Stupid capacitor question

[darw82]

This cap confusing me:
It is marked 10mfd 50WV (nonpolar)?
1. Does this mean it have 10microfarad capacitance and 50VAC rated voltage? or Is it mean it have 10millifarad?

2. Is it possible to make nonpolar capacitor from polarized capacitor?

Thanks.

[Netlist]

1) Most likely its a 10µF 50VDC cap. A picture would be nice.

2) Connect two say 22µF caps in series, negative poles together. Use both positive wires. You now have a 11µF nonpolar cap.

/Hugo

[darw82]

Netlist,
Thanks for your reply,

Since this cap connected in series, the rated voltage will be doubled. Am I right?

This cap will be use for 1Kw Air conditioner motor start.
I always use Polypropylene cap for this motor. But now
I only have 2psc of 10uf/400V (105C) alumunium cap.

Is it safe to use AL-cap?

[Netlist]

This is beyond the scope of an audio forum.
I strongly advise to take care with mains connected devices, caps in your case. Only use original spare parts and contact a dealer or qualified technician to tackle the job. From what I understand in your first post (hate to say it but I feel I must) you are not qualified.
Take care.

/Hugo

[mrshow4u]

Since this cap connected in series, the rated voltage will be doubled. Am I right?

I'd recommend putting a resistor divider in parallel with the series'ed caps. High values are ok. (1Meg etc)


I--------------^^^^^-------I------^^^^^^-----------------I
I----------------II-----------I-----------II--------------------I

[sreten]

Originally posted by darw82

Since this cap connected in series, the rated voltage will be doubled.
Am I right?

Hi,

No.

2 10uF polar 400V DC wired as 5uF bipolar is ~ 250VAC.
That is positive or negative peak AC voltage < 400V.

Not sure but sounds dangerous to me. Transients on the line
may pop one polar and then the other will follow very shortly.

/sreten.

[darw82]

Well, I think it is better to order 3.3uf/370VAC Polypropylene cap from Farnell.

[mirlo]

When you connect polar electrolytic capacitors back-to-back in series, the voltage rating does NOT increase, because only one of the capacitors is ever operating with correct polarity. At any given moment, the other one generally leaks and drops a very low voltage. It is not a safe thing to do at high voltages.

Furthermore, motor start capacitors require high current capability and low ESR in the capacitors -- that is why they are usually polypropylene. Even polyester (mylar) isn't usually robust enough.

Finally, the inductive kickback from the motor often causes momentary overvoltages that could cause insulation failure. Polypropylene capacitors have a "self-healing" property for moderate short term overvoltage transients. Electrolytics don't -- they might explode.

So I agree -- please use a polypropylene cap. Even the most robust electrolytics just aren't designed to be motor start caps.

[Netlist]

Let me again stress the importance of getting professional advice.
Electrolytic caps are indeed used as motor starters but they are made especially for this purpose.
http://www.bhc.co.uk/pdf/MS_Cat.pdf
Since we do not know exactly what environment the motor will be functioning in nor do we know what type of motor is used, we can not comment with enough accuracy to help the original poster.
At the very least do some reading on said material to prevent accidents.
http://www.google.be/search?sourceid...tart+capacitor

/Hugo