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.

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.

--

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