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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
Posted to rec.crafts.metalworking,sci.electronics.design
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Safely testing 22 kV capacitors
Ignoramus27088 wrote:
Got myself some capacitors for $10 apiece. http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7603382621 The specs are 22 kV, 1 uF, discharge capacitor. See http://igor.chudov.com/tmp/cap.jpg The seller was wrong in describing them as 22 VOLT capacitors. They are 22 KILO volt capacitors. (the seller is Fermilab). They are similar to this one: http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7598631638 but have twice more capacitance. Anyway, here is my question. I have a 9 kV DC power supply. (a Franceformer). How can I safely test these caps before selling them. At 22 kV, they can store about as much energy as a .22 bullet, according to my calculations. It would be less at 9 kV, but still, obviously, very deadly. So. What is a safe way to charge them, verify that they hold the charge, and then DIScharge them at 9 kV. i Why hasn't anyone mentioned yet that if you DO get a HV probe for your meter it will serve as a resistor to discharge those caps. It will take a while, but you can observe the progress on the meter it's hooked up to and when the voltage gets down to something that won't jump out and grab you, an easier to obtain lower voltage rated resistor can be used to pull them the rest of the way down quickly. MAKE SURE you watch out for "dielectric adsorption" which is the property of some capacitors to "recharge themselves" when left open circuited after you've discharged them. Needless to say, that effect can make for some nasty suprises. To play it safe, I'd leave a shorting link or low value resistor connected across them after they're discharged. Know anyone with a small Van DeGraf generator you could borrow? One of those could probably charge up those caps slowly enough for you to watch the voltage rise on the meter, and you could turn it off when you got to 20 KV. Just my .02, Jeff -- Jeffry Wisnia (W1BSV + Brass Rat '57 EE) "Truth exists; only falsehood has to be invented." |
#2
Posted to rec.crafts.metalworking,sci.electronics.design
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Safely testing 22 kV capacitors
Jeff Wisnia wrote:
Ignoramus27088 wrote: Got myself some capacitors for $10 apiece. http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7603382621 The specs are 22 kV, 1 uF, discharge capacitor. See http://igor.chudov.com/tmp/cap.jpg .... Anyway, here is my question. I have a 9 kV DC power supply. (a Franceformer). How can I safely test these caps before selling them. At 22 kV, they can store about as much energy as a .22 bullet, according to my calculations. .... So. What is a safe way to charge them, verify that they hold the charge, and then DIScharge them at 9 kV. .... Why hasn't anyone mentioned yet that if you DO get a HV probe for your meter it will serve as a resistor to discharge those caps. I agree with Jeff that using the HV probe is a safe way discharge the caps. Also, you probably should buy one on ebay, since that will give you a probe with a proper high-voltage-safe housing and high-voltage- insulated wire. If you are feeling adventurous and have some high voltage wire on hand, http://www.repairfaq.org/sam/hvprobe.htm has directions for making a probe, using a 200-megohm "bleeder resistor from a defunct video terminal". (Although in newer monitors the bleeder may be built into the flyback assembly and harder to use.) It will take a while, but you can observe the progress on the meter it's hooked up to and when the voltage gets down to something that won't jump out and grab you, an easier to obtain lower voltage rated resistor can be used to pull them the rest of the way down quickly. How long "a while" is depends directly on the probe's resistance. For example, if it is fairly low, like 200 megohms, the time constant RC is 2*10^8*10^-6, or 200 seconds. Voltage goes as V0 * e^(-t/RC). At time t+RC, you will have 37% as much voltage as at time t. If you start at V0 = 10000V, it will take 4.6 RC, or about 15 minutes, for the voltage to drop below 100V. If the probe's resistance is reasonably high, ie 1000 to 5000 megohms, the same drop would take 75 to 375 minutes - assuming the capacitor has no leakage. Finding out whether (or how badly) each capacitor leaks is probably the most important thing to test. If you measure the voltage, then detach the meter, reattach exactly a minute later and remeasure, you can figure out the capacitor's leakage resistance via the equation above: R = t / (C*(log V0 - log V1)). (base e logs) MAKE SURE you watch out for "dielectric adsorption" which is the property of some capacitors to "recharge themselves" when left open circuited after you've discharged them. Needless to say, that effect can make for some nasty suprises. To play it safe, I'd leave a shorting link or low value resistor connected across them after they're discharged. Know anyone with a small Van DeGraf generator you could borrow? One of those could probably charge up those caps slowly enough for you to watch the voltage rise on the meter, and you could turn it off when you got to 20 KV. .... I think you should read the high-voltage probe page mentioned above, and also http://www.angelfire.com/electronic/...store_cap.html which has a number of comments about testing capacitors. Also, I don't think you should start testing at 9kV all that quickly; ie, test at low voltages first to get your test setup in order, and to weed out any of the caps that are too bad to test at HV. AIUI, a Franceformer is a neon-sign transformer that delivers AC rather than DC; if so you will need a rectifier. Correct? If you use line voltage or some other kind of transformer, such as an oil-furnace igniter, you'll need to use a high-voltage resistor in series with it and your rectifier to limit the inflow current when charging a cap. A neon sign transformer probably doesn't need such a resistor. To avoid damaging terminals with sparks, you might want to attach some 6" wires to each cap, with hooks or loops formed on the wire ends to let you hang a test-leads carrier (fiberglass strip at end of leads) or a shorting bar from a distance. -jiw |
#3
Posted to rec.crafts.metalworking,sci.electronics.design
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Safely testing 22 kV capacitors
Jeff Wisnia wrote:
Know anyone with a small Van DeGraf generator you could borrow? One of those could probably charge up those caps slowly enough for you to watch the voltage rise on the meter, and you could turn it off when you got to 20 KV. Big Van DeGraf machines produce microamps! Charging a uF to 20,000 V at one uA will take 20,000 seconds. And, that's one of those 6 foot tall jobs with a 6" wide belt. Jon |
#4
Posted to rec.crafts.metalworking,sci.electronics.design
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Safely testing 22 kV capacitors
On Thu, 30 Mar 2006 16:19:47 -0500, Jeff Wisnia
Gave us: Why hasn't anyone mentioned yet that if you DO get a HV probe for your meter it will serve as a resistor to discharge those caps. If you were observant enough of the thread, you would find that even the original poster mentioned the HV probe loading. |
#5
Posted to rec.crafts.metalworking,sci.electronics.design
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Safely testing 22 kV capacitors
Roy L. Fuchs wrote:
On Thu, 30 Mar 2006 16:19:47 -0500, Jeff Wisnia Gave us: Why hasn't anyone mentioned yet that if you DO get a HV probe for your meter it will serve as a resistor to discharge those caps. If you were observant enough of the thread, you would find that even the original poster mentioned the HV probe loading. I guess my powers of observation are failing me Roy, but I always like to check out where I missed something and try and figure out why it escaped me. Would you kindly point out where the OP revealed that he realized a HV probe would serve as a safe discharging means, albeit a rather slow one? (Before I mentioned it that is.) Thanks, Jeff -- Jeffry Wisnia (W1BSV + Brass Rat '57 EE) "Truth exists; only falsehood has to be invented." |
#6
Posted to rec.crafts.metalworking,sci.electronics.design
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Safely testing 22 kV capacitors
Jon Elson wrote:
Jeff Wisnia wrote: Know anyone with a small Van DeGraf generator you could borrow? One of those could probably charge up those caps slowly enough for you to watch the voltage rise on the meter, and you could turn it off when you got to 20 KV. Big Van DeGraf machines produce microamps! Charging a uF to 20,000 V at one uA will take 20,000 seconds. And, that's one of those 6 foot tall jobs with a 6" wide belt. Jon Live and learn. I was remembering the ones I played with years ago which IIRC put out more like 20 ua, but I do agree that even that much current would make for an uncomfortably long wait. G I remember having to test the radiation hardness of some satellite equipment I designed. We went to a GE facility in Pennsylvania where they had a "flash x-ray" generator. It used a BMF horizontally oriented Van De Graf IIRC about 25 feet long and two feet in diameter which took many minutes of running to build up the requisite high voltage in a storage capacitor. The charge was flashed through an x-ray tube to produce a humungous blast of x-ray photons into my poor little circuits. Thanks for the mammaries, Jeff -- Jeffry Wisnia (W1BSV + Brass Rat '57 EE) "Truth exists; only falsehood has to be invented." |
#7
Posted to rec.crafts.metalworking,sci.electronics.design
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Safely testing 22 kV capacitors
On Thu, 30 Mar 2006 21:48:08 -0600, Jon Elson wrote:
Jeff Wisnia wrote: Know anyone with a small Van DeGraf generator you could borrow? One of those could probably charge up those caps slowly enough for you to watch the voltage rise on the meter, and you could turn it off when you got to 20 KV. Big Van DeGraf machines produce microamps! Charging a uF to 20,000 V at one uA will take 20,000 seconds. And, that's one of those 6 foot tall jobs with a 6" wide belt. How about a Wimshurst machine? http://www.google.com/search?q=wimshurst But wtih any kind of static generator, you'd need some kind of voltage limiting, wouldn't you? It wouldn't do to destroy the caps while trying to test them! Is there such a thing as a calibrated spark gap? Maybe a shunt regulator from an old, old TV - or the whole HV section, but again, it'd have to be calibrated somehow. Cheers! Rich |
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