Replacing capacitors

[kofisammy]

Hello
Is it healthy to replace a 100uf400v capacitor with 68uf400v capacitor?

[Geek-9pm]

Simple answer, No.
Do you hae a reson to make such a change?
If this is part of a critical assembly you must use the same value.
If this is only a classroomexperiment,  then you might be able to do it.

[DaveLembke]

First of all 400V Caps have quite a bite to them if you touch one that is charged with more than about 90 volts... Anything above 48 volts you might feel depending on how moist your fingers are.

Secondly you want to stick with the same value capacitor and same type. *Removed info on ways to make values with multiples of capacitors* Avoid doing that at all costs its too dangerous especially with the circuit your messing with which is probably a power supply or a flash for a camera which uses capacitors like this.

Whenever I work around high voltage capacitors, I avoid touching the PCB and any traces, and I take a digital multimeter to them. If a multimeter isnt available, with power unplugged I have shorted them with a screw driver before after the device hasnt been plugged into a battery source or outlet for about 10 minutes to give it time to drain. Not all capacitors drain out quickly and so I short them with a insulated screw driver between their 2 legs only and not to anything else, and usually get a small spark where it might still have a charge and sometimes nothing so it drained safely. You reintroduce the screw driver to short the capacitor until sparking stops and its drained out as for your reaction might be to jump back if it sparks and that might be only a partial drain time from momentary short.

BE VERY CAREFUL WITH THIS CIRCUIT IF YOU MUST FIX IT

The correct component with Radio Shack gone can be acquired online through Mouser, Digikey, or another source. You also want to look for the temperature rating on that capacitor such as 85C and match that too. 

The typical temperature range for aluminum electrolytic capacitors is –40 ºC to 85 ºC or 105 ºC. Capacitance varies about +5% –40% over the range with the capacitance loss all at cold temperatures. Capacitors rated –55 ºC generally only have –10% to –20% capacitance loss at –40 ºC.

One suggestion I have is ..... Its likely safer to just buy a new of whatever this capacitor goes to as for the risk of injury or death by electrocution or fire is present. 

[BC_Programmer]

-Some circuits have a drain resistor which will effectively drain the capacitor when there is no input power. Others don't. Shorting the two connections before fiddling with it is a reasonable practice either way.

-When replacing capacitors you can use a capacitor of a higher capacitance, but you'll want the same voltage and a temperature range that is either the same or wider than the original part.

-400V is pretty much only found in Switched Mode Power Supplies. Are you trying to repair a power supply? Don't. Even if the cap has failed, it's unlikely that it is the only failure point. replacing it would likely just blow the replacement as the original was likely to have failed due to another component failing first; which may be from another component, and so on. Repairing components like that requires either a lot of trial and error (and a lot of busted replacements) or a level of expertise that would mean diagnosing the circuit via circuit diagrams; and frankly somebody with that ability wouldn't be asking if you can replace one cap with another of lower capacitance.

[Salmon Trout]

I short them with a insulated screw driver between their 2 legs only and not to anything else, and usually get a small spark where it might still have a charge and sometimes nothing so it drained safely.

Use eye protection when discharging capacitors storing large amounts of energy. Better stil, make a probe with a 1 megohm resistor. I made one for colour TV CRTs.  I once shorted the capacitor in a small, consumer-grade electronic flash meant to fit on the hot shoe of an SLR camera. It was a 400 uF 350 V electrolytic. It hadn't been been used for weeks. There was a bright flash, and a sound like a pistol shot. A fair amount of smoke. The blade tip of the screwdriver was half melted away. Looked like it had been bitten off. Vapourised and melted steel had flown a foot or so on the bench, shown by burn marks.

If anyone is interested, the amount of energy in Joules (or Watt-seconds if you prefer) stored in a fully charged capacitor is given by 0.5 x (CV squared). In the brackets that's C in farads multiplied by the square of the voltage!!! So as the voltage gets higher the stored power increases exponentially. Around 10 to 50 Joules in a typical flash capacitor*. If you discharge it in a few millseconds that's a lot of Watts in the flesh of your hand or across your heart. Even more at the tip of my screwdriver!

*Tasers deliver around half a Joule per pulse, Most of those used by law enforcement are slightly less, around 0.3 J.

[Salmon Trout]

Also, capacitors can 'recover' after being discharged a first time, so do it again after a few minutes.