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 Post subject: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 3:05 am 
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I've seen many different versions on Youtube regarding this subject. However, I thought I'd go to the experts. When I work on a record player, it's usually been sitting around for months so I am still cautious regarding residual voltage. However, I may come across something that's been used more recently and wanted to know how to properly discharge the voltage before I started working on it.

Thank you, in advance.


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 3:11 am 
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The right way would be with a fairly low value resistor. Try 1K - 10K @ 10 watts wire wound resistor clipped on terminals of the cap.

Some of us use a screw driver but this isn't recommended. It will cause a bright flash, can open internal metal in a cap and damage the screwdriver. Once removed some voltage will build back.

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 3:21 am 
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Location: Maryland 20709, USA
If it's been off for a day, it's probably safe.

If it does have voltage, a 10K resistor will discharge most older (low-mfd) caps in a few minutes.

You can always check it with a voltmeter.
Get an old 1000 ohms-per-volt meter and it will discharge the cap itself.

or...

Hand it to your Mother-in-Law.

- Leigh

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 3:29 am 
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Location: Albuquerque, NM 87123
I use 10 ohms at 20 watts. I don't want any charge left on a cap, as it can damage modern electronics like capacitor testers.


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 4:59 am 
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Leigh wrote:

Hand it to your Mother-in-Law.

- Leigh



And ruin what could be a perfectly good capacitor?!


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 6:00 am 
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Dr. Radio wrote:
Leigh wrote:
Hand it to your Mother-in-Law.
- Leigh
And ruin what could be a perfectly good capacitor?!
Great comeback. ;-)

- Leigh

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 2:20 pm 
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Location: Charleston, W.Va.
Many years ago I made up an old 10K/2watt carbon comp resistor with short insulated leads attached to mini-alligator clips, and keep it hanging on a pegboard hook above my workbench. Very handy for this purpose and easy to make. As Norm says, value of resistor does not matter much.

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 3:36 pm 
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In the old shop days the best way to discharge a HV cap was against a butt cheek of a coworker.

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 4:19 pm 
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I just stick a long handled screwdriver in there and hit the positive lead to ground and wait to hear the 'snap'. Ta-da!


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 6:33 pm 
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Location: Santa Rosa, CA
Quote:
I just stick a long handled screwdriver in there and hit the positive lead to ground and wait to hear the 'snap'. Ta-da!


Try this method, then measure the cap voltage again. Some caps won't be totally discharged that way. You lose some energy in heating the screwdriver and the "snap," but you could end up reversing the polarity (on a non-polarized cap) and keeping a lot of the charge.

In the resistor method, I know I am dissipating the cap energy as heat in the resistor.

What do others thinks?

Rich


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 6:43 pm 
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Neat and tidy... → Constructing a Capacitor Discharge Tool
Image

Nine times out of ten I use a 100Watt light bulb :)

Greg.


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 7:17 pm 
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There's a reasonably informative and very entertaining thread on the topic on qrz.com: http://forums.qrz.com/archive/index.php/t-162986.html.

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 8:16 pm 
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Yup. The screwdriver option isn't the most technically correct method. Especially for discharging TV CRTs. You will often have to touch the electrode on the side several times with the tip to remove the charge. The other problem is that on some occasions you can damage your screwdriver. I restored an amp one time and ultimately found that one of the large power resistors was shorted and as a result all of the capacitors were not grounded at all. When I touched the ends of one of them the snap literally burned a chunk out of the tip. Ouch!


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 8:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Portland Oregon
In high school electronics class the teacher had a very big capacitor stashed away. It was the size of two car batteries and then he would charge it up over night. In class he would show us why we should be careful with caps and then drop a chunk of iron across the terminals. It was like a gun shot and would burn off a nice chunk from the end and shorten it some. He had saved the first big screw driver he used for that trick and there wasn't a hint of the tip end left, just a square rod with a plastic handle.
Ed


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 8:58 pm 
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Don't know if it's the BEST way, but some years ago, a co-worker showed me what must surely be the MOST FUN way. He would charge up this REALLY BIG industrial "power factor corrector" cap with the bench DC supply and then short it with a fine wire which would then vaporize with a bright flash and a loud BANG! I don't recall the voltage or the size of the wire or cap, but I do recall that he used a pretty long set of welding cables to keep a decente distance from the pop ... about like na M-80. Needless to say lots of caution if you try this method! ... not least: make sure The Boss is nowhere near.


^^;;^^
DD


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Thu 28, 2015 9:17 pm 
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Quote:
In high school electronics class the teacher had a very big capacitor stashed away. It was the size of two car batteries and then he would charge it up over night. In class he would show us why we should be careful with caps and then drop a chunk of iron across the terminals.


I translated that to mean that the shop teacher was secretly like a lot of us and likes to see spectacular pyrotechnics and used the "safety" lesson as an excuse to discharge a huge capacitor we'll probably never even see. The only time I've had something even close to this was when I disconnected 2 large deep cycle batteries and accidentally shorted the terminals with the wrench. It burned one side of the wrench off and the lead terminal on one melted almost completely.


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Fri 29, 2015 2:02 am 
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Location: Powell River BC Canada
Ed Jacobs wrote:
In high school electronics class the teacher had a very big capacitor stashed away. It was the size of two car batteries and then he would charge it up over night. In class he would show us why we should be careful with caps and then drop a chunk of iron across the terminals. It was like a gun shot and would burn off a nice chunk from the end and shorten it some. He had saved the first big screw driver he used for that trick and there wasn't a hint of the tip end left, just a square rod with a plastic handle.
Ed



Years ago, I used a similar technique. Two things happened on two occasions.

Someone in another classroom thought it was a gunshot.

On of my students said he lost his voice for a while , in the class, but he didn't know why.

Which is why that demo wont be done by me any more.

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de
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Steve Dow
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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Fri 29, 2015 3:09 am 
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All fine methods--just be sure to use PPE (including helmet and cup)

Norm

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Fri 29, 2015 4:34 am 
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I don't like direct shorting the cap. Besides the risk of damage, it scares me! I don't like *pops*
I suggest using a resistor. If the circuit is drawing current, the caps will discharge in only a few minutes.
It's best to test before touching no matter what.

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: May Fri 29, 2015 2:23 pm 
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Location: Charleston, W.Va.
I am sure many of us remember a standard practical joke in auto repair shops in the old days when cars used conventional distributors with points and condenser. Charge up the condenser, yell at a buddy a few feet away and toss it to him. Human nature, even if you know better, is to try to catch something being tossed to you. The result of catching the condenser is obvious! Of course this only worked with "new guys"; after having it happen once or twice you quickly learned NEVER to catch anything being tossed to you.

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