Condenser Capacitance

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  • Buccaneer

    US Member - 1 Year (includes $3 online payment fee)
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    #228107

    I happen to read Bill Mohat’s interesting article, chapter #3 in the new “Outboarder”
    today regarding condensers. It was timely as I was also going thru
    another magneto from a Scott Atwater 7.5 hp Bail-a-matic.

    The article suggested if you didn’t know the correct mfd value of your
    condenser, you could check your old condensers vial a digital
    capacitance meter, which I did on both Scott mags I’ve worked on lately.

    The specs for these Phelon condensers (FG-471-B) show they should
    be in the .12 to .16 range.

    Today’s condensers checked out at .28 and .31 mfd’s, and the two on the
    previous mag with the same condensers, checked out similarly.

    Mr. Mohat suggested checking the mfd’s on your condensers “before” any
    load testing on them, as further breakdown from load testing will effect the
    capacitance readings. I don’t believe the article said if the readings will get
    higher or lower as the condenser’s deteriorate.

    What’s been your experience? Do old condensers eventually read higher or lower
    on the mfd testing?

    While working on the previous mag, I tested my meter on new .15 capacitors and they checked
    out at .14, so my meter is “close”.

    The article also suggested from testing, there’s quite a bit of leeway in using
    the correct capacitance condenser, and a rating a little “higher” is better
    than “lower” than the specs state.
    Seeing how I was out of the axial .15 caps, I installed some .2 radial caps on this mag.
    I’m guessing it will spark just fine.

    DSCN0229

    Prepare to be boarded!


    frankr

    US Member - 1 Year (includes $3 online payment fee)
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    #228116

    The general rule of thumb is +/- 10%-20% is acceptable for most non-precision circuits. And paper capacitors certainly are not precision. So 0.15mfd + 20 % comes out to 0.18 mfd. I don’t think anybody is going to get upset over that extra 0.02 mfd.


    Mumbles


    Replies: 5764
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    #228117

    The uF rating of a cap gets higher as they age, never lower. And just like us, they start leaking with age.


    frankr

    US Member - 1 Year (includes $3 online payment fee)
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    #228119

    And at my age, I (ahem) leak pretty badly.

    Notwithstanding, I read Mr. Mohat’s article. He did a pretty good job at saying what he was trying to say, and I applaud him for it. Coil/Capacitance (LC) circuits are pretty complicated and magneto ignition circuits are no exception. There are a lot of things happening every time it produces a spark, and it is hard to explain it all in terms understandable to a novice. In fact, I get some more grey hairs every time I think about it. And I have a diploma that says I am a TV repairman (Not). But to share a grey hair, when the magneto points open, electrons flow into the capacitor and the voltage across the capacitor increases with time It is mighty fast, but not instant. When the time is up, the electrons flow back out in the opposite direction in series with the coil. It is that reversal in direction that induces a spark voltage in the secondary winding. A way too big (mfd) capacitor will not charge up to as high a voltage in the time available as would a small capacitor. Also, a leaky capacitor loses energy through the leak just as a leaky bucket takes longer to fill with water.. On top of all that, there is an issue with RPM.

    OK, I’ve got everybody confused now, including myself.

    KISS is a good thing.

    • This reply was modified 3 years, 2 months ago by frankr.
    • This reply was modified 3 years, 2 months ago by frankr.

    Buccaneer

    US Member - 1 Year (includes $3 online payment fee)
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    #228123

    Thanks for confirming my “guess” that the mfd readings on a deteriorating condenser goes
    “up” in reading. For fun one of these days, I’ll dig out the Stevens tester and do a leakage
    test on those four condensers. Not sure how good that function works on my Stevens,
    but it’s time to play around with it. I always change out the condensers on all my old
    motors, so I never really bothered with that test.

    I always wondered how the outboard builder’s figured out “what capacitance” condenser
    to match up with the coils they used, pre – oscilloscopes……. assuming they didn’t
    have said scopes at the beginning.
    Update……… just looked up “Oscilloscopes”, and it was invented by a French guy
    back in 1893……. but was an ink charting affair, not the current type with a monitor!

    Prepare to be boarded!


    jeff-register

    US Member - 2 Years
    Replies: 1562
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    #228124

    Bucc,
    If I suggest, use Vishay caps. A few years ago I spoke with a Vishay rep & he advised me to use these in a magneto setting. Look them up at Newark.com, an electronics warehouse. Part # is MKP1839/4/22/08/4/HQ for a .22 cap If you were closer I would give them to you. My health stinks so I’ll never use them. I’m way over in Arizona, Yea it’s a bummer to be here.
    Jeff


    labrador-guy

    US Member - 2 Years
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    #228125

    Ok guys what happens when you run the wrong capacitor in your motor? No spark? No performance? Coils get ruined? I see in most of the charts that the .2uF is quite common. Spark is generated the same way by magnetism.

    BTY some of us have not received our magazine yet!

    dale


    dave-bernard

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 2014
    Topics: 30
    #228126

    points pit quicker. to high they pit one direction, to low they pit the other way.

    • This reply was modified 3 years, 2 months ago by dave-bernard.

    frankr

    US Member - 1 Year (includes $3 online payment fee)
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    #228128

    Ok guys what happens when you run the wrong capacitor in your motor? No spark? No performance? Coils get ruined? I see in most of the charts that the .2uF is quite common. Spark is generated the same way by magnetism.

    BTY some of us have not received our magazine yet!

    dale

    As I tried to say, the capacitor charges up in one direction, then discharges in the opposite direction, in series with the coil. Several factors combine to affect the voltage that it charges up to. A higher charge voltage in series with the coil voltage creates a higher reverse current through the coil, which results in a higher AVAILABLE spark voltage. AVAILABLE emphasized because the actual spark voltage only rises till it jumps the plug gap. Then it’s all over till next time. In other words, the actual spark voltage is rarely the maximum it can produce. BUT if it it cannot produce enough to jump the gap reliably, you get no-start, hard start, missing, etc etc.

    And of course the pitting issue is another result.

    • This reply was modified 3 years, 2 months ago by frankr.

    Buccaneer

    US Member - 1 Year (includes $3 online payment fee)
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    Topics: 1072
    #228130

    Bucc,
    If I suggest, use Vishay caps. A few years ago I spoke with a Vishay rep & he advised me to use these in a magneto setting. Look them up at Newark.com, an electronics warehouse. Part # is MKP1839/4/22/08/4/HQ for a .22 cap If you were closer I would give them to you. My health stinks so I’ll never use them. I’m way over in Arizona, Yea it’s a bummer to be here.
    Jeff

    Jeff, I’ve been using DigiKey caps. Usually get them in 2 or 3 days,
    and haven’t had any problems..
    It’s 11 degrees here right now……… Arizona sounds pretty good, lol

    Prepare to be boarded!

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