Safe way to discharge a cap? - Page 1
Forum section: electronics / beginners

author: shane_95
author_rank: Regular Contributor
timestamp: July 10, 2012, 02:25:34 pm
content: What's a safe way to discharge a cap so i do some can work with out zapping myself? I'v done some googleing myself and I have been found that i need to put a load on it, what a good way to do so with out taking it of the PCB?The cap is 2200uf 25v, its charged with 14v

author: olsenn
author_rank: Frequent Contributor
timestamp: July 10, 2012, 02:36:46 pm
content: Connect both terminals of the cap through a resistor

author: digsys
author_rank: Unknown
timestamp: July 10, 2012, 02:43:44 pm
content: 14V is quite safe to touch, so you can use any resistor say from 100ohm to 10K or higher. Just bend the leads andtouch both terminals. 100ohm will take a sec or 2 max, 10K will take a few secs. If you want to watch, clip a voltmeteracross the resistor and watch the 14V drop (while the resistor is across the cap).

author: shane_95
author_rank: Regular Contributor
timestamp: July 10, 2012, 03:14:15 pm
content: now just to go through all my resistors and find out the right one

author: mariush
author_rank: Super Contributor
timestamp: July 10, 2012, 03:55:16 pm
content: It doesn't have to be an exact one ... anything over 10k ohm will do.Alternatively, you could use a computer fan - they run at 12v but they won't be damaged by the 14v in the cap as they'll drain it pretty fast. Or a 12v light bulb, like the ones used in motorbikes.

author: shane_95
author_rank: Regular Contributor
timestamp: July 10, 2012, 04:01:21 pm
content: yeah i discovered that about 10 min ago

author: SeanB
author_rank: Super Contributor
timestamp: July 10, 2012, 04:16:50 pm
content: I use a 56K 10W resistor, came as a few in a big bag-o-resistors-on-clearance, to discharge the capacitors I test on the insulation tester. I just hold it by the body and connect the leads across the terminals for around 10 seconds. It does not get warm at all, but be careful not to touch them. Just placing a screwdriver across the terminals tends to be bad for the screwdriver and the terminals, not to mention the hearts of people nearby from the bang.Normally i am testing motor run capacitors that are any value from 2u2 to 100uF, 400VAC or higher. I test them at 1000V for leakage and listen for internal arcing after doing a capacitance check on them. If you do not discharge them you hear the failing one crackling for minutes afterwards as they self heal.

author: G7PSK
author_rank: Super Contributor
timestamp: July 10, 2012, 05:26:36 pm
content: Just short it out with something metal like a piece of wire or an old screwdriver the cap wont come to any harm, some of the bigger capacitors may have a bleed resistor built in.

author: SeanB
author_rank: Super Contributor
timestamp: July 10, 2012, 06:06:25 pm
content: The screwdriver does tend to become smaller if you do this regularly.

author: codeboy2k
author_rank: Super Contributor
timestamp: July 10, 2012, 06:15:07 pm
content: Quote from: G7PSK on July 10, 2012, 05:26:36 pmI do this only on small caps, less than a 470uF and 50V. Although there is not enough energy in that cap to harm me, I still don't like letting the sparks out, so anything more than that gets a resistor for a few seconds.

author: Ferroto
author_rank: Frequent Contributor
timestamp: July 10, 2012, 06:22:45 pm
content: Personally I just use a screwdriver to short out the two leads but I'm far from the expert authority on safety.

author: T4P
author_rank: Super Contributor
timestamp: July 10, 2012, 06:32:01 pm
content: Quote from: mariush on July 10, 2012, 03:55:16 pmPretty sure most 12V fans can handle 14V no problem ...

author: David_AVD
author_rank: Super Contributor
timestamp: July 10, 2012, 10:02:38 pm
content: In the workshop, we have a 240V 25W incandescent lamp wired to a set of crocodile clips. This is often used to discharge capacitors in switch-mode power supplies - after disconnecting from the mains of course! And we measure the voltage on the cap afterwards just in case.Speaking of which, that's when you find capacitors sitting there with dangerous voltages on them. A 240V switch-mode PSU that fails to start can have up to 340V across the main filter capacitor.

author: notsob
author_rank: Frequent Contributor
timestamp: July 10, 2012, 10:21:01 pm
content: google it - I've even seen some with a resistor discharge network (with LEDs) so you can get a visual of the charge level.

author: Mint.
author_rank: Frequent Contributor
timestamp: July 10, 2012, 10:35:31 pm
content: Seems there are all sorts of fancy ways to discharge caps, and talk about the options - LED's, fans, resistors, lamps, shorting to create sparks...AWESOME

author: vk6zgo
author_rank: Super Contributor
timestamp: July 11, 2012, 03:58:02 am
content: Quote from: Mint. on July 10, 2012, 10:35:31 pmThe BIG caps with voltages like 14 volts will give you an impressive "ZAP",but are harmless to you.I've always tended to just short them,but nicety suggests you should discharge them more gracefully.With any caps with high voltages on them,I would not mess about trying to hold resistors or anything else across them,as you may get a very nasty surprise.You could make up a nice discharge stick with an insulated handle,series resistor & a clip lead to connect to a common point on the board or chassis which is hopefully one side of the cap,or just stick with a well insulated screwdriver,& short 'em!No doubt many people will throw their hands up in horror at this last comment,alleging that you will damage the capacitorMy feeling is that,if it's the capacitor or me,---------suffer, Capacitor!!PS: Be completely sure that you have removed the power from the circuit before you try any of the above!!!

author: Psi
author_rank: Super Contributor
timestamp: July 11, 2012, 04:05:09 am
content: Quote from: vk6zgo on July 11, 2012, 03:58:02 amUnless they're large farad supercaps, which will turn your metal screwdriver into an impromptu soldering iron and burn your fingers offQuote from: vk6zgo on July 11, 2012, 03:58:02 amNa, i agree.Most people just short them with a screwdriver. Unless it's something expensive or a REALLY HUGE CAP, there's no point wasting time discharging it slowly

author: David_AVD
author_rank: Super Contributor
timestamp: July 11, 2012, 05:23:53 am
content: That's why we like the light bulb. You usually see a brief flash from the lamp. And if it stays lit you know there's something wrong!

author: IanB
author_rank: Super Contributor
timestamp: July 11, 2012, 06:13:57 am
content: Quote from: Psi on July 11, 2012, 04:05:09 amWhen I was young and I found some 33,000 uF capacitors from a computer power supply I charged them up to 12 V and discharged them with a screwdriver. I ruined a screwdriver that way. Pretty much turned it into a pitted and deformed mess with the end missing.

author: Psi
author_rank: Super Contributor
timestamp: July 11, 2012, 07:08:14 am
content: Quote from: IanB on July 11, 2012, 06:13:57 amYeah, i'll rephrase... discharge them with your workmates screwdriver.

author: amyk
author_rank: Super Contributor
timestamp: July 11, 2012, 07:09:10 am
content: Quote from: IanB on July 11, 2012, 06:13:57 amThat's around 2.4J of energy, which may seem relatively small (contrast food calories - thousands of J) but when released in 1ms becomes 2.4kW of power. It makes a good demonstration of power vs energy.

author: Psi
author_rank: Super Contributor
timestamp: July 11, 2012, 07:11:45 am
content: Yeah, 30,000uF is starting to get a bit big, especially if you have a decent voltage there.

author: G7PSK
author_rank: Super Contributor
timestamp: July 11, 2012, 07:38:01 am
content: When I was about 12, I had an oil filled capacitor from the HT of a 1950's TV, not a very high capacity but it would charge itself just by holding it near a florescent lamp, and by that I mean just reaching up to the light on the ceiling with it in my hand, It would collect a high enough voltage to give you a shock, so the challenge to all my mates at school was to grab it by the brass studs at each end if you dare.

author: Rerouter
author_rank: Super Contributor
timestamp: July 11, 2012, 07:42:37 am
content: i use a 220 ohm resistor that i coiled up the leads and soldered so i have 2 wide pads (2mm diameter), i then grab the thing on the body with a pair of pliers and discharge them with that across the terminals,

author: eevblogfan
author_rank: Frequent Contributor
timestamp: July 11, 2012, 12:15:13 pm
content: heyI used to discharge my~30,000uF cap bank (400V rated - 300V charged ) via 600W lamp I've took apart from old HP Printer , just pair of cheap multimeter probs ( 18AWG ) .BTW , I've played with that bank [coil gun ! ] and I've shorted the leads BANG ! , that was aprox when I was 8 years old, and I've used proper gloves and clear shield etc , the charging PS was 555 timer +mosfet + 11.5V @ 5A spotlight transformer + bridg rectefier ,

author: Rerouter
author_rank: Super Contributor
timestamp: July 11, 2012, 12:18:10 pm
content: 30mF at 300V, that be some serious power for a short condition :|,

author: OndraSter
author_rank: Contributor
timestamp: July 11, 2012, 01:13:56 pm
content: All I can reply to that is thisWe have done that at our school. 820V on 2000uF capacitor shorted

author: G7PSK
author_rank: Super Contributor
timestamp: July 11, 2012, 02:00:07 pm
content: When Marconi built the trans Atlantic transmitter in Ireland he built a giant capacitor that was a large shed full of steel plates that were about 20 X 15 foot, hundreds of them they were charged to kilo volts any one walking into the shed would disappear like a fly in one of those electronic zappers, there is some video and photographs around of the construction of the station. All that remains now is a few concrete foundations for the generators and transmitter towers.

author: Jimmy the Squid
author_rank: Regular Contributor
timestamp: July 11, 2012, 07:04:34 pm
content: Quote from: SeanB on July 10, 2012, 06:06:25 pmI'm posting for the very first time in this forum, and I'm doing so because this statement had me laughing right out of my chair. I'm happy to see that the community here is just as funny, helpful, and quirky as Dave is.

author: T4P
author_rank: Super Contributor
timestamp: July 12, 2012, 04:34:40 pm
content: Quote from: Jimmy the Squid on July 11, 2012, 07:04:34 pmIn many places the humor is just only as good as the head is at humor!

author: SeanB
author_rank: Super Contributor
timestamp: July 12, 2012, 05:28:43 pm
content: Gee, thanks Dave for the compliment.I do have a few shorter screwdrivers, they keep wearing out, and need to be dressed every so often to get back to being other than a stump.