Wanted: Source for 3-Lead Electrolytic Capacitors

I believe the main rationale for three lead electrolytics was to avoid the danger of production inserting them with the wrong polarity. If you are repairing a unit it's unlikely that you will get them the wrong way round, so, just use normal (two lead) caps.

I'd be interested to know if I'm wrong!

The reason this power supply uses 3 lead caps is to achieve a certain seismic rating. The high level of inspection and burn in testing done would have detected any incorrectly installed devices. Replacing these with 2 lead caps would render it useless in this application. The replacement part must have the same form, fit, and function as the original.

If the issue is vibration resistance, then a simple answer would be a conventional 2 lead cap and liberal amounts of RTV to prevent it from moving.

Now I have seen three lead caps where the third leg is either an electrostatic drain (for high voltage applications) that is tied to the chassis ground bus, or the capacitor is actually two capacitors in series with one leg being a center tap. These are common in antique radios.

These caps are part of a critical safety system. The third lead goes to an isolated pad on the PCB and is not active. Since the original engineer selected this component I do not have the option of changing the style or adapting a different part.

The problem is this is not a repair, it is a rework. I cannot change anything on this board, even the solder I use must be dedicated by our metallurgical lab. No cleaners, solvents, not even tape can be applied to the boards. The parts used must be the original components, and they must pass dedication testing, or be an equivalent replacement that the customer's procurement engineering department approves of.

The approval process makes mating elephants seem fast, painless, and cheap.

It looks like these must be specials, made for this particular application. Are these multiple section or single section/value? The third lead must just be and anchor point.

I can not think of any modern type to fit your requirements, I am very good at tracing odd ball components. can you not use a radial type laid on its side and some hot glue to bond it down? Why is it so critical? Is it a weight/mass or voltage/leakage issue? There is always a away around any problem if only enough money and time are applied to it. What is the PSU used for? You mention seismic considerations and it being safety critical? Is this Nuclear related? Or is that going too far. Sorry for all the question but a vague question requires some more information, if I am going to be of any real help.

Yea!!! You get 2 gold stars!

Seriously it is part of a critical safety system from a nuclear power plant. That is why nothing can be modified in any way. If a part is not the exact part and exact part number that was in the original then a complete engineering review must be done. If a manufacturer adds NPB to a part number to show its lead free that becomes a different part. Even if it's the same part and it never contained lead it becomes a 'new' part and must be retested all over again.

I have contacted the company that made the original part and they are not willing to make any more replacements. The original module manufacturer is gone and I am hoping a custom manufacturer is willing to make what I need. I can get a new part approved as long as if has the same "form, fit, and function" as the original.

I also used every search engine I could find to locate a manufacturer with any 3 lead caps; several have them in their catalogs but no longer produce them. They are single section caps, 7.5, 25, and 50 VDC and really unremarkable spec wise except for the case.

Any Ideas?

I think you will have to go through the pain of getting approval for a replacement part. I used to work in avionics so I know about the problems of having to fill out all the paperwork. If you have turned over all stone an found nothing now is the time to up date and fit more modern alternatives. The thing to do is to find the nearest alternative and at least two other types that will fit if pushed. Then make a request to vary the spec. I have had to match components for many items of critical equipment were size value and voltage rating were vital. It can be done. It just requires patience. I have even completely rebuilt equipment from scrastch including the circuit boards. Etching them and drilling stuffing and soldering.

What sort of instrument is this capacitor in? Manufacturer? Model number? I would query the manufacturer first... to get the capacitors manufacturer and part number. Also, the instrument manufacture might be able to get you original documentation on the instrument. If you can get the cap manufacturers part number, from the instrument manufacturer OR it might be stamped on the case of the capacitor itself then you could go to the cap manufacturer to look for it. Also if we had case dimensions... capacitance values for each section at what working volts (for each section) who knows... someone might have one in his inventory archives (junk drawer).

More info please...

Bill

Hi Bill;

The original module manufacturer is no longer around, the company that made the original part no longer produces 3 lead caps. Due to some very specific age requirements we have on electrolytic caps I need to find a source of new parts that will pass the approval process.

I've looked all over the web checking for a manufacturer that carries or will make them and have struck out so far……..