A7 alternators again....

[Bob Culver]

Can any electronic whiz explain the  regulators used with pm alternators? Would expect the regulator to interrupt output not short it to earth. Voltage spikes might be a problem but not insurmountable. On a small engine the constant power drain must be significant. Most bikes now run with permanet lights on so the waste dissipation may not be great. I became aware of a few years ago when a colleague cooked his motorcycle alternator and found it common for the make.

[Stuart Giles]

Can any electronic whiz explain the  regulators used with pm alternators? Would expect the regulator to interrupt output not short it to earth. Voltage spikes might be a problem but not insurmountable. On a small engine the constant power drain must be significant. Most bikes now run with permanet lights on so the waste dissipation may not be great. I became aware of a few years ago when a colleague cooked his motorcycle alternator and found it common for the make.

I wouldn't claim to be an electronic (or any other kind of) whiz, but the way it was explained to me is that the shorting instantaneously drops the voltage but only for the moment that the voltage had exceeded the Zener value. once the voltage drops (near instantaneous) the Zener diode stops conducting and the battery receives charge as the voltage rises back up to exceed the Zener voltage; this short to earth rise to exceed Zener voltage is a continuing process. There is usually a smoothing capacitor in the regulator which means that along with the Zener diode switching in and out of the circuit at high speed, the output looks like a steady 14 point whatever volts for a 12V system. The particular alty I've used is a relatively low power single phase unit @168W output, so something south of 1/4HP power input even when it's working flat out. So the isn't much power to sink on this one.

More powerful permanent magnet alternators are usually two or three phase output and it's usual to switch one or more phase inputs off (ie disconnect them) when there is low demand. I had an oldish Triumph twin motorbike with a two phase alty, one phase fed the headlight and nothing else, whilst the other charged the battery.

[Dave Mann]

    60 watt quartz halogen is a bit of overkill, here's my 25 watt quartz halogen 12 volt lights which I find OK and I still get oncoming cars flashing me. they draw the same current as the original 12 watt 6 volt bulbs plus almost nothing for the rear LEDs and 2 amps for the coil and a vacuum wiper so I'm pulling a tad over 6 amps.

[Hedd_Jones]

A guy over the potteries way has done a very similar mod to his Ruby. Seems to be Successfull.


  foxfield1a.jpg (Size: 93.16 KB / Downloads: 185)

[Dave Mann]

What's the enormous heat sink for on the extreme left?

[Ivor Hawkins]

I’m no electronic wiz but I think that is a Zenor diode.

[Stuart Giles]

That'll be the regulator; has six wires on most Permanent magnet Altys. They need to have the ability to sink voltage/current to the chassis earth. From my experimenting with drive speeds using a lathe, it's unlikely that it would be shorting any significant current at the usual A7 road speeds: https://www.youtube.com/watch?v=bH4888jLD2w