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How do you discharge a capacitor safely.
Thanks for looking.
How does one discharge a capacitor when working on a PCB, so that one does not get electrocuted? Are there any other components that maintain a charge after turning off power source. Thank you. please answer to . |
How do you discharge a capacitor safely.
On Mar 5, 9:14 pm, James Sweet wrote:
wrote: Thanks for looking. How does one discharge a capacitor when working on a PCB, so that one does not get electrocuted? Are there any other components that maintain a charge after turning off power source. Thank you. please answer to . Usually just short it with a screwdriver. CRTs can store a charge too. You may get a big spark from a large capacitor, don't jump when it happens, you can do as much damage to yourself from ripping a hand on nearby sheet metal as from the actual electrical charge. I know from experience over manyyears and a few seconds of carelessness. H. R. (Bob) Hofmann |
How do you discharge a capacitor safely.
On Mar 6, 6:37 am, wrote:
Thanks for looking. How does one discharge a capacitor when working on a PCB, so that one does not get electrocuted? Are there any other components that maintain a charge after turning off power source. Thank you. please answer to . the safe way to discharge capacitor are through resistor, use the resistor in minimum 10 watts with resistor 10megaohms for AC capaitor from 110v to 440v for DC from 5 to 220v capacitor us 5 watts and 150kilo-ohms above 220v DC use 10mega ohms resistor value, use two long lead with insulation and one end ground to chase or PCB ground, and other lead to discharge the capacitor. take more care when discharging more than 110v capacitor which will produced high sparking when discharging. |
How do you discharge a capacitor safely.
"THERES RAJ, BLR" wrote in message oups.com... On Mar 6, 6:37 am, wrote: Thanks for looking. How does one discharge a capacitor when working on a PCB, so that one does not get electrocuted? Are there any other components that maintain a charge after turning off power source. Thank you. please answer to . the safe way to discharge capacitor are through resistor, use the resistor in minimum 10 watts with resistor 10megaohms for AC capaitor from 110v to 440v for DC from 5 to 220v capacitor us 5 watts and 150kilo-ohms above 220v DC use 10mega ohms resistor value, use two long lead with insulation and one end ground to chase or PCB ground, and other lead to discharge the capacitor. take more care when discharging more than 110v capacitor which will produced high sparking when discharging. I wouldn't argue with using a resistor to discharge caps, and also, on occasion, just whacking a screwdriver across them - although this can leave a bit of a metal deposit 'splat' on the PCB. However, the suggested 10 meg 10 watt resistor, or even the 150k, seems rather high to me. I think that you might struggle to even find 10meg at 10 watts. I have a metal film resistor of 2k at probably 3 or 4 watts, that has lived for years on the bench screw-magnet, and serves to discharge any caps that I ever come across that remain charged due to fault conditions. That includes up to 500 volts in some PA amps. This resistor has never even got warm during a discharge. If you work out the math, yes, at first the resistor may be massively overloaded from a high voltage cap, but it is for such a short period of time that the resistor never has a chance to get hot, which is what its power rating is all about. A higher value resistor than this ( though not as high as 150k even ) will do a gentler job of it, with less of an initial spark, but will take correspondingly longer to do it. Note also that many high voltage cap circuits have bleeder resistors, or voltage sharing resistors across them when they are stacked, and these should discharge the caps for you. You should always check that they are doing their job though, as they do have a tendency to go open. You can also discharge caps safely with an older analogue multimeter, which has a much lower input resistance than its modern digital counterpart. This has the added advantage that you can see the voltage dropping, but is somewhat slower than using a resistor of just a few k. In general, the following circuitry should discharge the caps for you, and caps remaining charged will usually indicate a fault condition such as a switch mode power supply not starting up, and leaving the main primary filter cap charged to peak line volts, or a filter resistor between two caps open circuit, leaving the first one in line, charged. Arfa |
How do you discharge a capacitor safely.
On 5 Mar 2007 22:26:46 -0800, "THERES RAJ, BLR"
wrote: On Mar 6, 6:37 am, wrote: Thanks for looking. How does one discharge a capacitor when working on a PCB, so that one does not get electrocuted? Are there any other components that maintain a charge after turning off power source. Thank you. please answer to . the safe way to discharge capacitor are through resistor, use the resistor in minimum 10 watts with resistor 10megaohms for AC capaitor from 110v to 440v for DC from 5 to 220v capacitor us 5 watts and 150kilo-ohms above 220v DC use 10mega ohms resistor value, use two long lead with insulation and one end ground to chase or PCB ground, and other lead to discharge the capacitor. take more care when discharging more than 110v capacitor which will produced high sparking when discharging. Just how much wattage do you think you are going to get out of a capacitor? |
How do you discharge a capacitor safely.
writes:
Thanks for looking. How does one discharge a capacitor when working on a PCB, so that one does not get electrocuted? Are there any other components that maintain a charge after turning off power source. Thank you. please answer to . See the capacitor FAQ at the site below. http://www.repairfaq.org/sam/captest.htm --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
How do you discharge a capacitor safely.
James Sweet writes:
wrote: Thanks for looking. How does one discharge a capacitor when working on a PCB, so that one does not get electrocuted? Are there any other components that maintain a charge after turning off power source. Thank you. please answer to . Usually just short it with a screwdriver. CRTs can store a charge too. This is generally frowned upon both due to the pits it will produce in the screwdriver, the noise, and potential damage to some types of capacitors. See the info at the site below. http://www.repairfaq.org/sam/captest.htm --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
How do you discharge a capacitor safely.
Sam Goldwasser wrote:
James Sweet writes: wrote: Thanks for looking. How does one discharge a capacitor when working on a PCB, so that one does not get electrocuted? Are there any other components that maintain a charge after turning off power source. Thank you. please answer to . Usually just short it with a screwdriver. CRTs can store a charge too. This is generally frowned upon both due to the pits it will produce in the screwdriver, the noise, and potential damage to some types of capacitors. See the info at the site below. Rightly or wrongly, I use a 240volt BC pygmy lamp with two leads fitted with prods soldered to the pads on the bulb. Works for me. Ron(UK) -- Lune Valley Audio Public Address Systems Hire Sales Maintenance www.lunevalleyaudio.com |
How do you discharge a capacitor safely.
xcellent choice, lamp provides indication and nice slope for current rate.
"Ron(UK)" wrote in message ... Sam Goldwasser wrote: James Sweet writes: wrote: Thanks for looking. How does one discharge a capacitor when working on a PCB, so that one does not get electrocuted? Are there any other components that maintain a charge after turning off power source. Thank you. please answer to . Usually just short it with a screwdriver. CRTs can store a charge too. This is generally frowned upon both due to the pits it will produce in the screwdriver, the noise, and potential damage to some types of capacitors. See the info at the site below. Rightly or wrongly, I use a 240volt BC pygmy lamp with two leads fitted with prods soldered to the pads on the bulb. Works for me. Ron(UK) -- Lune Valley Audio Public Address Systems Hire Sales Maintenance www.lunevalleyaudio.com |
How do you discharge a capacitor safely.
In article .com,
"THERES RAJ, BLR" wrote: the safe way to discharge capacitor are through resistor, use the resistor in minimum 10 watts with resistor 10megaohms for AC capaitor from 110v to 440v According to my calcs, the maximum power needed for a 10M0 resistor is 0.01936 watts. -- *He who laughs last has just realised the joke. Dave Plowman London SW To e-mail, change noise into sound. |
How do you discharge a capacitor safely.
Actually I'd like to get my hands on some 240V incandescent bulbs just
for that purpose. I've seen some sets use as high as 940uF for the primary cap (2X470). This is a considerable amount of power. I used to have a 75W 130V (long life) bulb for the purpose until it got used. I actually put it in the manifest as parts to be ordered but it is not going to happen. When I see a 75 watt bulb take over a second to discharge it, what goes through my mind is, just how long would my body take to discharge it ? Now I see that they make the PC so that the rectifiers and jumpers can be used for 120 or 240, in the US, Canada and Japan at least, they double the line voltage. For Europe it appears they just rearrainge the jumpers so it is a simple full wave bridge. They just wind up using higher voltage lower current devices. As it is, I have to discharge each filter in the doubler seperately, Going across the whole 320 DC would burn it out. I think a 240V bulb would handle it, it is not 320 VDC for long. Actually the bulb is the best choice, but a lazy choice. It is available and cheap. I am totally against using a screwdriver to short out the caps, it no doubt exceeds the filters' ripple current capacity, evidenced by the metal deposits from the spark as noted by one poster. Ideally we could have a constant current source with a low wattage bulb to give a visual indication. A regular lightbulb is less desirable because it's resistance is low when cold, thus there is a surge. If that filament burns out right away when you connect it, you might erroneously think the cap(s) were discharged when they were not. Put your solder wick on the drain output of an SMPS IC that still has 320VDC (or even 160) on it and you'll wish you wore gloves (and maybe goggles too !). Certain Mits RPTVs and maybe some Hitachis come to mind. There are plenty of direct view sets also but I generally don't work on them, except for 32" and up. But I have seen where the main cap failed because there was no load on it. All bulged, but it wasn't a power surge. See some places the power is "dirty". It is essentially 120 VAC but there are spikes and all kind of ****, it is not a perfect sine wave to say the least. In some of these sets there is no bleedoff if the SMPS doesn't start. Therefore the spikes will take the rectified voltage quite higher than one would suspect. They generally don't use high speed rectifiers for the AC line, which means they have a certain effective series inductance. Well that is an impedance, and even though a one meg resistor might keep the voltage from climbing, it doesn't even have that. Also, if the rectifiers outperform their specs, you might have a bit more. Do not be surprised if a diod outperforms it's specs. I had a case not too long ago, another tech had a set blowing the top damper diode. It was a recall, but like an 8 month recall. Well, somehow at the time the top damper was replaced with a 400V diode, like a 243636 (MI35 I think) but the application clearly calls for a 1500V device. I didn't do it, I have no idea how it happened but the 400V diode worked for months. Eventually the set came to me. Once this was figured out the set and customer have been happy together ever since. That's why, I suspect, that sometimes I don't get many answers. I am the dogshooter. And folks I got some big ones. But I can see as one reads my post they think "Oh yeah, he needs to......" and in the next few lines you see I already did that. Then at the end of the post you might be thinking "Yup, this dude is really ****ed". That's my job, ****ed. The newest part of my job of ****ed is those Sonys, the DX1 type chassis, the XBR400s and like that. I have had to fix enough screwups on those sets that I am now the alpha and omega of the DX1 at the shop. And I said it. Right after the words came out of my mouth I got this jumping off a bridge sensation, and rightly so. But in the long run, when I don't have to follow someone else I'll learn alot more about those sets. I would really like to get some 240V bulbs, any ideas for a source ? My 120V bulb disappeared shortly after the light in the hall leading to a bathroom burnt out, and now I am thinking, if I had a 240V bulb, that wouldn't happen. JURB |
How do you discharge a capacitor safely.
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