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pells61
02/06/2014, 06:36 PM
I have a coralife, garbage, 250 w DE metal halide ballast with a bad capacitor. The capacitor is 35 microfarads, 400VAC. The only capacitor I can find to replace it is 35 microfarad, 300 VAC. Since the microfarads are the same will the ballast function correctly with 300 VAC instead of 400 VAC?

kikoreefer
02/06/2014, 06:39 PM
It may function, probably will, but will more than likely shorten life and cause your power factor to drop. You can't find one online?

Coral Dilema
02/06/2014, 06:45 PM
No. it will explode most likely. 35 microfarads is the amount of capacitance. The voltage rating is that maximum safe voltage to operate the capacitor at. If you exceed that you take a chance on arcing plate to plate through the dielectric. It could actually explode. Most newer capacitors have a weakened spot in the metal can that is designed to release the pressure if this happens but you just never know.

You have to realize that even though you are only putting 120VAC into the ballast, the transformer in it increases the voltage considerably. And if it is only using a capacitor/diode voltage doubler instead of a transformer you still are looking at 2X the input voltage times the square root of 2, or about 340 volts. Without knowing more about how your ballast is designed it is hard to say for sure but if the manufacturer put a capacitor rated for 400 volt operation in there it likely needs it. Why pay for a more expensive capacitor rated for 400V if a cheaper one rated for 300V will work? No modern business would do that.

PaulErik
02/06/2014, 07:09 PM
If this is the Coralife 250-watt M80 HQI ballast (HPS-108SA-250) the capacitor should be rated at or above 280 Volts. The ballast has a maximum open circuit voltage of 205 Volts. This ballast comes equipped with a 280, 300 or 400 Volt rated capacitor. The capacitor rating will vary depending on demand, availability and cost. The microfarad rating must be matched.

pells61
02/06/2014, 07:56 PM
I opened the ballast case and on the inside on the ballast core I find that it is indeed an HPS-108SA-250, it came off a 3x 250w 72' HQI fixture. I like what Paul has to say but has anyone else had experience with such a ballast? Could I in fact put a 300VAC cap on this system?

pells61
02/06/2014, 08:28 PM
What do you guys think of trying this in my ballast?

http://www.grainger.com/product/PHILIPS-ADVANCE-Dry-Film-HID-Capacitor-5ZNG7?s_pp=false&sgAttributes=

pells61
02/06/2014, 08:30 PM
I think its a good fit, its a dry film just like the cap im replacing the only difference is the VAC the original is 400 this is a 300VAC

PaulErik
02/06/2014, 09:00 PM
That capacitor will work like the original one. The different voltage rating will not change the performance of the ballast.