Bridge Rectifier

[frankfast]

I would like to revisit a thread from a while ago. I currently have installed a bridge rectifier in lieu of a regulator. I've been told that with this installation, and I have no doubt that this is true, the battery will over charge. But what effect would having the lights on have on the charging circuit? Would the lights draw enough amperage to keep the battery from over charging? Also, one of the two yellow leads from the alternator could be disconnected from the rectifier. How would that affect the charging circuit. Both the coil and lights are connected directly to the battery. Would it be worthwhile to install an ammeter to check the charging circuit. I'm not sure if I have a 70 or 90 watt alternator on this '71 Scrambler. Any insight would be helpful. Thanks.

[ducwiz]

Hi frankfast,

first of all, you should find out the alternator type/power output.
Next, do you use a 12Volt or a 6V battery?
If the alternator is the original one in your '71 SCR, it should be a Ducati 90 Watt. These have 3 wires inside the cable, 2xyellow, 1xred. If a previous owner has changed the alternator to a 75Watt type, it could either be a Ducati or a Motoplat. The Ducati has 5 wires coming from the stator (2xyellow, 1xred, 1xgreen, 1xwhite). The Motoplat has only 4, (2xyellow, 1xred, 1xblue). The Ducati has 2 separate cables and hence a larger cable feedthrough with two openings. The '71 engine case has the small feedthrough, so the 75W stator can only be mounted/used after cutting the 2wire-cable at the stator. The Motoplat stator might fit, because the 4 wires are all routed inside a single cable.
Imho, it is a bad idea to run the charging system without a regulator. Cheap chinese 12V regulator-rectifiers are available i. e. on ebay, as well as a more expensive but versatile switchable 6V/12V R-R from some internet shop (no link or name at hand). An ammeter might cost you more than a cheap 12Volt regulator. And it cannot prevent overcharging ...
You might reduce overcharging of your battery by switching on the headlight as a continuous load, but you can never be shure your battery is 100% protected against overcharge, or on the other hand, getting sufficiently charged. And what do you do, if your headlamp fails, but there are many miles left to your home, and no replacement bulb is at hand?

cheers Hans

[frankfast]

The Scrambler has a 6 volt battery and 3 wires from the alternator, two yellow and one red. Therefore it must be the original 90 watt unit as you have described. You are absolutely correct in that I would never be sure if the battery is over charging or under charging. Since the bike is a play thing used only for short trips and regulators are costly, I thought I might be able to regulate the charging of the battery by either the use of lights or removing one of the yellow wires or a combination of both. An ammeter is an inexpensive option to a regulator and can be used elsewhere. But thank you for your comments. They will be taken in advisement.

Anther question. Has anyone ever modified this type of altenator to run AC without a battery?

[ducwiz]

OK, 6 Volt.
So, what type of rectifier do you use, and how it is connected between alternator and battery?

Hans

[frankfast]

Two yellow wires are connected to the two AC terminals of the bridge rectifier. The red is not used. The plus terminal of the rectifier is connected to the positive terminal of the battery and the minus terminal of the rectifier is connected to the negative terminal of the battery. The positive terminal of the battery is connected to a toggle switch which is then switched to coil and fuse box for the horn, brake light and lights. The lights are powered via another switch. It's a very simple arrangement.

[Jordan]

Has anyone ever modified this type of altenator to run AC without a battery?

That can be had by using a CDI type stator - the 75W.
A Motoplat CDI stator cable will pass through the hole in the crankcase without problem.
A Ducati CDI is fitted to singles that have a larger hole for its two stator cables. You have the smaller hole.
It might be possible to convert to a single multi-wire cable that would also fit through, especially if you convert to 12V which uses one less wire.

To modify your alternator (if it has 6 poles), you could do what I did and convert it to CDI.
It involves replacing one of the wired bobbins with a CDI bobbin (from an Indian made scooter).
It is not a trivial job but it works.

If you have not connected your red wire and use only the two yellows, you should have a 12V system?

[frankfast]

If you have not connected your red wire and use only the two yellows, you should have a 12V system?[/quote]


If that's the case then my connections to the bridge rectifier are wrong and it seems I would destroy the battery?

[frankfast]

If you have not connected your red wire and use only the two yellows, you should have a 12V system?

"


If that's the case then my connections to the bridge rectifier are wrong and it seems I would destroy the battery?

[Jordan]

Try this?

Remove battery and change lights to 12V.
Install a Lucas ZD715 "dumb" 12V regulator or equivalent.
Install a capacitor instead of the battery - approx 10,000 uF, 24V.

A digital voltmeter is cheap and easy to install compared to an ammeter.
It gives useful and reassuring real time info on charging status.

[veloduke]

If that's the case then my connections to the bridge rectifier are wrong and it seems I would destroy the battery?

Correct. You will be applying full alternator output to the battery with no control.

If I were you (which obviously I am not ), I would take the opportunity to change to 12V - 12V reg/rec, 12V bulbs, 12V battery, 12V horn and 12V coil
Two yellow wires to the reg/rec, the rest as you described.

Some expense, yes, but you should then have reliable light and charging.

Good luck