Zener Diode on Brums

[BRIAN MARSHALL]

To my knowledge, the last two Brum Scotts were OMV8K, registered in 1971, and WDO408L, registered in 1972, both assembled by Ted Murphy from kits of parts supplied by Matt Holder. I owned WDO408L for several years, and I can assure the doubters that both machines were SIX VOLT Miller alternator systems, with no heat-sink diode fitted or needed. (There was in fact a single bike made in 1978, but it was only loosely assembled, and was short of some components that were out of stock. I have a photo of it somewhere, and it is perhaps best forgotten…)
Brian

[Deleted User]

@dave bushell wrote:

Material cost is cheaper when using 12v as compared with 6v as current flow is half that of the 6v system and this means there is less voltage drop over a given length of cable arriving at it’s destination, I.e. the headlamp bulb. On a 6v system, this voltage drop can be reduced by using a heavier (and more expensive) cable. Thus when you’re manufacturing thousands/millions of vehicles, there’s quite a material cost savings with 12v
Dave

I agree when I designed my wiring harness using the Scott and Miller drawings I upgraded the wiring sizes. That became a small issue with routing as the harness became thicker. However it works real well.

[Deleted User]

I decided to go down the 12 volt path because the alternator on my bike was a mess. Broken spring washers inside the unit had caused the rotor to poll severely on the stator and the windings were torn to bits. I also purchased a negative to earth Bridge rectifier and Zenor Diode. There will be modifications to my wiring system but I will still be using a 6 position switch. I have a Classic Japanese enduro bike that I restored which has a 6 volt system. With a new voltage regulator it sill continued to produce to much charge for the headlight bulb to handle. We fitted, I think from memory a 1 ohm resistor in the circuit and the problem was rectified. The bike (Brum) shown is not mine, but I think the diode has been mounted underneath the bottom triple clamp. The diode will get warm but I don’t think it will be enough to cause a problem with the radiator. KP

[Deleted User]

I would have thought a 12volt battery as opposed to a 6volt one would cost more than 3 metres of thicker wire.

[Deleted User]

I would say that a 12 v system is a bit more efficient because it is using less amps in the system (ohms law) ,and also less susceptible to losses in the system due bad connections , poor wiring etc.
The better and efficient way to stabilise the power in a 12v system is to use a solid state rectifier/regulator which would simplify the circuitry on the machine . The zener diode method just turns the excess power into heat energy , it’s crude ,simple but also more expensive !

Here is the type of unit I would suggest, https://www.ebay.co.uk/itm/REGULATOR-RECTIFIER-12-VOLT-AC-ROYAL-ENFIELD-Bullet-Swiss-145377-/331591665948?hash=item4d3467491c:g:mcoAAOSw3ydVjWj7

One unit which does away with selenium wafer diode packs, heat sinks , zener diodes, and associated wiring

[Deleted User]

heres my exploded view of the original Miller and the 6 volt Lucas RM19 replacement alternator along with bearings and seals. I tried to find the least powerful three wire alternator to keep the loads on the flimsy drive to a minimum .

[Deleted User]

Two of the many advantages of a 12V conversion is the ability to buy replacement bulbs everywhere plus a greater choice of (cheaper) batteries. 😀

[Deleted User]

Your right Barrie. I went the antiquated way so that it looked akin to the period it was made in. KP

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