Film-Tech Forum ARCHIVE: Safely Discharging Capacitors on a Scrapped Rectifier

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Posted by Shane Hoffmann (Member # 1620) on 06-27-2007, 02:29 PM:

I have a 40+ year old 3 phase Strong rectifier/contactor (the bulky green box with the three dials on the front). Yesterday, we disconnected it and installed a new power supply.

I would like to disassemble it (firstly, to see if there is any valuable copper to sell, and secondly, because it's too heavy to move as is).

There are no longer any wires attached, but the two large capacitors next to the transformers are still holding a charge. Is there a safe way for me to discharge them, or should I wait until we have an electrician around?

Thanks.

Posted by Dave Macaulay (Member # 813) on 06-27-2007, 03:20 PM:

There's a bunch of copper in the transformers and chokes but the last time I tried the salvage yard guy wanted the insulating lacquer removed before he'd buy the copper - he'd take it as is but for nothing as he had to pay his guys to do the stripping. His suggestion was to hacksaw the coils to pull the wire out then throw the result on a fire to burn the insulation off. I didn't bother.
The caps will discharge if you just short the cap terminals or the main DC output lugs with something metal. There may be a nice spark. I would hold the tip of a large screwdriver against one terminal/lug and bring the shaft in to touch the other, if there's arc damage to the tool it'll be on the shaft rather than the working tip.

Posted by Rick Raskin (Member # 1561) on 06-27-2007, 03:22 PM:

The safest way to discharge a capacitor bank is gradually through a controlled load so that you limit the current. Unless you are familiar with how to do that, I recommend you wait for the electrician.

Posted by Bruce Hansen (Member # 281) on 06-27-2007, 04:43 PM:

The caps will self discharge over a short time, but to be sure, I have always used the screw driver trick. Be careful not to touch anything else with the screw driver while you are discharging the caps.

Posted by John Hawkinson (Member # 1135) on 06-27-2007, 04:59 PM:

Using a screwdriver isn't safe for you, the cap, or the screwdriver. The right answer is to connect a sufficiently large resistance across the terminals to bleed off the charge. Find someone who knows how to do it safely. Watch your resistor power dissapation limits, too.

--jhawk

Posted by Mark Gulbrandsen (Member # 72) on 06-27-2007, 07:20 PM:

quote: Bruce Hansen
The caps will self discharge over a short time, but to be sure, I have always used the screw driver trick.

Don't ever assume the first under any circumstance! Some low leakage electrolytic capacitors can retain a good part of their charge for weeks. Strong Switchers are a prime exapmple of this. Some of the older models had the bleeder resistors across the main capacitors left out of the design! I put em in when ever I get one of those units in the shop for repairs.

Discharging a cap with a screw driver or dead short is bad for the caps internal dielectric surface... discharging by short circuit can break down and even puncture the dielectric which is sometimes just a very thin oxide layer. Its also bad for the terminals and screw driver tip. There are amplifiers out there that have in excess of 100,000 mfd and close to 100 volts DC or more. Thats alot of Joules! . I have seen amplifier service manuals in the past that state not to discharge the main capacitor bank though a short circuit because it will vaporize what ever you use to short them out [Eek!]

. Indeed you are working with what is the equivelent of a defibrulator except that it is more likely to stop your heart than start it! The best way to discharge them is through a one thousand ohm 10 watt wirewound resistor and appropriate clip leads. Monitor the caps terminals with a DC voltmeter to be sure its gone down to zero.

Mark

Posted by Shane Hoffmann (Member # 1620) on 06-27-2007, 08:26 PM:

thanks for the advice everyone.

Mark, where can I find one of those resistors? Would RadioShack have one, or is it something I need to order?

Posted by Mark Gulbrandsen (Member # 72) on 06-27-2007, 10:50 PM:

Rat Shack might still have one in those cardboard parts drawers they still have around. Otherwise any local electronics parts place should have something close to that.

Mark

Posted by Phil Hill (Member # 371) on 06-27-2007, 11:02 PM:

I agree with the others that say DO NOT do a direct short with a screwdriver, etc. Not only may this pit and damage the tool, the cap my explode from the stress of a high-current discharge.

If you cannot find the proper power resistor for discharging the cap, a clothes flat iron or waffle iron will work just fine.

Following Mark's suggestion using (heavy) clip leads, connect the appliance across the cap and turn the appliance on and that will drain the cap. Use a voltmeter to check the progress.

And remember, depending on the type of cap, it may have a memory and "resurrect" itself after the initial discharging and get a another charge.

Posted by Stephen Furley (Member # 1277) on 06-28-2007, 03:40 AM:

quote: Dave Macaulay
His suggestion was to hacksaw the coils to pull the wire out then throw the result on a fire to burn the insulation off.

I wouldn't really recommend this either; some insulating varnishes can emit highly toxic fumes when burned; burning them off in uncontrolled conditions in an open fire isn't a terribly good idea, though common practice in the past.

Posted by Chris Slycord (Member # 4239) on 06-28-2007, 03:51 AM:

quote:
Indeed you are working with what is the equivelent of a defibrulator except that it is more likely to stop your heart than start it!

I know I'm being a little pedantic but a defibrillator does, in fact, stop your heart. When your heart gets into an unusual and/or weak rhythm a defibrillator will be used to stop the heart in the hopes that the normal pacemaker (sinoatrial node if memory serves me right) will take over and send the heart into a normal rhythm.

I'm happy to say that wikipedia confirmed my memory.

Posted by Mark Gulbrandsen (Member # 72) on 06-28-2007, 08:28 AM:

Chris,

You mean to tell me all those instances on ER where the patient went "flat line" and the drfibrillator was used to re-start the heart were not authentic?

Mark

Posted by Rick Raskin (Member # 1561) on 06-28-2007, 09:56 AM:

Assuming you want to proceed with discharging the caps, you could use a light bulb instead of a resistor. That is assuming the charged voltage does not exceed 120V. If so, you could put bulbs in series to accomodate the increased voltage. The lamps will dim out as the capacitors discharge.

Posted by Dave Macaulay (Member # 813) on 06-28-2007, 02:58 PM:

You guys are too much.
He's scrapping the rectifier - so what if the caps are damaged? Yes, they could explode... a deorbiting satellite could crash through his roof, lightning could strike the phone line and electrocute him while checking here for replies, Paris Hilton could drop by and blow him. All about the same probability.
This is an ancient brute force rectifier.
- The caps are probably about 25% of their labeled value.
- Unless someone turned the thing on and powered the contactors without starting a lamp, they only have about 25 volts maximum, and they've been self-discharging for how long?
- Most Strongs have discharge caps built in, I don't know about the antiques though.
Just use a screwdriver, it's probably got zero charge left anyhow. And you might see a nice spark!

Posted by Rick Raskin (Member # 1561) on 06-28-2007, 03:21 PM:

Yeah, we probably are to much. Safety first!

Posted by Chris Slycord (Member # 4239) on 06-28-2007, 03:22 PM:

quote: Mark Gulbrandsen
You mean to tell me all those instances on ER where the patient went "flat line" and the drfibrillator was used to re-start the heart were not authentic?

Depends on what they show you.

A defibrillator, ON IT'S OWN, can't save a flat line patient. In that case, they'll use a combination of CPR and cardiac stimulant drugs. Then, they get it to where they have one of the "shockable rhythms" (ventricular fibrillation or pulseless ventricular tachycardia). Though it's also possible that someone who is flat line actually has a tiny amount of electrical activity.

The defibrillator sends out electrical energy that depolarizes the beating section of the heart which stops the arrhythmia and then your SA node (pacemaker) hopefully starts up a normal rhythm.

EDIT: In fact, in both of those "shockable" cases the heart is still in activity and might even be beating (they're just in patterns that if left alone will have a major problem for the patient).

Posted by Phil Hill (Member # 371) on 06-30-2007, 07:42 PM:

So CB Shane, what's the final disposition of the PS?

Posted by Kyle Anderson (Member # 3504) on 06-30-2007, 08:57 PM:

Great way to kill yourself [Smile]

Posted by Shane Hoffmann (Member # 1620) on 06-30-2007, 09:33 PM:

I've been giving them a little extra time to discharge somewhat on their own...I'll probably give it a shot sometime during this next week when we're dead and I've got nothing else to do. I haven't decided if I want to go for it with a screwdriver or try the light bulb method. The angle I will have to go at them from is a bit awkward, which is the cause of any remaining apprehension. As far as not having much charge-- my little handheld voltage detector beeps when I put it on the terminals. I don't know what the threshold is for those things to begin registering voltage, but I know when it's minimal it doesn't beep at all...

Posted by John Hawkinson (Member # 1135) on 07-01-2007, 10:32 AM:

There is no guarantee that leaving them sitting there will discharge them. It may very well have no effect all (or worse).

You should measure the voltage with a DC voltmeter.

As for awkward angles, well, with two aligator clipsleads, you should be able to solve any such problem. (It's also good practice put a cliplead across the terminals after you have discharged them.)

If you've waited this long, take people's advice and don't use a screwdriver. I'm not so sure a lightbulb is a great plan, but I suppose if you burn out the lightbulb it's no big deal.

--jhawk

Posted by Michael Schaffer (Member # 1204) on 07-01-2007, 10:54 AM:

They should also slowly discharge when you simple leave the multimeter hooked up to it for a while. If you don't have one to check what's going on, you shouldn't attempt that nayway.

Posted by Mark Gulbrandsen (Member # 72) on 07-01-2007, 11:20 AM:

quote: John Hawkinson
I'm not so sure a lightbulb is a great plan

A light bilb is actually a fairly safe way to discharge them but is certainly not the prefered way. A light bulb is a little large and fragile to carry around in the tool kit.

quote: Michael Schaffer
They should also slowly discharge when you simple leave the multimeter hooked up to it for a while.

With most decent DVM's being greater than 10 megohm input it could literally take weeks to discharge them...assuming they are fully charged. Don't necessarily rely on a meter to do that job.

Mark

Posted by Michael Schaffer (Member # 1204) on 07-01-2007, 11:43 AM:

What's the typical capacitance of those capacitors?

Posted by Steve Guttag (Member # 268) on 07-01-2007, 12:47 PM:

Christie used 6000uF for the longest time...Strong was normally a bit higher and went to using two devices that when coupled were in excess of 6000uF...I'd have to check to see what they are. Suffice to say, they are not small

The screwdriver method is plain stupid...and can often damage the screwdriver too...depending on the charge in the cap at the time. Since the voltage is likely to be low, the wattage of the cap need not be particularly large...a few watts will do it since even if it was at full charage and the lamp didn't strike (125VDC on the high side)...it will quickly drop and with that the power dissapation the resistor must contend with. As for a light bulb it is a good one to use and most booths have them there somewhere...if on a trouble shooting type lamp...then you merely need some clips connect the troubleshooting lamp (Reel lamp)'s power cord to the cap.

Steve

Posted by Louis Bornwasser (Member # 3063) on 07-01-2007, 05:56 PM:

On large transmitters we used a shorting rod which was permanently grounded before touching any circuit. B+ was 7000 volts. I use a clip lead attached to the sacrificial screwdriver to create a mini shorting rod with good results in this industry.

Amplifying Steve: If the last time you powered up actually lit the bulb, the capacitors have 30 volts dc or less; up to 125 volts if no bulb lit. Louis

Posted by Stephen Furley (Member # 1277) on 07-01-2007, 06:43 PM:

quote: Louis Bornwasser
On large transmitters we used a shorting rod which was permanently grounded before touching any circuit. B+ was 7000 volts.

In the 60s, during hours when normal programmes weren't being transmitted the BBC wanted some sort of material which could be used for testing and demonstration of the new colour television sets. The answer they came up with was a collection of 'trade test colour films' still fondly remembered by many. These films came from various sources, 'It's the tube that makes the colour', about the making of shadow mask picture tubes was made by Mullard, who may have been owned by Philips by that time, I'm not sure. There were a a number of films by the Shell and BP oil companies, including 'Algerian Pipeline' and one of my favorites, 'The Home-Made Car. I have prints of a few of these films, including one about plastics, and 'We've Come a Long Way, about the design of oil tankers.

One of these films was about safety systems and procedures for working on high-Voltage equipment. I can't remember who it was made by, but the title was 'On the Safe SIDE'

Switch off
Isolate
Dump
Earth

In the film one of the characters has to make some changes to some high-Voltage experimental equipment. He follows all of the correct procedures, but unknown to him, somebody else has made an error in some previous modifications, and the coil he is about to touch is still live. At the last moment he says to himself 'I suppose that Earth is alright' and the camera zooms in on the Earthing hook. The man takes the hook off the point where it was attached, and which would have been directly connected to the coil if not for the previous error. He touches it directly onto the coil, with predicctable results, but at least he survives.

Does anybody remember this film?

Posted by Frank Angel (Member # 248) on 07-02-2007, 04:19 AM:

Shane, bring your girlfriend up into the booth -- cautiously tell her to stand back and explain how your are going to demonstrate the incredible power of the equipment you work with and how it stores huge amounts of electrical energy. Now take out a handful of steel wool, and while again cautioning her to stand back, dramatically drop the wad on the cap terminals. It will vaporize in a ball of electrical fireworks. She will ghasp. Reassure her everything is alright and that you have everything under control. She will think you are Mr. Wizard. You might even get laid. [thumbsup]

And if not, at least you will have discharged your caps. [Big Grin]

Posted by Mark Gulbrandsen (Member # 72) on 07-02-2007, 08:28 AM:

quote: Louis Bornwasser
On large transmitters we used a shorting rod which was permanently grounded before touching any circuit. B was 7000 volts.

Exactly! The tranmitter I used to work on... a TTU-1000(100kw UHF) had 6 air cooled high voltage supplies (cubicles) that were tied in parallel and could generate a combined total of 20kv at 18 amp [Eek!]

. I used to go into each high voltage cubicle(6X6X6 feet in size) and clean them out with a brush and vacuum once a month.... sure they are discharged but you still get the oogies while you're in there. Dust and high voltage don't get along! BTW: The transmitter had a permanently installed grounding rod system and they looked like gates, I believe there were three of them. Each had spring loaded mechanism to hold it grounded once you opened it. They had to be reset by hand to stay open. These were the main path to discharge all the H-V supplies before you could go into the back of the visual or aural cubicles to work. Most of the problems we had were just water leaks in the cooling circuit. This transmitter which went in in 1963 was so large it occupied 1/2 of the 94th floor in the John Hancock building.... The 2 heat exchangers for the klystron cooling circuits which were up on higher levels just below the roof were each the size of school busses.

Neat old stuff!

Mark

Posted by German Marin (Member # 918) on 07-02-2007, 11:52 AM:

I've discharged that capacitors a lot of times and frequently with a screw driver. When I don't want to see the big spark I've just used a big old wire (the one used for the lamp-rectifier) and touch the terminals without get sparks (just a little non-scared sparks)... if you think longer about this fact, the "fear" take your control and the panic won't let you do it.
Have you discharged the rectifier?
Saluti

Posted by Randy Stankey (Member # 64) on 07-02-2007, 12:49 PM:

Oh, quit being such a baby! Just lick it with your tongue!
That's what a real man would do!

[Wink]

Posted by William T. Parr (Member # 677) on 07-02-2007, 01:33 PM:

quote: Dave Macaulay
Paris Hilton could drop by and blow him.

If that happens.... Run out quick and buy a lotto ticket and a video camera. I hear she loves performing in those home porno's. [Wink]