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Ignition coil primary resistance

Discussion in 'Norton Commando Motorcycles (Classic)' started by hrd998, Jan 9, 2019.

  1. hrd998

    hrd998

    Joined:
    Aug 13, 2012
    How important is the primary resistance figure to the operation of the coil and subsequent ignition function. Most electronic systems stipulate to use a coil of a certain value ohm....why is this and what effect does it have?
     
  2. jbruney

    jbruney

    Joined:
    Jan 5, 2019
    I too would appreciate a bit more info on this topic seeing as I'd just installed a Vape Wassel on 44 year old coils with slightly under 2.5 ohms R. I had concerns which were allayed as soon as I cranked it off, so that is the total extent of my knowledge in that 2.5 works.
    I'm probably going to aide a buddy reviving a '70 Trumpet and would like to know a solid cutoff point on calling the coils trash in case of different readings up or down rather than piddle.
     
  3. kommando

    kommando

    Joined:
    May 7, 2005
    Recommended minimum primary coil ohms is 3.0, below that will work but will overheat the box which will then fail. I had a dual coil which was labelled 3.0 ohm but tested at 2.9 ohm, ran fine most of the time but on hot days the Boyer box would go for 20 mins, then needed 10 mins to cool before I got another 20 mins, cured by using 2 coils in series of 1.7 ohms giving 3.4 ohms.
     
  4. jbruney

    jbruney

    Joined:
    Jan 5, 2019
    Made me go back and read the instructions...….VW recommends 3-4.5.... I've got to stop cleaning that blackboard after each use.
    I forgot to mention mine were per each unit, so on the upper end.
     
  5. RoadScholar

    RoadScholar VIP MEMBER

    Joined:
    Dec 28, 2008
    Kommando hit the nail on the head. I am familiar with Boyer, TriSpark and Pazon. They like 3ohms of total primary resistance, they will tolerate 2.9 to 3.5. Below 2.9 the system unit flows more current then it was designed for and reliability suffers. Above 3.5 too little current flows through the system box and the spark energy decreases and may not be sufficient to start the engine.

    For a treatise on the subject go to DynoDave's website (Atlantic green), or, perhaps he'll chime in.

    An ignition system that uses points is considerably more tolerant, but even so, too little primary resistance will burn the points, too much reduces spark energy. My experience is that points based systems like 5 ohms; the contact surfaces will still burn, but you will have adjusted them several times by then.

    VWs "bugs" got points, condenser and plugs every 3000 miles, per the owners manual...
     
  6. KiwiNeill

    KiwiNeill

    Joined:
    Jul 25, 2011
    hrd998
    as far as I know the primary resistance determines the current flowing through the coil and depends on the type of system but if you stay within the recommended range all should be good.
    My Pazon Smartfire, Boyer MicroPower (and some modern Triumphs using the same PVL mini coil) have very low primary resistance twin lead coils of 0.6Ω but my Trispark uses a 3.0Ω Dyna twin lead mini coil from their recommended range of 3.0Ω to 5.0Ω.

    Pretty sure that swapping coils between systems will result in poor quality roadside time waiting for a lift.
     
    Last edited: Jan 11, 2019
  7. freefly103

    freefly103 VIP MEMBER

    Joined:
    Apr 17, 2013
    I have used a 5ohm Dyna coil on 3 Commandos fitted with TriSparks. Never had an issue. Discussed the issue of Primary resistance with Stephen Kelly at Trispark. He confirms 5ohm Primary resistance is fine.
     
  8. dynodave

    dynodave

    Joined:
    May 28, 2003
    correct: kiwineill
    "as far as I know the primary resistance determines the current flowing through the coil and depends on the type of system"

    which coil resistance? (actually impedance)
    1. It starts with the general type of ignition system. (that NO manufacturer openly declares)
    long dwell or short dwell. These are general categories and not an exact number/%
    2. more to follow:
     
    Last edited: Jan 11, 2019
  9. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    Too bad coil manufactures don't emphasize impedance instead of resistance.

    Impedance is the important number as it determines the time required to fully charge the coil. Impedance roughly referrers to the change in resistance over time.

    The best way to determine if you are making the best use of your coil is with a scope.

    If you watch the current draw during coil charging you will see the current increases as the coil nears full charge. When full charge is reached you will see a plateau or hook in the charging current. The time to the maximum current plateau is the correct charging time.

    That is why points can not supply maximum performance. The dwell time varies with engine RPM so the coil must have high enough impedance so it does not overcharge [and burn the points or overheat] at low engine speed.

    That means that as the engine RPM rises and the charge time is reduced the coil does not have time to charge fully.

    Performance electronic ignitions control the dwell time so a low impedance coil can be used and charged fully at any rpm.
     
  10. dynodave

    dynodave

    Joined:
    May 28, 2003
    2. The consideration of electronic ignitions that is almost unspoken is HEAT. Heat management is key in longevity.
    Each type of system attemps to control heat which is the prime killer of the drive transistor. In previous autopsies of failed EI, I as expected to find the output transistor was dead. However not always dead in the same way... some times they are non-functioning shorted and some times terminal death as a burned open. There are a few reasons they die this way.
    If they are properly designed and applied they would easily last longer than a human life.

    FWIW this topic is both electronics AND physics in nature. The general catagories of schooling are DC electronics and AC electronics where physics principles ares involved.

    3 more to follow:
     
    NorCat likes this.
  11. dynodave

    dynodave

    Joined:
    May 28, 2003
    3: The dwell which is a classic motoring term measured in degrees of crank rotation. This measurement of the period of the ignition coil primary being driven electrically can also be expressed as a %.
    In a norton points/coil ignition each coil is driven about 45% of the time. With a boyer analog it is around 80% at low speed and it/dwell reduces with faster RPM. (reverse of optimum)

    You would find the low resistance coil systems are quite "short dwell" time, like 17-22% more or less. The heat generated in the coils by these systems need to be mostly the same/similar. Most output transistors ratings I have observed in these system is around 12 amps, while the voltage runs 12.8 for a good battery and goes up eventually up to 14.3 volts.
    To sum it up the low resistance coils have a very high peak current but since the duty cycle is low the average heat/current is controlled.
    When you have a higher resistance coil the peak current is lower but runs longer %. The heat generated is thus moderated. The same long dwell ignition system hooked up to a low resistance coil may well over heat the transistor and start to fail.

    I find it interesting that the usually bullet proof norton Lucas RITA is mounted in the cool air stream between the coils.
    Counter that with the old analog boyer wraped in a antivibe foam stuffed up under the tank with no obvious cooling.
    Having dissected both systems, (for a period of time) I found both used the exact same transistor.
    External heat dispersion is a major consideration for these systems longevity.
     
  12. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    Many modern digital ignition systems do not use a degrees dwell or percentage time.

    They use a programmed charge time.

    IE- I use 1.7ms charge time in my system at full load. As the load is reduced and cylinder pressure drops then the charge time drops back to <1ms.

    Of course digital ignitions for a Norton are not going to be able to sense load so they will simply program in the time needed to charge the coil they recommend.
    That charge time will usually stay the same throughout the RPM range.
     
  13. YukoNorton

    YukoNorton

    Joined:
    Sep 4, 2017
    I found this on advrider, they were discussing coil oms and using ignition boosters on R65:
    by Lou Dudzik;
    Q: "...I asked him(Lou) why he specified at least 2.4 ohm coils in those diagrams, and whether I would get better spark if I used a lower-resistance coil, such as a 1.5 ohm or .6 ohm. Here is his reply:

    "As you may know, it's the dwell that should determine what coil you should use. Or usually they choose a coil then set the dwell accordingly. Since most people won't change the shape of the points cam, it's the points cam that will determine what type of coil to use. Generally speaking, the lower the resistance of a coil, the lower the inductance of a coil will be. The lower the inductance, the faster the coil will charge.

    People often think that a coil is fully charged when the current in the coil has reached it's peak possible limit. While that may be the case for slower coils, it may not be the case for faster coils. A coil is said to be saturated when the current has reached it's maximum based on the applied voltage and the DC resistance of the coil. But that should be specified as "current saturation". Another type of saturation is when the magnetism of the coil has reached its maximum. The magnetic saturation may occur long before the current saturation, especially on a fast coil. What this means is, for example, if a coil measures 1 ohm, and 12v is applied, the current saturation would be 12 amps. But the coil may be magnetically saturated at 6 amps. All of the current applied beyond 6 amps is wasted as heat. Also, once a coil is saturated, any time that the coil is held in saturation also only contributes to heat.

    A GM coil in the .5 ohm range might get over 20 amps on it if left unchecked with 12v applied. So GM put a current limit on the HEI modules. The current limit depending on which type of module you have may be in the 5 to 8 amp range. During the current limit portion of the dwell, the module is heating up very rapidly. I wanted to avoid this. And it is avoidable because motorcycles don't need fast coils. Since you are only firing once per revolution (on most classic bikes) you can use a slow coil and avoid the current limit entirely. This leads to a very cool running module, which is always good for reliability. So by specifying a high resistance coil, I am actually specifying a slow coil. The 2.4 ohm limit just ensures the current limit is not reached since it would only be wasted on a motorcycle as heat.

    It is important to note, slower coils, due to their higher inductance, often hold more spark energy than a fast coil, for a given current limit. Faster coils may get to a higher current, which may make it seem like the coil is giving more energy in the equation, but it doesn't. There are two reasons why... one is that the current will be limited by the igniter. That limit is what should be used in the equation. And secondly, once a certain current level is reached in the coil, the inductance relative to any increase in current reduces. This is because the coil is getting magnetically saturated. The coil starts to act more like a resistor once that happens, so the spark energy does not increase beyond that of the lower current value. You can see it on a scope by measuring spark duration. It's arguably the spark duration we are interested in once a certain amount of spark current has been reached, rather than actual energy..."
    https://advrider.com/f/threads/what-happens-if-you-install-a-low-resistance-coil.1238304/
     
  14. cliffa

    cliffa

    Joined:
    May 26, 2013
    This is all really educational - many thanks.

    Dave to quote you "I find it interesting that the usually bullet proof norton Lucas RITA is mounted in the cool air stream between the coils.
    Counter that with the old analog boyer wraped in a antivibe foam stuffed up under the tank with no obvious cooling.
    Having dissected both systems, (for a period of time) I found both used the exact same transistor.
    External heat dispersion is a major consideration for these systems longevity."

    Are you saying that the Boyer is more prone to failure than the RITA?

    Cheers,

    cliffa.
     
  15. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    Some o' of that sounds a bit fishy....
     
  16. dynodave

    dynodave

    Joined:
    May 28, 2003
    cliffa "Boyer is more prone to failure than the RITA"

    I am saying "External heat dispersion is a major consideration for these systems longevity"
    The boyer mounting position makes it more failure prone because of lack of cooling.= disregarding heat management.
    I designed a boyer mount adapter, which some folks have used my design to relocate the analog boyer into the cooling stream. (similar to rita mounting)
    Heat in the output transistor is usually more a problem than heat in the coil.

    The advrider article has jumped way ahead of where I was, the energy control concepts within the coil sounds ok, but the heat interaction of the proposed theoretical electronics and theoretical coil are probably not available for reasonable price.

    Microdigital controlled coil drive methods and associated terminology VS old analog is worlds apart, just like the multimillion dollar radar systems I helped to build and work on( for over 40 years) compared to a tinker toy rita or boyer analog.
    The basic physics principles are the same. Yet the applied physics are applied differently.
    heat control VS cost control remain the same.
     
    Last edited: Jan 12, 2019
  17. cliffa

    cliffa

    Joined:
    May 26, 2013
  18. dynodave

    dynodave

    Joined:
    May 28, 2003
    If you look at their offering, I have noted several items:
    The output transistor is an SMD mounted on a G-10 circuit board which acts like a thermal insulator. This is in direct contrast to the original rita TO-3 darlington transistor that was sandwiched against the aluminum heatsink case with thermal grease. But since I do not know what is the duty cycle % (98% for the original rita) I can't predict it's thermal performance.
    It may take the physical foot print but be very different and not actually mimic the original rita electrical characteristics or performance.

    Back then, right after John Carpenter and lucas quit the rita production, I had acquired all of the left over inventory from Micheal Moore/ Eurospares. That was with the intentions of reviving the product despite the TO-3 transistor becoming obsolete and directly causing the product to no longer be able to be made.
    The project waned since I realized it was a bit obsolete, and others would likely pick up the slack and also fix the boyer analog spark scatter problem that adversely affects the MKIII. That has obviously happened with mixed success.
     

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