How do I match condensers and spark coils?

[wmohat]

I have found listings of condenser values that
should be used for old outboards. However, what do
you do if you have an old outboard with a dead spark
coil? You can get a "general purpose" spark coil,
but at that point, how do you find a matching
condenser for it? Most manufacturers of
replacement coils do NOT list what size condenser
they should be used with. (This is especially true
for magneto coils.)

If you use too small of a condenser for a given
spark coil, you’ll burn your points. If you use too
large of a condenser, you’ll get too weak of a
spark, if you get spark at all. So, this is a bit
of a problem. I suppose I could just swap
condensers until I get one that seems to work, but
that’s hardly scientific or accurate in any way,
almost guaranteeing a less that optimal result.

Now, I do have an impedance bridge, so I could
measure the impedance of my spark coil primary.
But, even if I have that value, how do I match that
to a compatible condenser? Is that how it is done,
or is the process not that simple? ( I strongly
suspect it isn’t.)

Alternatively, I’m guessing that I could use
an oscilloscope, and measure the voltage spike on
the primary windings when the points open up. I’m
guessing that I should probably want to see between
50 and 100 volts peak across the
points………….so I could swap capacitor values
until I saw saw that measurement. That procedure
would probably work, and it’s what I would do if I
didn’t have any other information.

But, somebody, somewhere has to know how to
match coils and condensers. What’s the recommended
procedure? Who knows? Where can I find this
knowledge?

***********************************************************************
Bill Mohat MS/CIS, BSEET, CCNA

"You can avoid reality, but you cannot avoid the consequences of avoiding reality." --Ayn Rand **********************************************

[chris-p]

I cannot think of an outboard off the top of my head that doesn’t have its condenser value listed? Im sure there are the rare ones, haven’t encountered one as of yet though in my collecting and restoring.

Do you have a motor in particular?

[joecb]

Here goes my $0.02 on condensers. If you look at the listings of recommended condensers for outboards (and small motors) you will see that with few exceptions call for .19 – to .22 microfarads. Since the very best quality foil capacitors available these days have a rated value plus / minus 5 % , I think that the old paper /wax caps of days gone by are lucky to be +- 20%, if that. Based on this I have found that a high quality foil cap of 0.22 mf will work just fine I everything that I have tried em’ on.

Joe B

[frankr]

wmohat, you are thinking too much. If you are considering the OMC Universal Magneto coil, they use the 580321 condenser, 0.20 mfd. If some other mag coil, specify and one of us with a Merc-o-Tronic book can look it up for you.

BTW, you would be amazed at the peak voltage.

Actually, the points open at the time of voltage reversal as the magnets pass by the laminations. I’ve never seen it on a scope. Never got that concerned.

[george-emmanuel]

Those of us who are swapping coils on engines where there are no NOS or used replacements use coils from other engines. All you have to know is which engine you are borrowing a coil from and look up the replacement condenser. I agree with Frank. There are times when folks over think the issue.

George

[PugetSoundBoater]

I will admit i am very guilty of overthinking things. I like to call it "research" 😀
Jim PSB

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[wmohat]

In reviewing the replies so far, I have the following observations:

1) JoeCB: yes, .22uF will certainly work. However, if the original condenser was supposed to be
.68uF, you will get FAR more voltage across the points when they open than you should.
(about triple). This absolutely will shorten the life of the points. .22uF is far better than
nothing, but ….I still think you should be reasonably close.

2) FrankR: The points open AFTER the voltage reverses, and the voltage spike hits maximum (or just
a few degrees later, when coil current is at maximum.) I took pictures on a scope,
and wrote a technical article on the topic. (I believe it will be published in The Outboarder
in October). If you read that article, I guarantee you’ll find more information than
you want to know on this topic.

3) George: Yes, if you get a coil from another outboard, you can look up the matching condenser for it.
However, what you do NOT know is the strength of the magnets in the flywheel, and the
mechanical size of the poles. You might get up to 4 times more (or 4 times LESS) voltage induced
in the replacement coil compared to your original, and that might well determine if the
original condenser is appropriate or not. You might well have to use a condenser of a different
size than was used in the original application.

********* I still think that when the points open up, you want to see about a 50 to 100 volt "inductive
kick" across the points. With NO condenser, the "kick" will be much higher (several thousand volts,
which will burn the points rather quickly). And, with too large of a condenser, you might only get 10
to 20 volts across the primary, which won’t give you enough voltage to generate a spark at the
secondary. I’d like to see some documentation from either a coil or condenser manufacturer,
that describes how to "size" a condenser correctly.

***********************************************************************
Bill Mohat MS/CIS, BSEET, CCNA

"You can avoid reality, but you cannot avoid the consequences of avoiding reality." --Ayn Rand **********************************************

[joecb]

I’ll be looking forward to seeing that article. Often wondered about how to use a scope to evaluate the performance of a magneto.

Joe B

[chris-p]

Not sure if you saw my post, but which motor are you referring to? Spent a while trying to come up with a motor that doesn’t have the condenser value listed, but couldn’t think of one.

You may just be overthinking this.

Think the highest condenser I ever had to buy was for the P30, P35, P40, P45, PR40, TR40 which use the 72-651 rated at 0.4 (0.36 to 0.44)

As Joe states, good old 0.22 almost always gets her done. 90 percent of the time anyways.

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