Coil Tester Help

Discussion in 'General Electronics Chat' started by buygone, May 20, 2008.

  1. buygone

    Thread Starter New Member

    May 15, 2008
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    Im having trouble nutting out a coil tester, below is the schematic I was following its one put together some time ago by another member.
    [​IMG]

    Ive tried some dry test runs without success.
    My shopping list was as follows C2 .22uf 275vac40/100/21, the relay is a D12Volt DPDT 10A/24vdc/220VAC, the D1 diode was a 1N4007, the C1 cap is one from the type of ignition systems Im going to test the coils from. The R1 resistor is a 10W10ohms. Power supply is from a 12volt battery.

    If I try testing the circuit and runnig the + through the C2 resistor.... no joy, if I bypass the C2 the the relay buzzes furiously. Which I believe is not so good. Am I using the wrong resister in the C2 position.
    I will be testing a lot of coils and dont want to be burning out relays frequently.

    I'll be testing old magneto coils with this unit if I ever get it running and yes I am a novice so please be gentle.
     
    Last edited: May 20, 2008
  2. SgtWookie

    Expert

    Jul 17, 2007
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    That schematic is so small that I can't make out any details on it. Could you please post a larger version?
     
  3. buygone

    Thread Starter New Member

    May 15, 2008
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    Sorry about that, I hope this ones ok.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Here, try it like the attached schematic.

    Note that the relay's coil is now 5V.

    I'm not certain what voltage the newer electronic ignition coils run on.

    The older (pre-electronic ignition) coils would run on 12.6v when starting (for a hotter spark) and then 8v when running. Still, this should provide enough primary current to fire a plug that's gapped to 0.035"
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Why the change Sarge?
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    A coil driven with too-high an EMF makes for a better chopper.:) Since the relay coil current is not continuous, and the buzzing only lasts a short interval of time, the relay actually holds up. I had a spark generator in high-school using a similar principle.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Bill, I put together the 1st schematic about 8 months ago for someone else. When I first looked at it again, it confused ME! :eek: ;)

    Afraid I didn't draw it very well - just threw it together. It should still work, but it's somewhat confusing to look at. The OP was having problems getting it to work, for what reason I don't know offhand.

    The newer version is much more simple, and one can find a 5v relay at any Radio Shack. 12v double-pole relays can be tough to find in some towns.
    But in this new version, the relay won't buzz if the coil has an open primary winding.
     
  8. buygone

    Thread Starter New Member

    May 15, 2008
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    0
    I have had sucess, I persisted with the original design simply because I dont like to give up. I tok the whole thing to an autoleect near my work and asked him to double check.
    I had wired up the relay incorrrectly once sorted it works well.
    What I would like to know if anyone can help me is
    When I hook up a suspect coil the relay chatters for a short time without any arcing across the plug, then the sound from the relay changes folled by a few sparks across the contacts. the back to the original chatter and so on.
    Is this a indication that the coil has brokenbreaking down in the secondary winding and finally builds up enough charge to arc.
    I tried a couple of different (new) condensors with the same result. ?
    Any thoughts ?

    cheers
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Well, as soon as the relay makes it's first click, you should see a strong spark from the coil at the test spark plug, and you should continue to see a strong spark at the test spark plug as long as the relay is clicking. Letting the thing run for a while will warm up the coil. Sometimes, coils will work OK when they're cold but fail once they get warm; sometimes they'll work OK when they're warm but be intermittent when they're cold, and sometimes they won't work at all!

    It sounds like that particular coil has an intermittent secondary winding, and is a good candidate for the scrap heap.

    You might keep a "known good" new coil on hand for testing of the coil tester. After the coil tester has seen a lot of use, the internal switching contacts will get burned, and allow less current to flow through the coil under test. It's easier to have a known good coil to test the tester with than trying to use "real" test equipment such as ammeters, voltmeters and oscilloscopes.
     
  10. buygone

    Thread Starter New Member

    May 15, 2008
    4
    0
    Thanks for all the Help Sgt
    Last plea for help I promise:D
    I did as you suggested and tried the tester with a newly rewound coil, funny thing is it behaves the same, relay chatters like crazy then changes note arcs for a couple of seconds the repeats this pattern:confused:. The odd thing is Ive notice is the 10ohm resister gets hot... I mean smoking hot.

    Once again this site is fantastic its like being lost in the desert with a satalite phone and a chopper nearby.
    You set out with good intension and get lost but help is only a message away.

    Regards
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    I like the concept of it not working if the coil is open, an immediate audible feedback.

    If the coil is shorted it seems like the relay is at risk though.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    The resistor should be a wirewound "ignition ballast resistor", you get them from an auto supply store. I think they're in the neighborhood of 25 Watts! They look something like this:
    [​IMG]
    If they want to know what car you have, tell them it's a 1970 Plymouth 'Cuda. They will of course want to see it. Tell them you're building it a piece at a time, and this will be your first piece.
    My wild guess at 10 Ohms may not be very close.
     
    Last edited: May 24, 2008
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