How do I discharge main capacitor in switching power supply?

[Saull]

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?

[shakalnokturn]

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.

[ambrosia heart]

Use a light bulb 100W to 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 discharges
completely when it turns dark and no any light.
Now measure the voltage, nearly 0V.
See photos.

[blueskull]

Depending on the capacitance, I guess. If it is not particularly energetic (450V 330uF and up), I'll just short it with a disposable screwdriver dedicated for this.

[james_s]

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.

[Manul]

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.

[wraper]

Depending on the capacitance, I guess. If it is not particularly energetic (450V 330uF and up), I'll just short it with a disposable screwdriver dedicated for this.

Even 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.

[bdunham7]

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.

[S. Petrukhin]

Depending on the capacitance, I guess. If it is not particularly energetic (450V 330uF and up), I'll just short it with a disposable screwdriver dedicated for this.

Or an old insulated tweezers. Making bubuh is always fun, although it's wrong. 

[sean0118]

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.   

[fzabkar]

Use a light bulb 100W to discharge the large cap, not LED type.

That's what I do. Filament bulb.

[Haenk]

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...)

[akimpowerscr]

I use a wirewound 10W 100R resistor.