Re: gEDA-user: Polarized capacitor question

[Evan Lavelle <sa212+geda@xxxxxxxxxxx>]

Matthew Sager wrote:
> I was asked this question the other day and I did not have a good answer 
> to it, and have not been able to find a good answer for it.  So I 
> figured I would see if anyone here knew of a good answer.
>  
> All tantalum capacitor that I have ever seen marked the anode(+).  On 
> the other hand all electrolytic capacitors that I have seen marked the 
> cathode(-).  Is this an industry standard or is it done this way just 
> because it has always been done this way?

I think this is historical, but I can't find any references. I think, 
historically, Aluminium caps had the negative foil wound on the outside, 
and the positive one on the inside. A bar marked the terminal which was 
connected to the outer foil. This was important because you wanted to 
minimise the chances of arcing in your equipment, and you might 
potentially arc from the outer foil to a chassis, so you needed to know 
which terminal was connected to this foil. Tantalums were normally wound 
with the positive foil on the outside, hence the bar at the positive end.

I think, but again I'm not sure, that some chip tantalums may actually 
have a bar marked at the negative terminal.

Evan


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