How do I discharge main capacitor in switching power supply? - Page 1
Forum section: electronics / repair

author: Saull
author_rank: Contributor
timestamp: November 19, 2020, 12:15:15 pm
content: Hi,Some times I fix switching power supply. The biggest issue that I have is discharging main capacitor which is at 320v. If I just leave it to discharge on itself it can take hours.How do I deal with this problem in safe manner?

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 19, 2020, 12:39:17 pm
content: The answer others may give is: "If you're asking you shouldn't be in there."The safest way I know is to build a banana-resistor-banana cable with an appropriate resistor (i.e. 47k 2W), attach it to your multimeter and you can check that you've discharged to a safe voltage.Edit: Some designs include a 200k or so bleeder resistor across the capacitor, it can save you time for troubleshooting to add one, I would still recommend at least checking voltage before touching, discharge is slow and resistors can fail.

author: ambrosia heart
author_rank: Regular Contributor
timestamp: November 20, 2020, 01:44:53 am
content: Use a light bulb 100Wto discharge the large cap, not LED type.Unplug an ac inlet.Black probe to the cap. Hot chassis.Red probe to the cap. positive pin.A bright flash, a pop sound & a spark seen.Not more than 5 seconds, cap dischargescompletely when it turns dark and no any light.Now measure the voltage,nearly 0V.See photos.

author: james_s
author_rank: Super Contributor
timestamp: November 20, 2020, 02:05:52 am
content: I've used the screwdriver method before but I would not recommend it, especially for a beginner who lacks the knowledge necessary to determine what counts as "too big." An incandescent light bulb is the most common item readily accessible to most people that will safely do the job.

author: Manul
author_rank: Super Contributor
timestamp: November 20, 2020, 02:28:29 am
content: I think that shorting capacitors is kinda brutal and may cause internal damage. Such high current surge can not be healthy. Love your electronics, use light bulb.

author: wraper
author_rank: Unknown
timestamp: November 20, 2020, 02:35:36 am
content: Quote from: blueskull on November 20, 2020, 01:49:10 amEven 10 uF charged to 300V can cause some significant explosion/spark when shorted. 220uF will create a crater on the screwdriver and bricks falling out of some less robust asses. Not to say metal film deposition on PCB. The safe way is taking about 4.7k resistor with a few W power rating for discharging. It will discharge capacitors within a few seconds (unless it's some unusually high capacitance) and not cause any explosion.

author: bdunham7
author_rank: Super Contributor
timestamp: November 20, 2020, 02:38:18 am
content: An analog VOM is ideal. It drains the capacitor and tells you when it is done. The cheaper the better--less ohms per volt. But I just use a screwdriver.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 21, 2020, 09:09:02 pm
content: Quote from: blueskull on November 20, 2020, 01:49:10 amOr an old insulated tweezers. Making bubuh is always fun, although it's wrong.

author: sean0118
author_rank: Frequent Contributor
timestamp: November 22, 2020, 04:03:41 am
content: If your multimeter has a low impedance mode (Lo Z) you can use that to discharge it, just by measuring the voltage across it in low impedance mode you'll discharge it pretty quick.Switch back to high impedance DC voltage mode to check it has finished discharging.Even after discharging, if in doubt, short it with a screwdriver to be extra safe before touching.

author: fzabkar
author_rank: Super Contributor
timestamp: November 22, 2020, 05:54:30 am
content: Quote from: ambrosia heart on November 20, 2020, 01:44:53 amThat's what I do. Filament bulb.

author: Haenk
author_rank: Super Contributor
timestamp: November 22, 2020, 07:49:19 am
content: A sparking discharge (shorting out the terminals) will damage the solder joints, and the cap is certainly not constructed for an instant discharge. Don't do that, unless you are going to replace it anyway. Resistor is the way to go - but make sure it's properly insulated (otherwise death and other things might happen when accidently touching it...)

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 22, 2020, 08:47:40 am
content: I use a wirewound 10W 100R resistor.