Ballast resistor for low ohm coil
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  1. #1
    Member brewsky's Avatar
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    Ballast resistor for low ohm coil

    I ordered a new dual-fire coil for my dream and it only reads 1.6 ohms across the primary.

    If I add a 3 ohm ballast in series, that should bring the resistance back up close to the original 4.5 of the stock one?

    Am I missing a potential problem using this setup?
    78 CB550K, 78 CB750K, 66 CA77, 74 T500

  2. #2
    Senior Member Alan F.'s Avatar
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    Have a look at this for a good technical breakdown of why and how:

    Resistor Pack for 3-ohm Coils, SOHC4shop.com
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    Member brewsky's Avatar
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    So, reading that implies my setup would work, unless I'm missing something........

    Also, from the referenced site...."
    This Resistor Pack reduces this current to a safe level (2.8 amps) for the bike's stock wiring, while still allowing for a nearly 3-fold spark voltage at the sparkplugs from using these high-output coils.".

    Does this imply that a 1.6 ohm primary coil is necessarily "high-output"? The e-bay coil I have is certainly not a Dyna or Accel!
    78 CB550K, 78 CB750K, 66 CA77, 74 T500

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  5. #4
    Sensei 66Sprint's Avatar
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    Higher output is simply a larger ratio of output (secondary) turns to input (primary) turns in the coil.....
    In your example, 1.6 Ohms is the primary's resistance....
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
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  6. #5
    Senior Member mike in idaho's Avatar
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    Early Goldwings used ballast resistors in the coil feed circuit, they work great, hot sparks and less current draw on the points/ignition box. You could even setup a circuit to bypass the resistor, while using the electric start, to keep the sparks hot with a low battery.
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  7. #6
    Member pidjones's Avatar
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    Quote Originally Posted by mike in idaho View Post
    Early Goldwings used ballast resistors in the coil feed circuit, they work great, hot sparks and less current draw on the points/ignition box. You could even setup a circuit to bypass the resistor, while using the electric start, to keep the sparks hot with a low battery.
    This starting bypass is built into the GL1000 wiring.

  8. #7
    Member brewsky's Avatar
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    Simply adding a toggle switch across the resistor could bypass it for cold start purposes.

    What I am wondering is if the 1.6 ohm coil will have sufficient output for normal running with reduced voltage supplied to the primary due to the 3 ohm resistor?

    Doing some math, using the 3 ohm resistor should leave only 4.16 volts (with 2.6 amps) at the primary; a 2 ohm would leave 5.33 volts (with 3.33 amps), and a 1.6 ohm would leave 6 volts (with 3.75 amps)…..assuming there are 12 volts available to start with...….also assuming the math is right! ( I=V/R for the total circuit, and V=IR for each component)

    So the question seems to be,,,,does 2.6 amps to the primary produce the same result regardless of voltage used?

    Stock 4.5 ohm coil at 12V produces 2.66 amps

    Aftermarket 1.6 ohm (with 3 ohm resistor) coil at 4.16V also produces 2.6 amps?
    Last edited by brewsky; 11-03-2019 at 05:52 AM.
    78 CB550K, 78 CB750K, 66 CA77, 74 T500

  9. #8
    Sensei 66Sprint's Avatar
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    NO......
    A stock 4.5 Ohm coil @ 12Volts draws 2.66Amps of current flow, expending/using/requiring 31.3 Watts of power......
    A 1.6 Ohm coil @ 12V draws 7.5 Amps and uses up 90 Watts of power......
    The addition of the 3 Ohm resistor is to reduce the amperage draw, and thus the power usage..........
    The slightly diminished Voltage you read after a resistance is because unless the engine is running fast enough that the alternator can replenish the battery, its power is finite and being used-up......

    Wattage is the factor showing power usage, Amps show the speed at which the power reserve (the battery) is being reduced......
    Last edited by 66Sprint; 11-03-2019 at 07:52 AM.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
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  10. #9
    Senior Member mike in idaho's Avatar
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    The problem with a toggle switch is that,sooner or later, it will be left on and the points/ignition box will get pretty warm. better to use a cheap 4 pin relay in the starter circuit to switch the power.
    '65 YG1
    '65 CB160
    '66 CL160
    '67 CL77
    '68 TR6
    '69 T100R
    '69 T120R
    '72 Commando 750
    '78 XS650E
    '79 Gl1000
    '81 440 LTD
    My company car is a Kenworth

  11. #10
    Member brewsky's Avatar
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    Quote Originally Posted by mike in idaho View Post
    The problem with a toggle switch is that,sooner or later, it will be left on and the points/ignition box will get pretty warm. better to use a cheap 4 pin relay in the starter circuit to switch the power.
    NO doubt with me it would be SOONER!
    So maybe a momentary on bypass would be good....
    Last edited by brewsky; 11-03-2019 at 11:59 AM.
    78 CB550K, 78 CB750K, 66 CA77, 74 T500

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