Just wondering how long does a capacitor on a cobrahead hold its charge once it's disconnected from the power supply?
It's a 120v fixture.
Mine just has the terminals, it doesn't have one of those resistors between the terminals to help it discharge.
It's rated 20 uF 330v.

I think you could short it out with a well insulated heavy screwdriver or some other lump of metal. 

I would say a few hours, you could use a capacitor discharge calculator if you wanted to find a resistor that would work and solder it on.

I know some frown upon the practice but I usually discharge them with the short with a screwdriver across the terminals technique. I don't have to do it often though as I fitted most of my ballast capacitors with discharge resistors.

Direct-short discharging an AC film capacitor is probably harmless for the most part, they're robust enough to withstand it, especially if done infrequently.

Now how long does it hold a charge? In theory the DC resistance of the dielectric dictates how long it takes for the charge to leak through and the charge between the plates to get back to an equilibrium. The capacitance also influences this discharge time. With these two values it's possible to calculate the RC time constant.

Quick example:

Taking a random datasheet from Digikey of Panasonic AC film capacitors, I get an insulation resistance of more than 1 000 Meg-ohm. For a 20uF cap it gives an RC time constant of 20 000 seconds, or over 5 and a half hours! This is the time it'd take for a 20uF cap to discharge from 100% to 36.8%.

A cathode-ray tube has a conductive coating on both the outside and inside surfaces. This forms a capacitor capable of holding a charge for days!

But this is all too deep in theory LOL.





Long story short, several hours without a discharge resistor. 

Large HV oil film or paper caps can store a charge indefintly!You have to short them with a peice of wire across their terminals when in storage-not in use.Their charge can KILL you!!!Electrolytic caps have a high internal leakage so they won't store a charge for long time-usually not more than several min to an hour.If they are in a HV circuit best to discharge them before touching.In our transmitters hang a ground hook on the caps to be sure!!!!And check the bleeder resistors!!!!This is not so much of an issue with caps in ballast circuits.By code these caps have an internal resistor on them or one that is external across the cap terminals.

It can hold it really very long, weeks or months is not much problem for a good quality dielectric.
But discharging them by a hard short is not a good idea at all, it presents huge stresses to the capacitor internals (the peak current is very high, causing large magnetic forcess, which can rip the foil apart) and it tends to throw hot sparkles around (and into your eye). Having some ceramic resistor {some 100's Ohms / 10W or so} with wire leads in your tool pack for this purpose is a way better choice...