Best way to discharge caps?

[oneslowz28]

Today I was given a box with roughly 300 used disposable cameras in it. I plan on salvaging the LED's caps, inductors and other components that are in them. They have some fairly large caps in them. 330v 80uF. So the few that I have salvaged in the past I have just shorted the leads with a flat head screw driver, but I have always wondered if there is a better or proper way to discharge them?

 

[Boncuk]

330V out of an 80µF cap zap pretty good and it will eventually take more screw drivers to discharge all the caps. ("welded tips" )

Use one (isolated) piece of wire and a 2.7K resistor attached to one end.

Then short the cap. The current flow will be 122mA initially, decreasing with decreasing cap voltage.

After 5 seconds the cap should be fully discharged. To make sure it is really fully discharged you might short it directly after using this procedure.

Boncuk

 

[Mike_2545]

Push the button on the camera & make it flash?

 

[maddog7]

Nice explanation Boncuk, I would say only short them with a piece of wire, could you explain what's with the resistor? How does it helps? thanks

 

[Sicarii]

the resistor is to limit the current so the cap doesnt discharge all at once. Discharging high voltage caps by shorting them can weld the conductor to the leads of the cap, the resistor is used to prevent this from happening.

 

[ericgibbs]

hi,
This link explains it fairly well.

Capacitor discharge-discharging capacitors with right methods

 

[JBrock]

Surely those caps can discharge at one amp or much more. (Don't they power the flash? They probably are designed to discharge almost instantaneously.) Go with a 270 ohm resistor.
5 seconds to discharge a cap? Good gosh you've got 300 to do! Do the math. That's almost a half hour of your life holding a resistor on a cap to bleed off the charge.
They are designed to be thrown away! How well can they hold a charge, and what are they really good for?
OK, I'm caving in and begging you to tell us what could you possibly expect to do with those capacitors (and inductors) !???.

 

[Nigel Goodwin]

Using a resistor is a 'nicer' way - but a screwdriver (preferably someone elses screwdriver ) is fine. Particularly so for flash gun capacitors which are intended for a very rapid discharge through the flash tube.

 

[Hero999]

330v 80uF.

That doesnt' sound right to me, are you sure that isn't 330µF 80V?

I've never seen an 80µF capacitor before, it's not a standard value.

Generally capacitors aren't considered dangerous unless they're charge to more than 120V as the shock won't bad enough to injure you.

Smaller capacitors charged to higher voltage are also considered to be harmless, 100pF charged to 5kV will only give you a static shock.

I can't remember the exact value but the IEE regulations state a maximum energy level capacitors charged to a above 120V need to be below after a certain amount of time after removing the power.

 

[Nigel Goodwin]

That doesnt' sound right to me, are you sure that isn't 330µF 80V?

I've never seen an 80µF capacitor before, it's not a standard value.

It will be 80uF 330V, it has to be that voltage in order to work a flash tube - the capacitance is probably the highest they good make it at that voltage in the available space.

Standard values don't matter to a manufacturer, only to you and me.

 

[Hero999]

You're right, I should've read the whole post and not just jumped on it.

Still I'm surprised they didn't label it as 82µF.

You're right, some manufacturers do make components in non-stanradrd values but they're just uncommon.

 

[Cobalt60]

Just want to say, I repair digital cameras for a living. We have audible dischargers. There is a few resistors and a speaker. It generally takes like 3 seconds to discharge a flash cap, and with an audible discharger, you pretty much know when its discharged. I say pretty much because even though you may think the noise has stopped, it has really just become so quiet that you dont hear it. So after using a discharger you can then use a screwdriver (or whatever) to make sure. This will result in a small static like spark (if anything), as opposed to the virtual explosion that happens when you short a fully charged flash cap.

I have to imagine that these dischargers are cheap enough to be worth purchasing one, but if you are going to make your own, I definitely would want it to be an audible style.

 

[Sceadwian]

Screw driver's work fine, but don't think a quick tap is all you need, holding it for a few seconds at least is a must. High voltage caps will hold some charge in the dielectric material and slowly self charge if you don't fully deplete them, especially flash caps. I've seen a 'discharged' 300 volt cap charge back up to 50 volts after a quick tap, they read 0 on a meter briefly, but slowly gain it back. Even after a 'complete' discharge from a flash they're not empty either, the two flash units I played with we cheap over the counter disposables, the caps both had close to 80 volts on them even discharged.

 

[Cobalt60]

Heh well chances are, if you use a screwdriver, the resulting blinding flash of light and deafening sound will cause you to instinctively remove the screwdriver from the cap. At which point you will connect it a 2nd and probably 3rd time.

I definitely do not recommend the screwdriver technique, but it does get the job done I suppose.

 

[Sceadwian]

Camera flashes aren't that bad, especially discharged. A small tick is the worst you get from a 'discharged' one. Charged.. that's another story.

 

[q5101997]

with a screwdriver.