Saturday, 21 March 2015

A100 Restoration: Electrical Investigations

The date of the inspection draws nearer and I've checked most of the items on the list. Brakes seem good, a new chain and sprockets are on the way, brake lights work, signal lights work, transmission works, etc.

The one thing which I am seriously worried about at this point is the headlight circuit. As it stands, the headlight only comes on VERY dimly when the engine is running. 

Free Wiring Diagrams - Suzuki A100 & AX100

I managed to track down some wiring diagrams from online sources and also from the Haynes Manual I purchased. They are provided here just in case they are of use to someone else.

After examining the wiring diagram and getting some insight from a person on advrider I came to the conclusion that the voltage regulator was faulty. Funny thing: if you look at the Haynes diagram, the regulator is actually not shown.

Electrical Engineering with Tom

Back in 1975, voltage regulators were simple. This particular one is constructed with a resistor an a zener diode. It is called a shunt regulator and the diagram is more or less like this:

Vs represents the output from the magneto. Vs on the bike is alternating. Dz is the zener diode. R2 represents a headlight. 

Here is my super simplified explanation about what is going on: The zener diode is a diode in reverse. Basically, as the voltage across the zener gets high enough, the zener will go into breakdown, allowing current to flow across it such that the voltage across the zener is fixed to Vz (in this case it should be  ~6V for our headlights) 

It is important to note that the zener still acts as a regular diode in the other direction, so when Vs goes below 0V (is negative) the output will be clamped to a maximum of about 0.7V. Here is what we would expect the input and output functions to look like if everything were ideal:

Note that at its peak, the engine is outputting Vs_max (probably ~18V or so) and Vz is about 6V. Youmight ask, where does this voltage go? It is actually lost over R1 such that VR1 is Vs-Vz when Vs is greater than Vz. Also note that the zener does not come into effect when the Vs is between -0.7V and Vz.

In any case, I believe that the zener on my bike is faulty and actually breaking down much sooner than 6V. My suspicions seem to be confirmed by multimeter readings of about 2V across the headlight when idling. I've since ordered a new zener diode with a Vz of 6.3V from someone in Florida. 

My backup plan to pass the inspection if the zener does not arrive is to omit it so the voltage will be unregulated. I pulled it and started the engine in the garage and the headlight lit up very brightly indeed. The only concern is burning out the bulb.

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