Will these Capacitors Work?

[Tubs]

There is more to it than just the voltage as Tom explains.
I bought some .1mf that were 1000 volts for the Super Elto
ignition. Testing I could get a spark but the motor wouldn’t
run. I guess they were too slow as Tom describes.
Let us know how it works out.

Tom wrote:

There’s one more specification for a condenser, that’s the dV/dT, or how fast the component and move the voltage. To function in a magneto circuit, the value has to be pretty high, a lot higher than most uses. I think if you search this site a bit more, you can find a little more information on which ones have worked well in magneto applications. (Try “MKP 1839”.) I don’t know what the markings on the one shown above mean, so I can’t rule it out. The Vishay MKP 1839 HQ series should be OK.

Tom

Tubs

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

• This reply was modified 4 years, 5 months ago by Tubs.
• This reply was modified 4 years, 5 months ago by Tubs.

[outbdnut2]

There are also specs called power factor, leakage, etc.; and some internal chemistries respond in different ways to fast-rise pulses. You will drive yourself crazy digging into all those specs, trying to understand how they apply to magnetos and it still may not work. Of course you want non-polar capacitors, but darn near all of them below 1 ufd are non-polar so not an issue. Most metal film capacitors will work, so buy them and try them. I’m a retired electrical engineer/circuit designer, and for magneto use, I don’t bother with anything beyond capacitance, voltage, and temperature (which is ambient temperature. Temp rise from current inside the capacitor is insignificant by comparison. Things get really picky if you are designing a circuit for high frequency radio waves in the megahertz region, and there is a whole ‘nuther set of rules for capacitor selection if the capacitor sees the 120 volt AC line in a UL or CSA approved device. Keep it simple!
Dave

[Buccaneer]

Out of curiosity, I looked for a dV/dT number in the specs, but only
found that the “big ones” in question, 338-1160-ND
and the smaller one’s I’ve been using, the BC2609-ND both show
“Applications – High Pulse, DV/DT”.

I presume that means they both are suitable. I imagine
there’s an actual spec number somewhere for the DV/DT, but
I couldn’t find one.

I’ll let you know if the big caps work when I get that far.
Thanks for all the replies!

Prepare to be boarded!

[RICHARD A. WHITE]

This stuff fascinates me to no end…

http://www.richardsoutboardtools.com
classicomctools@gmail.com

[Tom]

The dV/dt value on the capacitor in question here is something like 1200, if I recall from when I looked it up yesterday. That should be fine. I believe the MKP1839…HQ series is somewhere around 1000. I’m sure there are other specifications that apply, but since you have them on hand, give them a shot on a hot day and report back.

Regarding a capacitor tester, the Heathkit IT series tester cited above is an excellent way to go if you can afford one. They are quite expensive when compared to the lesser Heathkit C-2 or C-3. I’ve rebuilt several C-2 units and they are more than adequate for my purposes. They test up to something like 400V and can also be used to measure resistance and capacitance with better than 10% accuracy if capped correctly. Even the C-2 and C-3 units consistently go for $75 to $100 on the bay, although one slipped by for less than $40 last weekend.

As mentioned in a previous post: “As an aside, I recently went through a bin of condensers that had been saved by a friend. There were over 50 condensers. I tested them at 200 V with a Heathkit C-2. There were two good ones in the pile. One was a new OMC, the other was an original condenser from a Johnson A.”

One final note: I’ve never found a used brass can WICO condenser that was useable. Even after removing the internals to use the can as a housing, the leakage across the “insulator” at the top could cripple a rebuild. So if you’re rebuilding any of those, measure the leakage of the empty can before you stuff new guts in it. I’ve had success doing this with the Vishay capacitor after learning that I had to test the empty can. (These are the stand up condensers with the screw terminal on top and the tab about halfway down for mounting it upright. In many Scotts and Martins, replacement with a standard configuration condenser is difficult.)

Tom

[Buccaneer]

Tom, thanks for all the information!
I’ve been installing new electronic capacitors on just about all projects,
so haven’t really been testing them. I have checked some new caps
mfd ratings against their stated rating, with a digital meter, and they’ve
been spot on.

I do have a Stevens tester for checking coils, but not sure how good
it is for checking condenser leakage. Perhaps a Heathkit would be
a handier and less compact unit for testing condensers.

I agree that the Wico condensers with the screw top post are a pain
to put a cap inside. I did manage to solder the cap lead about 1/2 way
in the threaded post on one attempt, but I had to clean up the top
threads with a tap to remove some of the misplaced solder.
It’s hard to fit the cap inside the Wico can if you run the lead
on the outside of the threaded post, and end up with too sharp
a bend in the cap’s lead wire.

I did buy a couple of those tiny ceramic caps talked about recently,
but have not tried them yet.

Prepare to be boarded!

[Buccaneer]

I installed the big caps today, and put the mag and flywheel on the power head.
It appears to have good spark, but I’m nowhere near test running this motor.

Prepare to be boarded!

[crosbyman]

neat job 🙂

Joining AOMCI has priviledges 🙂

[rudderless]

If ya still can…bed them in a small amount of polyurethane glue or similar. They might move off the clamp over time….just a thought..

[Buccaneer]

rudderless wrote:

If ya still can…bed them in a small amount of polyurethane glue or similar. They might move off the clamp over time….just a thought..

I was thinking about that same thing, but was thinking clear silicone.
Any reason polyurethane glue would be better?

Prepare to be boarded!

[Author]

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