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3-Lead Electrolytic Caps?
Old 9th November 2009
  #1
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3-Lead Electrolytic Caps?

Hi,

I was replacing caps in a power amp recently and had an appropriately sized pair which were pulled from duty at a very early age. Problem is, they have a negative pin, a positive, then a third unknown. It registers nothing on my ESR meter. Can the two normal connections be used as a regular polar electrolytic or is this a whole different animal?

Thanks!

- forgot- > These are big caps BTW (10,000uF 50V)

Last edited by Jidis; 9th November 2009 at 02:58 AM.. Reason: specs
Old 9th November 2009
  #2
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JohnRoberts's Avatar
 

On pin may be purely mechanical for more robust physical attachment to PCB.

May be attached to outer can or not... If you don't measure anything from either other pin, I'd ground it and see what happens.


JR
Old 9th November 2009
  #3
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Quote:
Originally Posted by JohnRoberts View Post
I'd ground it and see what happens.
That sounds frightening John. heh

I'm not sure I could get a ground connection to the pins easily. An anchor does sound right though. All I could find on the net was junk about smaller three wire caps for ceiling fans or motors. These things look totally normal other than the third leg.

Thanks!
Old 9th November 2009
  #4
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djmukilteo's Avatar
there's no such thing as a 3 wire cap
capacitors have 2 leads.
if these are the large silver can types....that third tab is in fact for attachment to a circuit board...in fact that tab typically has detents at the end of it that can be twisted after insertion into a circuit board for stability
Old 9th November 2009
  #5
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djmukilteo,

It's just a regular style pin on these (I'll attach a picture), but I'm figuring it's as you guys guessed now and just there for extra support.

BTW John- I scratched into the plastic when I was removing some glue and the can definitely has no connection to that extra pin on these. I put a meter on it.

Take Care
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3-Lead Electrolytic Caps?-100_3151.jpg  
Old 9th November 2009
  #6
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Hi
Old catalogues for Philips and many other manufacturers showed caps with 2, 3, 4 and 5 pins, all for single caps. They are added for rigidity and I would actually leave the extra pins soldered but not connected to anything as there may be a possibility of inadvertent internal connection.
You could go to the website for your particular cap to get the 'lowdown'.
The current trend seems to be to use 2 pin caps and throw some glue on it after assembly if it may be subjected to rough handling.
Matt S
Old 9th November 2009
  #7
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JohnRoberts's Avatar
 

In case it isn't apparent, large radial caps put a lot of stress on the solder connections due to mass and moment arm and such... After shaker table testing you can often find parts like this rolling around inside the chassis.. It is not uncommon to use hot melt or some other mechanical attachment for extra strength in high vibration environments.

Axial capacitors were much preferred for mechanical attachment strength but they fell out of favor for multiple cost reasons (parts more expensive and use more PCB area). At one point after they were in decline, my old employer was about the largest customer for axial caps in the US, but they were not immune to the cost pressure, and went radial too.

JR
Old 14th November 2009
  #8
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Weird.... Got a pair of 1970's Pioneer amps I was recapping. One was the destination for these three lead jobs. I bought brand new Nichicons for the other. I get to pulling the caps to put in the new Nichicons and lo and behold... more 3-leaders. I think the destination amp for those two I was holding is the same deal if I'm looking at the correct pads. Might not have to clip any pins afterall.

BTW- The amp the 2-lead Nichicons went into had the third anchor pin on an isolated pad. Looks like in this other amp they may be tied to the cap's negative leads.

Take Care
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