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Thread: Ballast resistor for low ohm coil

  1. #31
    Super Moderator ancientdad's Avatar
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    That would work too. The only thing with that connection would be the distance (as opposed to the already available wiring and space for a relay right near the coils) and the large ring terminal to a small gauge wire at the solenoid, but it would have the same effectiveness

    Edit: no it wouldn't work, as the starter motor would be getting feedback from the coil voltage
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  2. #32
    Member brewsky's Avatar
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    Sitll thinking............(dangerous I know!!)
    Last edited by brewsky; 11-06-2019 at 02:38 PM.
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  3. #33
    Super Moderator ancientdad's Avatar
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    When the starter solenoid releases and the bike is running, the ballast voltage would be fed back through the starter cable to the starter motor... low amps and volts, but still constantly feeding the brushes in the starter motor trying to make it turn
    Tom

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  5. #34
    Member brewsky's Avatar
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    Quote Originally Posted by ancientdad View Post
    When the starter solenoid releases and the bike is running, the ballast voltage would be fed back through the starter cable to the starter motor... low amps and volts, but still constantly feeding the brushes in the starter motor trying to make it turn
    Yep, you are right.....I knew it was too easy all along!

    It also would disable the "crank with no ignition" feature of the key switch.

    I deleted the diagram to prevent anyone from trying it and saying "they saw it on the internet so it must be right"...…………….
    and also to prevent future embarrassment!
    Last edited by brewsky; 11-06-2019 at 02:42 PM.
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  6. #35
    Sensei 66Sprint's Avatar
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    Wire it so an automotive relay separately closes both the solenoid circuit and the jumper circuit when the start button is pushed.....(start button powers relay)......
    Releasing the button opens both circuits, so no back-feed effect occurs....Simple, and inexpensive to do....
    Last edited by 66Sprint; 11-06-2019 at 03:07 PM.
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  7. #36
    Senior Member mike in idaho's Avatar
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    I would wire the relay trigger wire(#85) to the starter motor side post of the solenoid, the supply for the relay(#30) to the other post(battery side) of the solenoid. The other trigger lead(#86) can go to any convenient ground connection, and relay output (#87) can go to the coil terminal.
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  8. #37
    Member brewsky's Avatar
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    Thanks for the info guys......I've got a couple different resistors coming and will take some readings before deciding which one to use. I've got 2 different oem coils...one reads 4.5 ohms and the other 3.6 and can't find the design spec listed anywhere.
    I did find the design ignition load to be 3.5 amps so maybe work back from there.

  9. #38
    Member brewsky's Avatar
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    Picked up a 1.4 ohm ceramic ballast RU 4 ($4.99) from the local auto parts place, and a 2 ohm 100W aluminum version (2 for $4.99) online.

    Had trouble finding specs for the ceramic versions, as they are listed by auto make instead of resistance value, so I just brought my meter with me.

    The only ohm/temp spec I could find online for the ceramics was an RU 11 and it listed the "cold" resistance at 0.5 and the "operating" at 1.75!

    So who knows what the "operating resistances" of the ones I have are?

    At any rate, the manual says points closed, not running amperage should be 3.5, so that would equate to a coil resistance of between 3.4 and 3.6 ohms depending on the supplied voltage (12-12.7V) (Which happens to be the value of the old OEM one I have which is not melted)

    So either one of the resistors I have will put the coil + ballast at at least 3 ohms Plus whatever the "operating resistance increase" happens to be.

    So I believe one of these will be suitable amperage wise, still to be determined if there will be enough voltage left to adequately fire the coil.

    More when the garage temperature increases! It's 25 now outside and the state just put a bunch of #@&^% salt all over the roads!!
    Attached Thumbnails Attached Thumbnails Ballast resistor for low ohm coil-coil-resistors.jpg  
    Last edited by brewsky; 11-09-2019 at 06:49 AM.
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  10. #39
    Senior Member Rod Fryatt's Avatar
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    For what it's worth, I have been using a ballast resistor on my CD175 for a few years now with no problems at all.

    The 6v coil is 2 ohms and draws 3 amps on 6 volts. When I changed to 12v I kept the old coil and fitted a 2 ohm resistor in series. I fitted a small relay from the junk box with its coil in parallel with the starter relay coil. When energised, contacts close and short out the resistor.

    The fact that the coil gets extra current when the starter is running guarantees instant starting so it only gets that boost for a fraction of a second. Also remember when the starter motor is running the battery voltage may well fall to nearer 10v so a bit extra for the coil is a good idea.

    Ballast resistor for low ohm coil-relay.jpg
    Last edited by Rod Fryatt; 11-09-2019 at 07:27 AM.
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  11. #40
    Member brewsky's Avatar
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    Neat installation,, and good to know!

    Looks like the identical ballast I have.
    78 CB550K, 78 CB750K, 66 CA77, 74 T500

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