Capacitor

Has anybody out there made Jan Ridders ignition device from a battery powered gas lighter? If so, where did they get the capacitor from? Radio Spares are to expensive, you have to buy a pack of five. Farnell are better they will supply just one. This capacitor is polyester whereas Jan Ridders is foil. Will this do?

What value and spec? Maplins are worth trying - sell you one if they have in stock.

It should still work but not quite as well (reduced output). Foil capacitors are preferred for circuits producing high current pulses, like an ignition. They seem to be harder to find and/or more expensive than other types. I'd be tempted to try a polyester and stick with it unless there's an obvious problem.

What is the supposed performance difference between a polyester (film) cap and a foil cap? There are some very good polyester (PET) caps about so it's hard to generalise. Or is it easy to generalise?

Try Rapid on Line, the are on the web, and deal with normal people, and sensble prices.

Yup, it's much too easy to generalise. Using thicker foil is supposed to reduce internal resistance, but I got that from a last millennium handbook. It's so hard to keep up-to-date with electronics. Those audiophile forums know all about capacitors though, NOT!

I would guess that foil has a much lower ESR (equivalent series resistance) allowing more rapid discharge=bigger spark. Certainly comparing the ESR figures for two capacitors would be the way to decide all other things being equal.

Not really. It's more about how the connections are made and the loss tangent (quality factor) of the plastic itself. Very difficult to quantify the ESR of a high quality film cap. Even electrolytics can have ESRs in the mR region.

It's usually ESL (inductance) that's the problem. When you have to realistically expect ESLs of the order of 10s of nH, the ESR becomes a secondary limitation. Check out "bus bar capacitors" used for IGBTs to get an idea what they look like.

Edited By Muzzer on 10/12/2016 13:58:21

I have a couple of surplus 1uf 400v caps, I believe they are metallised polypropylene, they came off a couple of circuit boards that were used on Worcs Greenstar boilers, they were part of the ignition firing circuit, as they provided the pulse to the ignition electrodes. I used to work there as a mechanical engineer in the R&D.

They are similar to Farnell part no. 121-5552

If you drop me your name and address, I can post them to you to try

Angus

A polyester capacitor IS a foil capacitor, but the converse is not necessarily the case.

Foil refers to the electrode material and hence implies the construction, polyester refers to the dielectric.

I don't know details of the application, but in general polypropylene has better ESR and pulse performance than a polyester, and will be physically larger for the same rating, and more expensive. So I am sure polypropylene would be an excellent choice for the application, but polyester would probably be OK providing you are not pushing the ratings too hard.

Martin

I would like to thank all those experts who answered my request for information regarding two different types of capacitor. I now know the difference between foil and polypropylene, also what ESR means. Though I must point out I'm not an electronics expert. Merry Christmas and a Happy New Year to you all.

Victor.

Foil can be a lot better for various reasons but a lot could depend on who made them. Wima make decent polyester capacitors. I think I might be inclined to try one of those with a suitable rating. They did do other types too and various ranges.

It's been a very long time since I worked on ev's where controlling and even adding spikes was often important but I vaguely recollect that wima quoted volts per sec ratings or something that gave an indication of esr and inductance. If so the higher the better.

One rather large capacitor that would probably blow a 1/4" steel shafted screw driver to bits if it was used to discharge it had to be foil.

John

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