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Author Topic: Safely Discharging Capacitors on a Scrapped Rectifier Shane Hoffmann Film Handler Posts: 68 From: Fond du Lac, WI, USA Registered: Feb 2003 posted 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.  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 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.  |  IP: Logged Rick Raskin Phenomenal Film Handler Posts: 1100 From: Manassas Virginia Registered: Jan 2003 posted 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.  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 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.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 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  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 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 . 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  |  IP: Logged Shane Hoffmann Film Handler Posts: 68 From: Fond du Lac, WI, USA Registered: Feb 2003 posted 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?  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 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  |  IP: Logged Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 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.  |  IP: Logged Stephen Furley Film God Posts: 3059 From: Coulsdon, Croydon, England Registered: May 2002 posted 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.  |  IP: Logged Chris Slycord Film God Posts: 2986 From: 퍼항시, 경상푹도, South Korea Registered: Mar 2007 posted 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.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 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  |  IP: Logged Rick Raskin Phenomenal Film Handler Posts: 1100 From: Manassas Virginia Registered: Jan 2003 posted 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.  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 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!  |  IP: Logged Rick Raskin Phenomenal Film Handler Posts: 1100 From: Manassas Virginia Registered: Jan 2003 posted 06-28-2007 03:21 PM                     Yeah, we probably are to much. Safety first!  |  IP: Logged

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