How to discharge a parallel of very big e-capacitors ?

[kagliostro]

My son and his friends purcased this professional flash unit that requires the sostitution of a pair of connectors (two of the four on the left)

EDIT: I forgot to post the image



so the unit has been opened, there is a parallel of very big e-capacitors that has still voltage on it

the question is

for this parallel of large e-capacitors which will be an appropriate discharge resistor value in resistance and in power ( ohm & W ) ?

Usually a 22-47Kohm 5W resistor is enough for the e-capacitors we find on audio amp, but here ?

Thanks

Franco

[shooter]

my experience is to start BIG n work down, so ~=>100k
I believe most flash units were ~ small K volts, maybe 4,000 with little current
monitoring the voltage discharge is tricky at high volts, you gotta come up with a high V probe, or build a backyard one.

the systems I worked on that had both Big k volts AND large currents used ~ 1FT long, 1.5" dia wirewound ceramic R's (can't recall values), It sucked cleaning up after a catastrophic fail 

[kagliostro]

Thanks Shooter

Franco

[DummyLoad]

ohms law still applies franco - what is the charged DC voltage of the cap? you have a meter; measure. determine the time constant if you know size of the caps. the first time constant is 63% you need 5 time constants to consider the cap discharged. size the resistor power handling capacity accordingly.

ex: a 100milliFarad cap charged to 100V with load resistor of 22KΩ will take 2200 seconds to discharge 63% - therefore 5 TC x 2200 secs = 11000 seconds (3.05 hours) to fully discharge through 22K resistor - but it's a 1W resistor!

100V / 22K =  4.55mA  or .455 watt but 3 hours to discharge = that's TOO large.

100V / 1K = 100mA = 10W - i'd use a 1K 25W resistor in this example. TC would be 100 seconds to 63% or 500 seconds (8 minutes 20 secs) to full discharge.



https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-time-constant

--pete

[kagliostro]

Thanks DummyLoad

The capacitors, inside the cabinet, are mounted with a metallic shield around and only part of the top of the capacitor is visible

the unit isn't on my town, is in one other town (not very far but not here) where my son and a pair of his friends are arranging a craft photo lab

the other day I was there but I had no instruments under hand, so I didn't measured the voltage and was not able to read the voltage at which the capacitors are labeled

I'll try to have more info about the situation to apply your suggestions and use the useful calculator you linked

Franco

[PRR]

A large flash unit can be 300V-600V at 1,000uFd-6,000uFd.

If they show ANY voltage, after not being charged for days, the is some life in the caps and it is worth trying to re-use them. One of my favorite hi-fi amps I re-capped with four 470uFd 450V flash caps. (In the days when we all built disco-strobes.) They are specified for FAST discharge with small size, not steady ripple-filtering, but they seem to do well.

[kagliostro]

Thanks PRR

Franco