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XOIIO's avatar

Should I use diodes?

Asked by XOIIO (18146points) March 6th, 2010

I am working on a project that involves 3 camera flashed that are charged up to be discharged at once, each at 300 volts to the same metal contacts. Sould I put diodes to prevent then charging eachother or frying the circuit and what kind would I need?

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12 Answers

grumpyfish's avatar

So you have three camera flashes that are attached to a single capacitor, or three camera flashes attached to different capacitors?

XOIIO's avatar

I’m hoping to fit the 3 individual circuits.

grumpyfish's avatar

As long as you disconnect the charge circuit from the discharge circuit before discharging, you should be OK.

300V isn’t too much for a diode to deal with, but the current is going to be the trouble.

You should be able to work out the discharge current (based on the series resistance of the capacitor and the capacity of the capacitor) and use that to size your wiring.

ARE_you_kidding_me's avatar

Yes, using them will not hurt a thing.

XOIIO's avatar

@grumpyfish SoI need to find a way to disconnect the capacitors before discharging them? I can only think of relays but we only have one kind here.

grumpyfish's avatar

@XOIIO Thinking about it further, you may not need to. The resistor on the charge circuit should be enough to keep the discharge from causing problems there.

Could you post a circuit diagram of what you’re currently doing—that would help. Doesn’t need to be graphically correct, just needs to tell the story.

XOIIO's avatar

All It really is is wires soldered to the capacitor leads, and two metal contacts for the person or object to be touched with. I just don’t know if there is a risk that enough of the voltage will be used up vreating a spark, but I think it will.

Actually this is the design but modified to make a mild EMF just take out the coil.

grumpyfish's avatar

So wait… you’re planning on applying 380v to a person?

grumpyfish's avatar

So… here’s the quick math on this: A standard flash capacitor from a disposable camera is something like 160uF. At 380V that’s 60mA seconds of stored energy, you have 3 capacitors? That’s 180mA seconds.

The old saying is “It’s the volts that jolt, but the mills that kill.”—60mA is enough to kill you if you’re not lucky. 300mA will kill you if it goes through your heart at all. Having seen someone (amazingly not killed) electrocuted, it’s not something you want to mess around with.

Anyway—for the calculation, we can assume the ESR of the capacitor is 0.5ohms, and dry human skin (holding a wire). Skin resistance is at worst case around 3k, 1k if wet. Let’s call it 1.5k ohm for good measure.

So our total resistance in the circuit (ignoring wires) is really around 1500 ohms. So:
R = 1500
V = 380

V/R = I

So I = 250mA—which is enough to kill someone, cause burns, etc.

We also have W = V*A which gives you 95W. I really don’t want 95W to be dissipated through my skin.

XOIIO's avatar

@grumpyfish I plan on having 3 capacitors discharge though, so wouldn’t that be 1140 volts like I said on the not so legible design? And yes, I know one camera flash can burn you and male small holes with blood in your skin.

I don’t think this would kill you, and I manly like to vaporize craters out of metal :)

grumpyfish's avatar

Ah, if you put them in series, you get 1140 volts and only 60mAs—the diagram you linked to shows them attached parallel.

Making craters in metal is an excellent application of this! Above ”...and two metal contacts for the person or object to be touched with” freaked me out a little =)

XOIIO's avatar

LOL

Your right, I should have them in parralell. I think I will have one camera flash circuit charging several capacitors in series.

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