capacitor questions

[Gary Majchrzak]

*besides the obvious (ie: ballast isn't working)*
is there an easy way to tell if a ballast's capacitor is shot?

can the capacitor from a 400w M59 (probe start) MH ballast be wired into a 400w M135 (pulse start) ballast?

TIA :)

[on the spot]

is there an easy way to tell if a ballast's capacitor is shot?


You can test a capacitor with an ohmmeter set at a high resistance scale, after you turn off the branch circuit power and disconnect the capacitor from the circuit. Of course, always safely discharge the capacitor first. If the ohmmeter measures a very high resistance, the capacitor is open, so replace it. If the reading is zero or a low resistance, the capacitor has a short. You should also replace it. If the reading is zero, or if a very low resistance shows initially, and the resistance slowly increases, the capacitor is good.

[Gary Majchrzak]

thanks.

I'm no electrician but I seem to recall that capicitors store energy and can be harmful if improperly handled.

Still wondering if I can use the cap from a probe type ballast in a pulse start.
Are there different different size capacitors used in 400w ballasts?

[PaulErik]

The capacitor microfarad rating depends on the ballast itself. They are not interchangeable by wattage. The rating required varies by ballast type, design and manufacturer. Most HID ballast capacitors are equipped with a shorting resistor for safety. This discharges the capacitor to a safe level once powered off. Ballast capacitors are required to have a shorting resistor for safety approval in North America.

[Gary Majchrzak]

many thanks once again, Paul :thumbsup:

[DaveMorris]

That's scary. I have two dual 250w and one dual 400w PFO ballasts and none of them have shorting caps on them.

[Gary Majchrzak]

if/when the capacitor goes it will likely explode and cause a noticeable smell. It happens. Replacement kits are sold for this purpose.

[PaulErik]

The shorting resistor will be internal on a dry film capacitor. The shorting resistor will be on the top of the terminals on an oil filled capacitor. A few ballast models are equipped with a lamp ignitor that houses the discharging resistor for the capacitor. All of the PFO magnetic ballasts should discharge the capacitor. For testing purposes the capacitor does need to be fully discharged by shorting the leads. The resistor discharges the capacitor almost completely for safety.