Bridge Rectifier

[frankfast]

A while ago I had the pleasure of discussing the use of a bridge rectifier with De-Cat-I Bob (bless his soul). I have a '71 350 Scrambler whose electrics and harness has long since vanished. I wanted to employ the simplest charging system that I could use so I came across a very small 4 terminal bridge rectifier. With that and a small 6-volt battery I managed to get the bike running with the help of Bobs schematic. There was to be no regulator with this setup. The bike has a 3 wire 60 watt alternator. Bob suggested that I run the two yellow wires to the AC terminals on the rectifier and the red wire directly to the battery. The + terminal on the rectifier was not used. I ran a ground to the - terminal. Recently I read an article suggesting that the + terminal should run to the battery so I abandoned the red wire from the alternator. That made more sense because the + terminal must be providing DC to charge the battery.
The bike ran both ways. I'm not sure the difference between the yellow leads and the red lead from the alternator and can only assume that somehow the red lead was charging the battery with DC current. Wish Bob was still around to explain this phenomena.

[ducwiz]

Pardon me,

but it seems that you are absolutely not knowing what you do while tinkering on your electric system. If you continue to run it without a regulator, your battery soon will die from overcharging, after spilling lots of acid over your precious bike.

Notes:
- is the Ducati 6 Volt regulator-rectifier unit still present?
- original 1971 Scrambler alternators are rated 90 Watt, not 60. Or in case of the electronicignition version (from later years) 75 Watt. In the attached top figure you can see the basic layout of the Scrambler's charging circuit. The alternator is a 2phase or half-bridge version (phase wires = yellow) with a common tap (red = central/neutral). The latter is used as the positive DC output of the alternator-R-R combination (B).



In the bottom figure, a 12 Volt system is depicted which was used on later V-twins like the Pantah. The alternator now lacks the red neutral wire, and the R-R is extended with 2 additional rctifier diodes. Their common connection (B) now is the positive DC output to the battery.

If you want to stay with 6 Volt, the old R-R ist mandatory. Afaik, no 6 Volt R-R from an old japanese bike exists, which will do the job as a replacement.
Lacking the 6 V R-R, I recommend to convert to 12 Volt. But this needs a modification of the original R-R - you have to add 2 rectifier diodes, a small converter electronics, and abandon the red wire. A 6 Volt Ducati R-R, modified for 12 Volt operation in a Scrambler or MkIII (12 Volt converter inside R-R box, hence invisible; new diodes with cooling fins on top):



As a replacement, you may use a Shindengen SH-221-12 (older Kawasaki bikes, KZ400 etc.). It is also possible to use a Ducati 12 V R-R, but the versions for so called "2 wire alternators" exclusively.
Of course you can use a bridge rectifier, actually for both the 12V single or 6V dual phase version, but never whithout a regulating device. The most simple solution is to connect a LUCAS 12 V Zener diode (BSA/Norton/Triumph) in parallel to the battery. But beware, there are versions for positive and negative ground available, so you have to ground your battery according to the Zener type.

Imho, there is no option for a 6 Volt single phase oder 12 Volt dual phase system.

Further questions?

cheers Hans

[frankfast]

Thanks Hans.

Technically you are correct. The bike needs to regulate charging current to the battery and I intend to eventually employ a zener diode as the least expensive means to do that. However, in practically, the bike does not seem to overcharge the battery with only one yellow wire connected to the bridge rectifier, lights on, and running at constant revolutions of 3000 to 4000 RPMs. If city riding were the norm, the second yellow wire could be switched on in order to provide enough charge with the lights on. I've been experimenting with this system for a while now and it seems to be working and Dew-Cat-I Bob, being a proponent of less is better, agreed that it would work. Note that my Ducati Scrambler is not a touring bike and I expect that I might incur a problem over long distances but for the type of riding that I do, It seems to work.

Thank you for you knowledge and insight.

[ducwiz]

Frank,

well, this minimalistic approach seems to work for you. Using only one yellow (and the red) wire cuts the available power in half, 45 Watts maximum. In turn, with headlights switched on you are of course safe against overcharging your battery, but at risk with undercharging. As long as your bike runs with an original CDI (Ducti or Motoplat) this does't matter, because a blank battery only affects your lighting system.

I remember Bob (r.i.p.) as a very profound adept of Ducati singles, and their electrickery in particular. Mostly, I deeply agreed with his explanations and statements. Only once, our differences could not be adjusted - about basic working principle of the w/c singles alternator system.

To increase charging power, you proposed to "switch on the second yellow wire". To be honest, i do not understand how this could be done. A simple circuit schematic would be very useful here, or a reference to an older post of Bob, where he explained his ideas/suggestions.

Hans

[frankfast]

Hans

Search the topic "It's Running". I started the topic when I finally got the Scrambler running. It's a long read starting out discussing a broken oil pump shaft and then transitions to the use of a bridge rectifier. Dew-Cat-I Bob has a lot of comments about the use of a bridge rectifier without a regulator.

By the way, I've ordered a regulator today. - Frank