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Posted by Jeremy M Smith (Member # 4913) on 07-30-2010, 04:32 AM:

I got two bloody good shocks off one of my projectors while changing a bulb and yes it was off so must of been the Capacitor.

Has anyone had this happen to them? ive been pretty worn out since is it big enough to do any damage?

Posted by Monte L Fullmer (Member # 2797) on 07-30-2010, 05:03 AM:

...lol..you got bit since the cap on the recifier didn't get discharged quick enough. That's 400VDC that you get bit with from a fully charged cap.

Next time, take a screwdriver, a long handled one, and touch the cathode cable and then to chassis ground with that screwdriver shaft. You can also do this with a good hank of 14G solid copper wire long enough for one end to tough that cathode cable and the other end to touch chassis ground.

You'll see a fat blue spark jump from the screwdriver shaft (Or wire end) to ground.

That discharges the cap so you won't get bit again ...

-Monte

Posted by Jeremy M Smith (Member # 4913) on 07-30-2010, 05:07 AM:

thanks Monte I will do that next time got someone else to do it after the second shock but I had discharged it by then lol lesson learned

Posted by Marin Zorica (Member # 1716) on 07-30-2010, 05:10 AM:

I did get shock even from ignitor, but that's nothing compared to DC from rectifier [Razz]

One tip, don't discharge cap with short circuit if isn't necessary, better is to use 10Ohm or so resistor and short DC with it, first is better for cap, and second it won't spark new screwdriver, as I did melt couple of them doing short over some wires [beer]

Posted by Ian Parfrey (Member # 5122) on 07-30-2010, 05:32 AM:

I hope you're going to show us some photos of your cooked screwdrivers, Marin [Big Grin]

I have heard that discharge thru a resistor is better, but I'm also guilty of doing the screwdriver trick as well. It livens a boring shift and keeps one entertained...

...but for some giggles, throw a mouse or other vermin type critter on those cap terminals and enjoy the show.

Posted by Joe Redifer (Member # 3) on 07-30-2010, 10:16 AM:

I had this happen once. It was enough to startle me, but not enough to hurt.

Posted by Monte L Fullmer (Member # 2797) on 07-30-2010, 12:44 PM:

..course, who has a 100 Ohm resistor at their disposal?

I do believe that it's something that one isn't going to do all the time when changing out bulbs, is to do a bulb change shortly after it's been on - it was just one of those things that just happened.

But, when one needs access to the bulb area after the rectifier was on, yet no bulb strike, this trick is the quickest safety valve to prevent injury.

Posted by Tony Bandiera Jr (Member # 2365) on 07-30-2010, 01:31 PM:

Were these Strong lamphouses with the DC ignitors? I hate those bastards, I got bit good off one even after I discharged it. (I didn't hold the short long enough to fully bleed it down.)

Most rectifiers I have run into bleed down on their own fairly quickly, so unless you dig in right after a shutdown, you shouldn't be getting hit from the rectifier itself. There are always exceptions though.

Another good precaution for bulb changes,is to use the volt meter function on your lamphouse (if it has one) to take a peek first.

Monte, what rectifier are you using that charges to 400 volts DC? The highest no-load I have ever seen on a rectifier cap (measured at the cap is around 200...majority read around 165-180.)

Posted by John Hawkinson (Member # 1135) on 07-30-2010, 02:02 PM:

"..course, who has a 100 Ohm resistor at their disposal? "

THAT IS A VERY BAD ATTITUDE.

Umm, who doesn't? They cost what, 1 cent each? Personally I tend to use an incandescent light bulb (say, 60W) in a socket with two clipleads. The bulb lights up and dims and you know you're safe.

There is no substitute for safety. If you don't look at the voltmeter and worry about the voltage before you mess around inside the lamphouse, you are being unsafe, and could do serious damage to yourself.

Most lamphouses or rectifiers should between them have a bleeder resistor, so the voltage should drain on its own. But it's not always true. So it's always better to check.

Better safe than sorry.

Better safe than dead.

--jhawk

Posted by Monte L Fullmer (Member # 2797) on 07-30-2010, 02:48 PM:

quote: John Hawkinson
I tend to use an incandescent light bulb (say, 60W) in a socket with two clipleads. The bulb lights up and dims and you know you're safe.

Now, that's a tasty tip to follow since the filament in the bulb doe act like a resistor ..gee, thx for the info.

Just that some of us think of the obvious and "old fashioned" methods - a screwdriver shaft across the terminals of a cap to discharge it since we're not in the practice of making a daily habit of discharging caps.

Sorry, got the 400VDC in my mind of larger caps..

Have a great weekend - Monte

Posted by Louis Bornwasser (Member # 3063) on 07-30-2010, 04:58 PM:

The discharge rod in broadcast transmitters is about a #6 with a wooden handle and a LARGE "J" hook. Just hang it on the circuit on which you are working. Totally safe.

400 volts? must be a malfunctioning Strong switcher?? (Normal rectifiers are probably not over 115 volts dc. Louis

Posted by Rick Raskin (Member # 1561) on 07-30-2010, 05:29 PM:

Roger jhawk on the light bulb method. There may still be some residual charge after the bulb dims, so leave it on for a minute or so to be sure its clear. Also it pays to verify with a voltmeter.

Unbled power supplies [Mad]

. I remember an HF transmitter I worked on in the Army. Unbled 5KV supply. When you opened the interlock door a shorting bar would fall over the capacitor bank. BAM

Posted by Joe Elliott (Member # 4006) on 07-31-2010, 02:09 PM:

I remember working with my dad one summer, after this third time retiring, he was working as a maintenance man at an apartment complex. He had a shelf of large (2 foot tall) capacitors that were used for the rooftop AC units. He reached across the shelf to grab something, and one of the caps bit him. Jolted him hard enough to launch him over a table and against the wall on the other side (he was crouching, so the muscles tensing helped his launch). Being the calm collected guy he was, he simply stood up, dusted himself off, and said "and that's why you need to discharge capacitors before storing them.". He then walked out of the shop, and I am sure quite a display took place once he was out of my sight.

Posted by Monte L Fullmer (Member # 2797) on 07-31-2010, 03:43 PM:

..and to be mean...while I was in college living in an off-campus apartment, there were 4 of us in this one apartment and one was an electronics geek with a mean sense of humor.

He charged up, I think it was a 400uF/200V cap, and just to get me up so I'd be in time for my class. touched that cap across the calf of my leg while I was asleep...."DANG!" That was worse than getting bit by a 7 cell cattle prod!

I carried those burn marks for quite the time...

Posted by John Hawkinson (Member # 1135) on 07-31-2010, 03:58 PM:

"and that's why you need to discharge capacitors before storing them."

Just so we're clear here, the real lesson is that you tie a wire across both terminals (shorting wire / shorting bar) on capacitors before storing them.

Because believe it or not, a cap sitting on a shelf can build up a nasty charge all by itself. And of course, it's key in these things to never trust the guy who put them away anyhow. Because you never know.

--jhawk

Posted by Stephen Furley (Member # 1277) on 07-31-2010, 04:15 PM:

Part 1

Part 2

Made in the '60s; used to be shown by the BBC as a trade test colour film. When regular colour transmissions started here in 1967 there were no programmes for mush of the day, just a test card, so the BBC started running films from various industrial film units, Shel, BP, Naional Coal Board, Post office etc. Itwas a cheap source of colour material for testing and demonstrating the new colour television sets. I used to have prints of some of them, but not this one, which was made for he United Kingdom atomic Energy Authority.

Posted by Jim Cassedy (Member # 4115) on 08-02-2010, 11:02 AM:

I've been either smart or lucky enough not to have ever
gotten a cap-jolt when changing a bulb. Perhaps, like some
others here, I've had past experience working with large
transmitters and know the importance of making sure all
large caps are discharged before going anywhere near them.

But I HAVE gotton a 110V jolt from the igniter input
terminals on a Xenix II, which are apparantly still "live"
since they're fed by a separate circuit.
(At least they are in the booths at the theater I'm at)

I thought I was "safe" because I'd pulled the main switch
to the rectifier and discharged the caps, but there was
still a live 110V input to the igniter circuit board in
the back of the Xenix II. My hand came in contact with
something on the board while I was reaching behind the
mirror to to something-or-other before changing the
bulb. (And before I'd put on my safety gloves) Ouch!

Posted by James Westbrook (Member # 3690) on 08-02-2010, 03:09 PM:

I was replacing a triac on a Rentec dimmer and I thought I had ALL the power off - and didn't -
Something good came out of this:
I have been experiencing shoulder pain, like arthritic, which none of the over-the-counter pain relievers could ease.
The electrical shock helped my shoulder pain tremendously. [Smile]

Posted by Monte L Fullmer (Member # 2797) on 08-02-2010, 07:29 PM:

..and how many of us use the "lock-out/tag-out" system?

(I like that SIDE procedure...)