Capacitor Check - bi-polar??

[pelagos]

Hi

This may be a silly ques but

I have this cap out of a compressor - it doesnt have any indication of the +/- side and no marking if it is bi-polar?

Can I assume it is?

Cant find any info on the deets from the cap itself?

All the other caps in the compressor are like this - i cant imagine they are all bi-polar? Or maybe they are?

If not how to determine the +/- side?

There is no indication on the PCB of the correct capacitor orientation

thanks for any info

 

[Evertide]

+ 0 - 25 V +...

 

[pelagos]

+ 0 - 25 V +...

YEah i saw that on the cap and sorry if that is obvious - but it means?

 

[capacitorless]

Reverse google image search came up with https://resi.store/products/290/ It isn't a great match but it shows something similar.

A non-polarized cap will (typically) say NP somewhere: https://www.santacruzelectronics.com/tormach/Tormach.cfm?do=detail&q_part=NPR22M50&q_StartRow=721 just in case you didn't already know

You can try testing with a multimeter (scroll down most of the way - the shortcut links don't seem to work at the top): https://www.nextpcb.com/blog/capacitor-polarity#:~:text=a dangerous situation.-,How to Determine the Polarity of a Capacitor?,-To determine the

See if there's an obvious ground trace under one of the two legs (hopefully/probably the opposite of the leg marked +0-25V+), check a few other caps in the same way, and get a consensus based upon that. Probing with a meter with one side connected to ground might help too.

If you can power the compressor up without causing problems, you could check the voltage across the two legs, to determine what the circuit is set up for.

Edit: the vast majority of radial electrolytics I've seen have a black stripe/pattern, indicating the negative leg. Also, I didn't come up with anything based on the other markings, as you likely already tried. Sometimes Google isn't super helpful with part numbers, just ime. Good luck, I'm curious what you find out!

 

[pucho812]

there are a couple of hints here.

First is size. It’s a 22uF cap.
That eliminates a lot of options.
So we know it is probably an electrolytic.
Those are usually marked “np” or “bp” to indicate they are bipolar. But polarized caps have marking to denote which lead is positive and which lead is negative.
The 0-25vdc literally means it’s a 25vdc cap and can handle voltages up to 25vdc.

Can test it with a dmm for capacitance. Perhaps that may give further clues

 

[scott2000]

But polarized caps have marking to denote which lead is positive

 

[Script]

See if there's an obvious ground trace under one of the two legs

+1
Caps on power rails are polarised, not BP.

Ripping all caps out at the same time is a bad idea. Maybe mark side with legend with pen on PCB.

 

[gyraf]

+ 0 - 25 V +...

..the pin side where this is printed is the positive...

 

[pucho812]

Please quote the full quote as I have seen it both ways

 

[scott2000]

Please quote the full quote as I have seen it both ways

Not in the OP pics have you

 

[transient]

Pretty sure the suggested method on that website of checking polarity with a multimeter won't work!

"Use a multimeter: A multimeter is possible to use to determine the polarity of a capacitor. Set the multimeter to the continuity or diode test mode, and touch the positive probe to the positive terminal of the capacitor and the negative probe to the negative terminal. If the multimeter reads a positive value, then the positive probe is connected to the positive terminal, and if the multimeter reads a negative value, then the positive probe is connected to the negative terminal."

Any initial reading on the multimeter will depend on residual charge on the capacitor, if any, and will soon be replaced by charge from the multimeter, even if nominally reverse-biased.

 

[transient]

Set me thinking ...

I used my insulation tester on its 50 volt range on a couple of sample capacitors. When correctly polarised, the capacitors charged up enough to give a nice spark when shorted with a screwdriver - don't try this at home folks! When reverse-polarised, there was no significant charge.

Why anyone would do this, I don't know.

 

[RoadrunnerOZ]

What make and model is the compressor? Is there component numbering on the board and if so what C number is the capacitor? A quick look at the schematic would answer this.

 

[RoadrunnerOZ]

There appear to be more numbers/letters below the 8317H on the cap - what are they? Photo?

 

[capacitorless]

Not to beat a dead horse, but I wonder if there's another way of doing it with a multimeter. Perhaps a resistor in series with a voltage source, with the meter measuring current draw, and looking for it to charge up faster in one direction? Discharging between tests, of course.

 

[transient]

Perhaps a resistor in series with a voltage source, with the meter measuring current draw, and looking for it to charge up faster in one direction? Discharging between tests, of course.

That's essentially what my insulation tester was doing. 50 volts was enough to force a result, whereas I think that the terminal voltage of most multimeters will be insufficient, even on the diode range.

 

[RoadrunnerOZ]

If the voltage is too high at the correct polarity it’ll blow holes in the electrolyte and foil, if reversed the capacitor will blow out the pressure sealed base even well under rated voltage - do not apply reverse voltage to polarised electrolytics. I accidentally put one in the wrong way round the other day - 100uF 25V rated in a 15V circuit - the bang was impressive

 

[capacitorless]

Makes lots of sense. I just started geeking out on the wikipedia article (which is super informative). I originally figured that a current limiting resistor would prevent damage, but it's not like that.

 

[RoadrunnerOZ]

When reverse voltage is applied the capacitor will start to boil inside - the small voltage applied by a multimeter won’t cause damage but a higher voltage will - resistor or not. Vishay indicate that a reverse voltage (Urev) up to 1.5V is permissible but no more than that - see page 3:
https://www.vishay.com/docs/25001/alucapsintroroederstein.pdf

 

[transient]

If the voltage is too high at the correct polarity it’ll blow holes in the electrolyte and foil, if reversed the capacitor will blow out the pressure sealed base even well under rated voltage - do not apply reverse voltage to polarised electrolytics

Most electrolytics will survive reasonable short-term overvoltage and short-term reversal for testing purposes, and the OP only required to test one, even if sacrificial, to verify the marking. It is necessary to exceed the recommended 1.5 volt threshold to establish that the capacitor is reverse-biased.