Motobecane zener diode / electrical questions

So, the zener diode is the latest in the string of bad parts on my Moby. But there is something that I'm not understanding very well..

When I had the bike running, I noticed that the tail light was on - and quite bright. I tried the brakes, but pulling either brake had no effect on the tail light (at least not that I could tell). From this I assumed that one or both brake light switches were stuck on.

I had to repair the steering on this bike, so I had the zener diode off of it. I was ready to re-install it, and I thought I'd check the zener, just for drill. Guess what? It measures a dead short! This didn't make any sense, so I rigged up a little zener diode tester with a 16VDC wall-wart, and a 200ohm 5W resistor. The tester confirmed a dead short (and yes, I tried it with both polarities). If anyone wants to know, I can tell you how to make & use the tester.

From the wiring diagram, I don't see how this is possibe - to have a tail light lit, with a shorted zener diode in the system. Does anyone know how this could be? And is the motobecane a negative or positive ground system?

If anyone has a spare zener, I could really use it at this point. Any help appreciated on either count.

Re: Motobecane zener diode / electrical questions

A zener diode will act like a short when enough voltage is applied (in either direction). Zener diodes are made to operate in what is called the "reverse saturation" region. At that point, the zener diode will provide an almost constant voltage, no matter how much current you push through it. This is why zeners are used as voltage regulators.

Re: Motobecane zener diode / electrical questions

I'm already familar with the characteristics of a zener diode, which is why I rigged up a tester to check it.

I tried biasing the diode in both directions, and in each case, there was near-zero voltage drop across it. It's behaving like a piece of wire, not a voltage regulator.

Unless this is not a simple zener device, I'm pretty confident that it's shorted out.

My main question, is how did the tail lamp manage to light if the diode is shorted? The way the motobecane is wired, there should be no voltage to run the light. It's kind of bizarre.

Re: Motobecane zener diode / electrical questions

A zener diode failing in such a way that it becomes a short doesn't make a great deal of sense (given the semiconductor physics at work), it'd be much more likely to "pop" and become an open circuit. The actual part in question here may be more complicated than a basic zener diode, however.

But yeah, if the motobecane lighting circuit is reasonably standard, a shorted zener diode would kill the lights entirely. The Dempsey book has more information, but I don't think it covers motobecanes specifically (chapter five, available here )

Re: Motobecane zener diode / electrical questions

I have some further thoughts on this.

I know that it's more common for zeners in small electronics to open up, than short, but this part is exposed to the elements. It would only take a small breach in that old sealing compound, and the part could get corrupted. I could see this casuing it to short over a long period of exposure (which this machine clearly had).

And the fact that there are no anode/cathode markings makes me wonder if it's truly a simple zener. Google search on the part numbers gave me nothing.

Re: Motobecane zener diode / electrical questions

Steamboat Aka J. R. Stevens /

If you place an ohm-meter(X10 range,both polarities) across a a zener or any diode and the meter reads zero ohms both ways, it is shorted unless the breakdown voltage of the zener is less than the the ohm-meter battery voltage(rare). That said, It is nearly impossible to determine why it shorted. Most likely it was caused by a surge of voltage from the charging system or the voltage dropping resister shorting. Also lightning can do strange things to electronic components. Jim

Re: Motobecane zener diode / electrical questions

Yep, I'm with you Jim.

At least on the Motobecane, there isn't a dropping resistor per se (at least not in the wiring diagram). It seems that the internal impedance of the exciter coil is the only current limiting in the system.

Unless this was very carefully designed, a system like this is bound to abuse the zener diode. Guess that's why it's a 50W part - pure brute force.

Re: Motobecane zener diode / electrical questions

It's not that rare at all, really. The Ohm-meters I use apply a test voltage of 5 volts, and the zeners I work with enter reverse breakdown at 3.4 volts. The forward threshold voltage is 1 volt.

Check the applied voltage of your ohmeter with a voltmeter or oscilloscope.

Re: Motobecane zener diode / electrical questions

Steamboat Aka J. R. Stevens /

You must be using laboratory type test equipment as most cheaper(garage type) VOM use 1 1/2 to 3 volts and are used on 6 and 12Volt systems.. Jim.

Update #1

Sounds like MoPedLar has a replacement for me. When I get the part, I'm going to make some bench measurements on it and post the results here. I'll also try it on my tester setup.

I still find it odd that (if it's truly a simple zener) that there are no anode/cathode markings on the device. Maybe they assumed that the typical mechanic wouldn't know or care about it, but it's such an easy thing to do.

Re: Motobecane zener diode / electrical questions

As usual, no update ;)

'nyone got some infos or a diode for muah?

Re: Motobecane zener diode / electrical questions

Try Radio Shack but you must know what you want they most likely don't know.

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account