Zener diode polarity

Classic Norton Commando Motorcycles.

Zener diode polarity

Postby B.Rad » Wed Apr 24, 2013 11:35 pm

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

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Re: Zener diode polarity

Postby Old Bloke » Wed Apr 24, 2013 11:52 pm

B.Rad wrote: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


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?
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Re: Zener diode polarity

Postby L.A.B. » Thu Apr 25, 2013 2:30 am

B.Rad wrote: Regarding the zener diode, the standard big brass fitting to me, are they all polarity specific


Yes. Zener diodes are polarity specific. The polarity is usually marked on the Zener.

B.Rad wrote:For example, can a zener off a truimph or BSA being negative earth


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.
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Re: Zener diode polarity

Postby B.Rad » Thu Apr 25, 2013 7:29 pm

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

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Re: Zener diode polarity

Postby Torontonian » Thu Apr 25, 2013 7:36 pm

The best moment is when you pull the primary cover and an atom bomb of smoke comes out.
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Re: Zener diode polarity

Postby kraakevik » Thu Apr 25, 2013 9:04 pm

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



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Re: Zener diode polarity

Postby Old Bloke » Thu Apr 25, 2013 11:57 pm

kraakevik wrote: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



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Useful info. How was the Zener diode mounted?
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Re: Zener diode polarity

Postby kraakevik » Fri Apr 26, 2013 10:02 am

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



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Re: Zener diode polarity

Postby Old Bloke » Fri Apr 26, 2013 1:47 pm

No need. I can visualise that. I always wonderd if it would survive with no heatsinking.
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Re: Zener diode polarity

Postby Nater_Potater » Fri Apr 26, 2013 2:12 pm

Old Bloke wrote:No need. I can visualise that. I always wonderd if it would survive with no heatsinking.


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
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Re: Zener diode polarity

Postby kraakevik » Fri Apr 26, 2013 6:20 pm

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



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Re: Zener diode polarity

Postby Nater_Potater » Sat Apr 27, 2013 8:42 am

kraakevik wrote: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



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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!
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Re: Zener diode polarity

Postby bluto » Sat Apr 27, 2013 11:00 am

(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

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