I've recently replaced my central A/C system and now ended up with some motor run capacitors from the old A/C. (I tried to salvage the relay, but it's already worn so it's no good.) My question is, will a motor run capacitor withstand a direct connection to the mains for a long time? I think that one connected in parallel with the input of a UPS will help absorb electrical noise and help prevent nuisance tripping. The capacitor I'm planning to use is 5uF, 370VAC. BTW, is it possible to use a capacitor as a ballast for a fluorescent lamp?
Motor run capacitors are designed for continous duty, but they aren't going to help in the way you want. They are there on motors to energise a second phase winding on the motor. Are you sure it's a motor run capacitor and not a motor start capacitor (not designed for continuous duty). The ballast for a fluorescent lamp is inductive not capacitive (hence the heavy inductor!). The capacitor is just there for RF interference suppression.
I'm very sure that they're motor run capacitors. I also have a 10uF, 370VAC and a 40uF, 440VAC. I think the 10uF would provide slightly better filtering and the 40uF would draw too much current to be useful for this application. BTW, the 5uF was mislabeled "5MFD". I knew that the value was wrong because it was physically about half the size of the 10uF and it tested about 5uF on my capacitance meter. Remember, "u" is for micro (well, it's actually mu, but a lowercase "u" is easier to type). A "m" is for milli. (And since the label used a capital "M", it would be mega and that would be very, very far from the actual value.) I actually used a permanent marker to correct the label! All three capacitors also say "protected 10,000AFC". What does that mean? I've seen a design for a small fluorescent lamp ballast without any inductors or transistors that used a capacitor to limit the current. A diode with a resistor in series was used as a starter (it would work as a voltage doubler until the lamp starts, then the capacitor would be used to limit the current).
The "M" is common on capacitors in equipment like appliances, motors, etc. It may just be a legibility thing, or simply easier to print, anyways, I have seen it for years and still do. The capacitor is not suitable for what you want to do, it is designed to be used in series with the line to induce a phase shift in a winding to help start or keep and AC motor running. I'm too lazy to type out a full explaination, but if anyone is interested: http://forum.allaboutcircuits.com/lofiversion/index.php/t2012.html
Iirc you would want an inductor for mains, instead of a capacitor. Tying one loop in the mains cable is suppost to be enough to protect against lightning, so might stop some niose?
I'd say the 5uF size is too big to be of use, a faster 10n-100n film type would be more effective. They're commonly added across transformer primaries in audio psus. However, as Rod Elliot advises here,
