Are film capacitors directional?

Thank you, Al. My multimeter doesn't go to AC millivolts. I will borrow a better meter from a friend. Please forgive my ignorance, but once I have the meter, what exactly do I do?

bc

Please forgive my ignorance, but once I have the meter, what exactly do I do?

There's no ignorance to be forgiven, aside perhaps from my own :-)

Thinking about it a little further, I believe that to get meaningful readings with a battery powered DMM you MIGHT have to provide it with an AC ground reference. That could be done by connecting a piece of ordinary hookup wire between the screw on the wall plate of an AC outlet, and the capacitor lead to which the DMM's "common" terminal is connected. I'll describe the procedure as if that wire is necessary, although I'm not completely certain that it will be necessary.

What you would do is as follows:

Set the meter to read AC voltage, on a fairly sensitive scale that would resolve a millivolt or so. If the meter has a "LoZ" (low input impedance) provision, disable it. Connect the meter's com (common) input lead (or whatever the equivalent nomenclature may be on the particular meter) to one end of the capacitor. Connect the wire from the wall plate screw to that same point, perhaps holding it in place with an alligator clip that may be provided with the meter, or that may be attached to the end of its lead. Connect the other lead from the meter to the other end of the capacitor.

Put your hand on the capacitor, and see if you get a reading. If not, drape the AC power cord of some powered up electrical device across the capacitor, and see if that gives you a reading. Note the value of the reading.

Remove the three connections from the capacitor. Connect the com lead of the meter and the wire from the wall plate screw to the opposite end of the capacitor than the end they were originally connected to. Connect the other lead of the meter to the opposite end of the meter than it was originally connected to. Repeat the voltage measurement.

The end of the capacitor that was connected to the com lead and the ground wire when the lower of the two measurements occurred is connected to the capacitor's outer foil.

Best,
-- Al

Hey Al....once again, thanks for your valuable input on all technical subjects. I don't mean to sidetrack this thread - but since audiogon disabled the ability to communicate otherwise I'd like to ask & hopefully this is valuable to others as well...is it worth buying a really quality grade multimeter? I've used a $20 Radio Shack digital multimeter for years to bias my amps & often wondered if I'd get different readings from a better grade meter?

Hi Pehare,

Thanks! I don't know the answer to your question, though, as I haven't ever used a RS DMM. FWIW I did use a RS FET-input (high input impedance) analog multimeter many years ago, and it seemed to work well, and lasted a long time.

If the opportunity arises, perhaps you could borrow a Fluke or other good meter from someone, and compare readings.

Believe it or not, the only meter I've had here in recent years is a not particularly accurate Triplett analog multimeter, supporting both my audio and antique radio endeavors. I do have a low end oscilloscope, though. I'll have to get myself a good Fluke DMM at some point, probably one that also measures capacitance and perhaps inductance as well.

Best regards,
-- Al

Thanks, Al! Very helpful. I bought a better meter today, so I will try the procedure you described tomorrow and report back.

Bryon