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Ok maybe not all, but the Nichicon UES series of bipolar "audio-grade" electrolytic through-hole caps have a short lead and a long lead..

Why?

The datasheet has nothing to say about it. This fairly recent question: Orientation of "bipolar" capacitors on PCB seems somewhat related, and I did some searching and found a boatload of speculation but nothing that stuck. So far the only thing that seems likely to me is that perhaps the manufacturing process is built around the more common polarized electro with the uneven leads. Or that the reason is common knowledge and I am once again tragically out of the loop.

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    Yeah. I think it is a carryover from polarized E-caps. – user57037 Mar 24 '19 at 05:49
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    @mkeith that seems sensible, by chance can you cite a source for this info or point to some published supporting evidence? I ask out of scientific curiousity, not to challenge your assertion. –  Mar 24 '19 at 06:09
  • No it is just a guess. If I knew the answer definitely, I would write an answer not a comment. – user57037 Mar 24 '19 at 06:10
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    The leads are different lengths as the cost of changing the machine is not worth it, or the machine is swapped between types of caps so different lengths are still relevant for the other types... – Solar Mike Mar 24 '19 at 06:54
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    This is speculation on my part but may be relevant: Rolled axial-lead capacitors (both paper and plastic-film) in days long ago were marked at one end with a band on the body of the capacitor near where the lead exited the capacitor. This band marked which lead was the most outer layer of the roll. This was occasionally important if the capacitor was used in a filter circuit - one could ensure that the outer layer was grounded just by observing which lead was connected to ground. – Dwayne Reid Mar 24 '19 at 09:49

2 Answers2

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Apart from using standard machine setup for polarized and non-polarized, the different lead lengths aid manual insertion.

The long wires will seldom be exactly parallel making it tricky to align both simultaneously. Having different lengths means that one can be inserted and the other aligned by pulling on the body of the capacitor.

enter image description here

Figure 1. Nixie tubes. Source Vannadiy on eBay.

A similar trick can be used to insert a nixie tube into a PCB where, otherwise, trying to align eleven pins at once would risk loosing one's sanity. A chosen lead is left at full length and working around the tube each wire is shortened by a mm or two. Now insertion becomes a mater of engaging each wire, one at a time. The wires are cut to length after a couple of solder tacks to keep the tube in place. I think I first read of the technique in Elektor magazine.

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The vast majority of electrolytic capacitors are polarized, so manufacturing is optimized for those. Creating a separate process for equal-length leads would increase costs, without being actually necessary.

And the capacitors are likely to be placed by a machine (or by persons who do not know about the intricacies of the circuit). Polarized caps with equal-length leads would be rejected as unplaceable and defective, so creating bipolar caps in the same shape avoids the need to have a separate process.

CL.
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