Zenor Diode

Christopher Kilby /

My headlight blew on my 78 Piaggio Bravo, but when I tested the voltage at the headlight it was just over 6volts at idle and would climb to around 16volts at higher RPMs.

I've got the manual, which mentions the double Zenor Diode being attached to the turn signal relay by a green wire. Mine looks grey and my best guess as to where the Zenor Diode is, is next to the battery below the turn signal relay.

MY problem is that I can't find any information about the location of the Diode on the bike.

Does anyone know where to get a new one or where exactly it is on my moped? My local Radio Shack didn't have what I was looking for.

Re: Zenor Diode

Does the manual specifically say that if the headlight blows out, the Zener diode is suspect? I looked over the wiring diagram for your bike, and it appears that the only function of the Zener is to provide a stable voltage to the turn signal circuit - it needs this, or the flashing rate would not be stable or reliable.

From what I see, the headlamp is on a seperate circuit (black wire), but of course, the diagram is not 100% clear on this.

If you were measuring the voltage after the light was already blown out, you were reading what is called the open-circuit voltage. While 16V is way to high for a 6V lamp, the actual voltage under load (with a good bulb) might be much lower. Maybe the bulb just burned out on it's own - they do that. I also see somehting odd in the diagram - it looks like the taillamp is in series with the headlamp.. kind of odd. If the taillamp is missing/burned out, then the headlamp would not light. Again, the diagram is not clear on this.

The diode itself should look a lot like a short bolt, but it has an electrical terminal (with a wire) sticking out of it. It might be attached to the frame itself, in a location where it's hard for water to get at it - and difficult to see. On a Motobecane, it's attached to a heatsink, which is mounted below the headlamp.

Hope some of this is helpful.

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