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Grounded ATV generator coil

  1. May 3, 2016 #1
    I had a friend contact me regarding helping him out with lights on his ATV. I don't get to work on powersports vehicles much but occasionally folks contact me when they have electrical issues. Here's a link to a wiring diagram for the vehicle (it's a late 80s TRX250R)

    http://www.trx250r.net/forum/attachment.php?attachmentid=320&d=1222835980 [Broken]

    My friend is wondering about hooking up different lights. After taking a look at this diagram I said, "OK seems like a pretty simple circuit; a single-phase coil with the chassis as the circuit common for the coil and loads", to which he replied "I don't know about that I just don't know much about these half-wave systems". I pointed out that the diagram doesn't contain a rectifier therefore it is not a half-wave; it's simply single-phase AC powering the lighting circuit. It would only be half-wave if I were to wire a diode in series with the output wire. He insisted that it is indeed half-wave, specifically "any stator that goes to ground is half-wave". This assertion to me seems insane; "ground" in this case is nothing more than a convenient circuit common (the engine and chassis). There's no way for something that's just a chunk of metal from an electrical standpoint to act as a rectifier. But he and a couple other guys that work in that field are all telling me "that's a half-wave system". So I called a couple certified Honda shops and they gave me the same line, "any stator that goes to ground is half-wave". One actually defined it as a "half-wave AC system". Now I feel I've entered the twilight zone. Isn't "half-wave AC" a contradiction in terms? The wave will certainly fluctuate, but by its very nature a half-wave output remains on one side of neutral. Plus there's no rectifier on this vehicle, or several others of the old batteryless system that I looked up. There's a regulator certainly but it's in parallel with the load; my guess is that it's simply a transistor or thyristor that "shorts" the two ends of the single-phase coil to each to control the AC voltage across the load.

    Anyway what started out as doing a quick favor for a buddy has now morphed into my exploring some old ATV and dirtbike systems. Apparently the wisdom in the motorsports industry is that if you connect one side of a stator to the chassis it becomes a half-wave system, but I just don't see how a piece of machined metal acts as a rectifying component. Am I missing something blindingly obvious here? It's hard for me to imagine that multiple professionals in the field are all giving me the same bad info, but at the same time it is what it is: bare metal's not a semiconductor.
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. May 7, 2016 #2
    The diode is part of the regulator... When the stator is grounded, there is only one half of the wave that is usable. The other half would turn the ground into positive, so it must be open circuit until the wave makes the next pass. From what I googled to think of the best answer, because I wasn't sure either, said that this grounded low-output system isn't used much anymore.
  4. May 7, 2016 #3

    jim hardy

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    I agree that it is an oxymoron.

    With no battery there's no reason to rectify at all.
    The wiring diagram doesn't show any rectifier between the lighting coil(left one) and the lights, just a switch.
    If there's any rectification it is in the COI box up top that's fed by your right hand coil. My guess is that winding makes a few hundred volts and that gets rectified for the capacitive discharge ignition.
    But the lighting cicuit doesn't even go up there. Lighting is plain old AC.

    The lighting i would call an unrectified AC system. You can verify that, read voltage across taillight with a DMM. It should read ~13V AC and less than 1V DC.

    See if the guys like that term. After you demonstrate it and a decent interval has passed they'll come around.
  5. May 7, 2016 #4
    You're correct; with the exception of a few dirtbikes and racing ATVs (these days almost everything has a battery). However I'm not sure how a diode in the AC regulator would effectively rectify the system since it's in parallel with the load. Even if there WAS a diode in the regulator the single-phase circuit from the two sides of the lighting coil across the load wouldn't be rectified. Although I don't have an internal topology for the AC regulator my assumption is it's just a transistor that "shorts" the two sides of the AC coil to each other to control the voltage across the load, similar to the shunt style regulators in the permanent magnet regulators on the street bikes. I'm also not sure how the stator being "grounded" makes half the wave unusable since the "ground" is just using the chassis as a circuit common. Isn't running a single-phase AC loop through the engine block the same as just running a wire? That's the key question I keep coming back to every time I look at this.

    I ran a test earlier this week connecting an LED string in place of the OEM headlight to test it. If that's the only change the LED will not turn on, which I would expect. If I wire a diode in series with the LED it shines bright, so my assumption is that that circuit (the lighting coil) is indeed as Jim Hardy put it unrectified AC. The CDI I know is a whole different story, I know that's rectified for the primary side of the ignition coil.

    Thanks to both for the responses, much appreciated.
    Last edited: May 7, 2016
  6. May 11, 2016 #5
    An LED is a diode by itself so how does an additional diode make it light?
  7. May 11, 2016 #6
    It doesn't; it was an error in my test setup that I've since corrected (we all have those face-palm moments every once in a while). I likewise got the vehicle repaired to the satisfaction of myself and the owner.

    Thanks again to all who responded.
  8. May 11, 2016 #7

    jim hardy

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    @XZ293 did you check the lighting voltage? Is it AC like we suspected ?
  9. May 12, 2016 #8
    Yes it was. I checked with an oscilloscope to be 100% certain. BTW to clarify my previous post that JerromyJon responded to, it was mis-wording on my part. I'm using a diode to rectify to half-wave before getting to the LED headlight to prevent shortening the lifespan of the LED by running AC to it. The on/off part was miswording (I have a couple different projects going on and got them crossed up in the malfunctioning piece of erratic machinery between my ears when I was writing the post).
  10. May 12, 2016 #9

    jim hardy

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    Thanks ! Dont worry i make mistakes all the time. You did right - out with it and along.

    Good to know that about those machines. I get asked often for help, that's one more factoid in my arsenal. Thank you.
    I need to make a solid state replacement for the old fashioned electromechanical voltage regulator on a 60's Evinrude....

    old jim
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