Dave, How to safely discharge a capacitor?

[wblackledg]

Dave, 
Yet another idea for a blog video.  I have no idea how to do this safely.  Saw one video on YouTube of someone holding he cap in his hand when sparks were flying. Didn’t look safe at all.
Thanks.

[c4757p]

I'm going to preempt the safety brigade and say "bigass screwdriver", as long as we're not talking "supercapacitor" or something. But I will be disagreed with. If you don't like that, then a bulky power resistor, about 100 ohms 5W, discharge for about 10RC.

And do it two or three times.

[AG6QR]

The simplest way is to discharge it with a resistor.  The common term is "bleed resistor" because the resistor "bleeds" off the charge.

Taking into account the max voltage on the capacitor, choose a resistor with enough resistance so that the power dissipated by the resistor (v*v/r) won't exceed the resistor's power rating.  For big capacitors, that is, high voltage and high capacitance, this may mean it will take a while to discharge.

[alm]

I would not be too worried about safety for most caps, but it's not very nice for the cap to draw a huge surge current, depending on its ESR. That's why I think it's good practice to include a series resistor (although a well insulated screwdriver is an excellent handle).

Because of dielectric absorption, keeping the terminals shorted is not a bad idea for HV caps.

[SeanB]

I use a 56k 10W wirewound resistor, though with the 56k I guess it is more a thick film metal film unit rather than wire, hard to draw wire that fine and do a cheap resistor with it. They do not get warm at all discharging a 100uF motor run cap charged to 1kV for testing, and I have a few spares just in case anyhow from a grab bag of assorted power resistors I bought for $2. Takes 10 seconds before I stop discharging and check the capacitor further. The ones that fail are broken open and tossed, some are very poor inside.

[wblackledg]

I'm going to preempt the safety brigade and say "bigass screwdriver", as long as we're not talking "supercapacitor" or something. But I will be disagreed with. If you don't like that, then a bulky power resistor, about 100 ohms 5W, discharge for about 10RC.

And do it two or three times.

I had in mind the caps in my newly acquired Vintage SB-200 Linear Amp for Ham Radio.  A guy at my ham club told me to use a wire taped to a long wooden dowel rod, and connect the wire to earth ground. He said expect a sound like a rifle shot.  That also sounded a bit catastrophic. 

I assumed that people who regularly work on such things have a method.  The resistor idea sounds good.

[David_AVD]

Do a search and you'll find plenty of discussion and previous suggestions for this topic.

[IanB]

Long ago when I was young, I had some smoothing capacitors I salvaged from a computer power supply. They were about 20,000 uF and had big screw terminals on them.

When I charged these up to 12 V and shorted them with a small screwdriver they made a giant bang and destroyed the screwdriver.

A resistor to moderate the current is a good idea, assuming you don't want to destroy things.

[David_AVD]

A resistor to moderate the current is a good idea, assuming you don't want to destroy things.

Well, the OP is reading EEVblog where a lot of people seem to enjoy blowing stuff up!  lol

[c4757p]

Yeah, screw terminals and screw drivers don't mix! Definitely don't go discharging 20mF into a screwdriver!

[wblackledg]

A resistor to moderate the current is a good idea, assuming you don't want to destroy things.

Well, the OP is reading EEVblog where a lot of people seem to enjoy blowing stuff up!  lol

yes, but I got all that out of my system when I was in the Marine Corps.   

Now that I have mellowed somewhat, I just want to safely discharge a big capacitor(s) without calling Ordnance Desposal or Homeland security.. 

[Smokey]

I love it.   I read the title and intended to reply with "big @ss screwdriver" but I'm wayyyyy too late to the party.  You guys are great!!!

A high value power resistor is usually permanently installed across big caps in equipment so they won't hold a charge forever.  Just be aware of the bleed time (or measure the voltage) before you go poking around.  Google turned up some good capacitor bleed time calculators.  Also be aware that unless you wait a LONG time, there will usually be a bit of voltage left even with a good size bleeder resistor.

You didn't say the voltage or capacity of the cap you want to discharge.  The higher value resistor you use, the smaller the spark when you bridge the terminals,  but the longer it will take to bleed.  I'm envisioning a knife  switch with a resistor in series with the terminals of the garbage can sized cap and closing the switch with a wooden broom handle from across the room.  Then again the op might have a 12v 10uF tiny thing in which case I might have to grab my favorite multi-use plastic handled screwdriver/pry-bar/chisel/box-opener/paint-stirrer/cap-discharger and make some new weld marks on it

[Stonent]

[westfw]

http://repairfaq.cis.upenn.edu/sam/strbfaq.htm#strbcap