capacitor discharge rate

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Jim - 31 Dec 2005 03:58 GMT

Is there a way to calculate the amps a capacitor dischages through a short?

I have a "spot welder" made up of five large 150,000 uf (5 volt)computer
capacitors in parallel, which are charged to up using a 1 to 5 volt supply
and dischaged through a SCR to 2 beefy probes. The purpose is to spot weld
terminal tabs on batteries in place of soldering them on.

How many amps are being produced across the terminal probes, ususally spaced
5mm or so apart? Is there a math formula to calculate this? My 70 amp SCR
still survives, so I guess it must be less that that.

thanks
Jim

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BobG - 30 Dec 2005 17:10 GMT

something like CV=IT maybe?

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Bob Monsen - 30 Dec 2005 19:25 GMT

> Is there a way to calculate the amps a capacitor dischages through a short?
>
[quoted text clipped - 6 lines]
> 5mm or so apart? Is there a math formula to calculate this? My 70 amp SCR
> still survives, so I guess it must be less that that.

You can't tell unless you know what the resistance is. If you know the
resistance, then

V(t) = Vo * exp(-t/R/C)

and

I(t) = V(t)/R

so

I(t) = Vo * exp(-t/R/C) / R

where Vo is the initial voltage across the caps.

If you can measure how long it takes for your voltage to get down to 1/2
the original voltage, you can figure out the resistance; it will be

R = t/(ln(2)*C)

plug that back into the I(t) expression.

For example, if it takes 0.1 second to discharge down to 2.5 volts from a
5 volt charge, then

R = 0.1/(ln(2)*.75) = 0.192 ohms

Then, the initial current will be I = 5/0.192 = 26A, and after 0.1s, will
be 13A.

Signature

Regards,
Bob Monsen

Our minds are finite, and yet even in those circumstances of finitude, we
are surrounded by possibilities that are infinite, and the purpose of human
life is to grasp as much as we can out of that infinitude.
- Alfred North Whitehead

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Mike - 30 Dec 2005 21:26 GMT

>Is there a way to calculate the amps a capacitor dischages through a short?
>
[quoted text clipped - 6 lines]
>5mm or so apart? Is there a math formula to calculate this? My 70 amp SCR
>still survives, so I guess it must be less that that.

Not necessarily, Most SCRs can withstand at least 10 times their rms
current rating in a discharge application such as this. If you have
the data sheet look at the peak one cycle surge current rating

>thanks
>Jim

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Jasen Betts - 30 Dec 2005 21:35 GMT

> Is there a way to calculate the amps a capacitor dischages through a short?
>
[quoted text clipped - 5 lines]
> How many amps are being produced across the terminal probes, ususally spaced
> 5mm or so apart?

what's the resistance and inductance of the whole setup? (the limiting
factor is probably the resistance of the capacitors)

Bye.
Jasen

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Electronics Forum / Basics / December 2005

capacitor discharge rate