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What cap. do I need? Answered

I was looking at this instructable ignition coil and I built it without a capacitor, it worked fine except the coil gets hot after even very short run-times.  So he confirmed that I needed a capacitor to stop the overheating. So I added one.  I used the kind he told us to use (film) and I tried multiple ones, but none worked.  He told us not use the metal can type so I didn't at first, but later I got a really big one and tried it.  It was the only one I got to work, but then it exploded, so now I don't know what to do.  He hasn't gotten back to me, although it hasn't been very long so I don't accuse him, I would just like to know.  The capacitors are storing a charge, its just not running the coil for some reason. 



Best Answer 7 years ago

The lamp dimmer has a strong DC component ( Heating your coil ).
A capacitor blocks DC current flow through the coil.
The capacitor must be an AC type.
I would use A motor start cap just like in the picture.
If you want more spark put two in parallel.



My goal is to avoid buying, but anyway, will two capacitors with different polarities work? And also why would a can-type capacitor (don't remember the real name) work if they are the most polarized caps I have ever seen.

If they are AC capacitors, "different polarities" doesn't make any sense !
Electronics is a strong, verbage specific language, please use it well.

You can join in parallel, different capacity values ( yes they add ) but
the voltage,  that is the lowest, is the weak link.

I hear you don't want to spend $$.  I, nevertheless recommend you do !
Start caps can be had at motor rewinding shop$.

Sorry, without pics, can't advise about your descriptions of a can-type
which you remember not.


I mean't two DC caps hooked positive to negative to form one capacitor that could take voltage from either side. The can type are the blue or black ones and the right name is electrolytic

Congrats, I understand you now, a very much more detailed description.

But I have to nix it. Even if the capacitors were Identical they would not
share charge and especially Voltage in series without a pair of
resistors to balance the capacitors.

PM me your address and Ill give you a couple caps.    Alex

Wow, Thanks Man!!!

I have a big orange box looking cap, it says .47 M and 275V~ and also has 40/100/21, X2, and some other stuff. It should be AC, and should work, but it doesn't. It held a charge one way, but not the other. It says MPX/TSC along with MKP and GMF.

It looks like you need a 1µf-3µf high voltage (>100v) mylar film capacitor. Have a look on ebay if they don't have any in your local electronics store.

I have a bunch and none worked, voltages from >1k to 100

Are you sure the value is correct though? What number does it say on the side?

Well, I have a bunch, one rated for 1600v, one that says DC400, a big orange box one that says .47 M and 275v~ so it should be an AC cap, I have a big ceramic disk (penny size) that is rated for 1kv, I have a blue 100v film cap, and a couple more.

You don't say what cap you are expecting to use, but you need one which is designed to work with at least 400V.

Well I am using the mylar film type ant I have a 1600v one, 100v, 400v, and some others.

Um...what input amplitude are you using? Automobile coils want to see a 12V peak waveform. (or in the neighborhood of 12V) If you overdrive it, you will get a higher output voltage, but you will also get overheating.