Capacitor Polarity how to tell

[ahenkel]

I have a power supply board for an LCD with bad caps. I desoldered the caps. I however failed to mark the polarity before I pulled them. Is there a way to determine how to install them. The board has markings where the caps go. Its a half shaded circle. I'm wondering does the shaded side indicate negative and the non shaded positive.

 

[PottedMeat]

i believe the shaded side is positive like this picture:

http://www.vonwentzel.net/ABS/Repair/PSupplyCloseUp2.jpg

when in doubt use one probe of a multimeter to known ground and the other to the pins of the capacitor holes.

 

[ahenkel]

ah ok that makes sense. Doesn't look too complicated thanks

 

[Colt45]

I'm pretty sure I've seen boards where negative is shaded, it's kinda up to manufacturer I suppose.
Try and trace from ground with a multimeter, if possible...

 

[PottedMeat]

I'm pretty sure I've seen boards where negative is shaded, it's kinda up to manufacturer I suppose.
Try and trace from ground with a multimeter, if possible...

or check the silkscreening under unreplaced electrolytic caps, reverse connected caps can fail in a spectacular way. i had a pair of ~20mm diameter caps pop through their pressure release tops along with a load of smoke and sparks. the enclosure was covered with spattered paper and stank of ammonia.

 

[ahenkel]

Hmm well seeing as how its just a PSU for a monitor I'll assume its gonna be standard positive is shaded. Hell if nothing else i'm out a broken monitor I got for free.

 

[PM650]

Asus mobos use shaded for positive, but every other silkscreen I've seen uses shading for negative. It's almost certain that the caps on the secondary side will all share the same ground in an lcd supply, just check to see if it's shaded or unshaded that are all connected together. The negatives won't be coming out of any diode packs either, usually goes directly back to a transformer winding (for a positive supply).

 

[William Gaatjes]

i believe the shaded side is positive like this picture:

http://www.vonwentzel.net/ABS/Repair/PSupplyCloseUp2.jpg

when in doubt use one probe of a multimeter to known ground and the other to the pins of the capacitor holes.

Indeed, the best way to be sure. Measuring is knowing... Guessing is missing...

And, when using this technique on other similair looking capacitors still present on the pcb , you might find out what the ground is and therefore the marking of the capacitor.

 

[William Gaatjes]

I have a power supply board for an LCD with bad caps. I desoldered the caps. I however failed to mark the polarity before I pulled them. Is there a way to determine how to install them. The board has markings where the caps go. Its a half shaded circle. I'm wondering does the shaded side indicate negative and the non shaded positive.

A normal legged electrolyte capacitor :
http://www.farnell.com/datasheets/14936.pdf

On this pdf, the ground is connected to the pin of the capacitor closest to the white stripe.






A tantalium SMT capacitor :
Who ever invented the casing of this capacitor did not like standards. Tantalium capacitors have the pin with + polarity marked with a stripe.
Confuse a tantalium with a aluminium elctrolyte capacitor and you get a very large BANG !

http://www.farnell.com/datasheets/80703.pdf






A aluminium electrolyte capacitor SMT model 1:

On this pdf, the ground is connected to the pin of the capacitor closest shaded half of the capacitor. In the datasheet it is the purple spot.
http://www.farnell.com/datasheets/31865.pdf





A aluminium polymer capacitor SMT :
Who ever invented the casing of this capacitor did not like standards.
On this pdf, the + is connected to the pin with the + marking. Therefore, the ground should be connected to the other pin.
http://www.farnell.com/datasheets/437475.pdf


The only way to be sure is to measure and look up some datasheets. Although it is all pretty standard. At www.farnell.com you can usually find a lot of components and corresponding datasheets.

 

[silverpig]

Can't you just see how the other capacitors are coded?

 

[Shawn]

Well you'll know if you have them in wrong. They explode.

 

[bobsmith1492]

Well you'll know if you have them in wrong. They explode.

Probably not, actually, but it IS possible. And by explode you mean swell up and rupture with little fanfare.

 

[ahenkel]

I got it figured out. On the PCB silk screening the shaded portion is negative so all I need is to find time to get some new caps.

 

[bryanl]

I got it figured out. On the PCB silk screening the shaded portion is negative so all I need is to find time to get some new caps.

Good luck, but try to measure the voltage across some of the replacements because when installed backwards they don't always explode immediately but can take several minutes. And some boards are marked oppositely, like those made by Asus.

 

[bobdole369]

One side will tend to be more or less short to chassis, or another known ground. That is the negative side.

 

[davesaudio]

Probably not, actually, but it IS possible. And by explode you mean swell up and rupture with little fanfare.

When the wife comes screamin' down the stairs see if you are still alive:
well, we call that "explode"

 

[johnnydrinkawater]

I have a power supply board for an LCD with bad caps. I desoldered the caps. I however failed to mark the polarity before I pulled them. Is there a way to determine how to install them. The board has markings where the caps go. Its a half shaded circle. I'm wondering does the shaded side indicate negative and the non shaded positive.

In electric components, usually anything with a distinguishing mark (stripe, etc.) is the positive lead/terminal.

 

[bobsmith1492]

In electric components, usually anything with a distinguishing mark (stripe, etc.) is the positive lead/terminal.

Nuh uh.

Diodes - stripe marks cathode, which is the negative side when current is flowing unless it is a zener in which case a typical "zener" configuration will have the cathode positive.

As Gaatjes pointed out, capacitors can go either way.

The only thing that holds true for electronic parts it to read the datasheet; then, test it. Then test a few dozen. Then test them over temperature. Then you might be ok...