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Zener diode polarity

Discussion in 'Norton Commando Motorcycles (Classic)' started by B.Rad, Apr 25, 2013.

  1. B.Rad

    B.Rad

    Joined:
    Feb 28, 2012
    hello forum

    Regarding the zener diode, the standard big brass fitting to me, are they all polarity specific or can any zener be put on a Norton. For example, can a zener off a truimph or BSA being negative earth, be put straight on a Norton with positive earth.Wired appropriatly of course. I dont know much about electrics and appreciate any advice.
    Best wishes
    Bradley
     
  2. Old Bloke

    Old Bloke

    Joined:
    Feb 14, 2013
    I would say that a Negative earth Zener could not be "put straight on a Norton with positive earth" as the body of the Zener, i.e. the bit that gets bolted to the frame forms one of the connection points. You could use it if you isolated the Zener from the frame, but you will, as you say, have to wire it appropriately and have to bolt it to(I would imagine) a fairly large heatsink. I assume you have a Mk2 commando?
     
  3. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Joined:
    Nov 20, 2004
    Yes. Zener diodes are polarity specific. The polarity is usually marked on the Zener.

    Only 1979-on Triumphs had negative earth electrical systems, before then they were positive earth, and BSAs were also positive earth.

    If you've seen a pre-'79 Triumph or any BSA with a negative earth electrical system then it has been converted from pos. earth. This refers to '60s or '70s machines with alternators and Zener diode regulation and not anything dating back to WW2 or before.
     
  4. B.Rad

    B.Rad

    Joined:
    Feb 28, 2012
    Thanks to all for this info. it has been helpful.

    I know enough that when the smoke escapes from electrics, it does not seem to work. it seems hard to stuff the smoke back in too.

    thanks again
    Bradley
     
  5. Torontonian

    Torontonian

    Joined:
    Dec 28, 2009
    The best moment is when you pull the primary cover and an atom bomb of smoke comes out.
     
  6. kraakevik

    kraakevik

    Joined:
    Oct 12, 2009
    The 850 E-Bay chopper I sold recently had a negative-ground wiring-harness conversion with the Zener-diode polarity reversed accordingly--it charged the battery just fine with the standard Commando rotor, stator and rectifier



    Tim Kraakevik
    kraakevik@voyager.net
     
  7. Old Bloke

    Old Bloke

    Joined:
    Feb 14, 2013
    Useful info. How was the Zener diode mounted?
     
  8. kraakevik

    kraakevik

    Joined:
    Oct 12, 2009
    The diode was mounted outboard on the Z-plate, with the hot wire connected to the terminal, I believe--if you'd like a photo just send me an e-mail



    Tim Kraakevik
    kraakevik@voyager.net
     
  9. Old Bloke

    Old Bloke

    Joined:
    Feb 14, 2013
    No need. I can visualise that. I always wonderd if it would survive with no heatsinking.
     
  10. Nater_Potater

    Nater_Potater VIP MEMBER

    Joined:
    Apr 7, 2013
    Two things: One, must have heat sink. The Zener "regulates" the voltage by bleeding off excess current to ground; It does this by converting it to heat. We can't regulate the alternator in our bikes in the sense where a car's voltage regulator can actually modifiy the alternator's output, since the Lucas alternator puts out power in direct relation to the crank rpm. Picture no headlight, running at six grand. That power from the alternator has to go somewhere. Lots of heat to dissipate.
    Two, positive to negative ground conversion requires reverse-polarity zener AND rectifier. Nathan
     
  11. kraakevik

    kraakevik

    Joined:
    Oct 12, 2009
    Nathan

    You might very well be right about the polarity of the rectifier--my chopper may have had an aftermarket rectifier--please weigh in, you folks who have converted Commandos to negative-ground systems.

    The chopper I sold had a conventional rotor, stator and diode--and it worked perfectly.

    I don't remember whether the rectifier was stock--but the diode--which may have been hooked up backwards--was a standard unit



    Tim Kraakevik
    kraakevik@voyager.net
     
  12. Nater_Potater

    Nater_Potater VIP MEMBER

    Joined:
    Apr 7, 2013
    I need to revise my earlier statement about needing a different rectifier. When I first responded, I was looking at one from my junk drawer that has only three connections; two for the alternator, and one for the negative lead to the battery (the mounting post is the fourth connection for the positive ground to the frame). After pulling the seat off my '74 Interstate, I see that its rectifier has four distinct connections. In this light, all you would need to change is the Zener(s), and swap the red and green wires from the rectifier. Cheaper yet!
     
  13. bluto

    bluto

    Joined:
    Jul 15, 2010
    (quote) I need to revise my earlier statement about needing a different rectifier. When I first responded, I was looking at one from my junk drawer that has only three connections; two for the alternator, and one for the negative lead to the battery (the mounting post is the fourth connection for the positive ground to the frame). After pulling the seat off my '74 Interstate, I see that its rectifier has four distinct connections. In this light, all you would need to change is the Zener(s), and swap the red and green wires from the rectifier. Cheaper yet![/quote]


    I don't think that is correct, I'd thought the same thing but it looks like the four terminal rectifiers have one labeled "stud"....so if it were mounted to the frame you would indeed need the correct polarity and they are sold in negative and positive ground versions...you had it right the first time.

    see http://www.oldbritts.com/17_49072r.html

    [​IMG]
     

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