not discharge but if I do the same for a capacitor, it will.

Thanks

star882 6th December 2009 09:14 PM

Some capacitors use polarized dielectrics. When voltage is applied, the molecules in the dielectric align to the electric field, effectively increasing the capacitance. When the capacitor is discharged, the molecules unalign and release their stored energy. It takes some time for that to happen, so one way to model that is a second capacitor in parallel with the first with some series resistance.

A friend of mine has seen that happen with a 2F (yes, 2 Farads), 16v capacitor he bought for his solar energy projects. To test and condition it, he charged it using a current-limited bench power supply at about 500mA up to 15v. He mentioned that instead of fully charging in about a minute as expected, it quickly charged up to 15v in about 25 seconds and then continued to draw current at a roughly exponentially decreasing rate for a few minutes. He asked one of his friends about it and she said something about the dielectric fluid having a high viscosity which means it polarizes relatively slowly.

Mark245 7th December 2009 05:15 PM

I set up a simple experiment to learn more about this. I charged up a 470uF cap to 10v. I can connect either terminal to earth ground and it has no effect on the voltage across the cap. How then is it possible to receive a shock from a capacitor by only touching one terminal? It seems like you would have to touch both. The reason I am asking is that if you touch a CRT where the anode from the flyback touches the tube, I have been warned that you could receive a fatal shock.

Also, by touching the "dag" on the back of the CRT, why is this safe but you cannot touch where the anode from the flyback connects? Following the logic that if one side of the cap is charged then both sides are charged, it seems like touching either side of the cap should be harmful.

Thanks for the replies so far.

mark


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