Discharging a capacitor to safely work on a camera

[riley]

Hi there,

I have a fleet of old digital cameras that I would like to turn into IR cameras:

http://www.hars.de/2009/05/convert-a460-infrared.html

I went for it and took one apart but didn't realize I could get zapped by the flash capacitor. Needless to say it scared me and now I need to learn how to discharge the cap for the rest of the cameras. I came across this tutorial:

http://camerarepair.blogspot.com/2007/11/important-warning-camera-flash.html

It suggests buying an analoge voltmeter to discharge the cap. I went to Radio Shack and bought the closest thing, an analoge multimeter. The tutorial specifies setting the meter to the range closest to 50 Volts but doesn't specify 50 AC or 50 DC. Any thoughts on which one I would use?

Thanks in advance.

 

[mikey5791]

Hi Riley,

My advice on discharging the capacitor(cap) is to be extra careful as the cap still able to store quite a huge amount of electric shock.

Step 1:To discharge safely you need to get a 5W or 10W 5K to 10K ohm ceramic resistor (likely it is a square type) connect one end to one lead of the cap, then the other resistor end to the other lead of cap. Connect it for about 10 minutes. Be sure to insulate yourself with preferably a thick rubber glove before touching any of the cap leads.

Step 2: After the 10 minutes, disconnect both leads. To confirm no more charge, use a heavy screwdriver holding the rubber handle, touch & short both terminals of the cap to discharge any remaining charge. Hopefully you don't see any spark by now. To check use a cheap analog multimeter, set to a highest DC setting and measure for any volt reading. By now the DC reading should be lower than 10 volts. If your meter reading is more than 20 volt, redo step 1 & 2 to discharge.

Hope this helps.

 

[kpatz]

If you use a voltmeter, use the DC setting. Depending on the meter, it may not discharge the capacitor very quickly, but it will tell you the voltage charge remaining on it.

I would also use a higher setting initially (250-300V or higher if available) and take a reading. Once it drops below 50V, switch to that range and take another reading. You should see the voltage drop with time.

A resistor is the fastest way to discharge the cap. Just make sure to wear insulated gloves so you don't zap yourself.

 

[(*steve*)]

I made a device something like this for discharging flash capacitors.

I'm pretty sure I used 4 diodes in each direction. The resistor (R1) was chosen to limit the current to a reasonably safe value. Of course the current depends on the voltage across the capacitor, and the peak power dissipation was way higher than the resistor rating.

The LEDs will indicate polarity and the fact that the capacitor is discharging. The LED will go out when there's less than a couple of volts across the capacitor.

One probe was connected to an alligator clip, the other was a shrouded probe affixed to the body of the unit. It was designed to be used one handed.

edit: the design came from here (about 1/4 the way down). And it's been do long I forgot about that other resistor.