AC/DC ground planes

[sluggo49]

Not XX related.

 

I have a bike (XR650R) that has  AC lighting and AC to the CDI box. The alternator has separate coils for the two circuits and neither is grounded at the alternator. Each AC circuit is grounded to the frame, though at separate physical points.

 

I am converting the lighting circuit to DC by running the output of the lighting coil through a regulator/rectifier. I intend to use the frame as the DC ground as well as retaining it for AC ground.

 

Anyone tell me why sharing the frame for the two ground planes wouldn't work? There is plenty of advice about this on other forums, but I haven't seen a good explanation from the doubters as to why it won't work. I understand electricity pretty well, so don't hesitate to get technical.

 

For what it's worth, I'll be running LED lighting, horn and fan on DC.

[XXitanium]

...just pull it straight from the (is there one?) battery?

[superhawk996]

How and why did someone put an alternator on the bike?

[redxxrdr]

Without thinking too much.

Quick google using I phone to get these schematics.

The stator looks to be a 3 phase, like most bikes of today. But, they used each phase/coil separately.  maybe to allow operation with a dead battery?

 

I assume that your goal is to switch to LED lighting.  If so, the modern r/r is a full wave bridge. Input to the bridge would be the output of one of the coils.  Output of the rectifier would be plus and minus DC.

If the coil, also has a connection to ground, then  half the rectifier could be bypassed by a short to ground.  Lower output.

 

since LED are still diodes, I bet that they will work on the AC circuit. Diode being a built in rectifier.  But.....  you may experience a dimmer output, since you would be using a half wave rectifier instead of a full wave, as is used on newer systems.  Also, a/c output on 3 phase systems are around 60 vac. The regulator helps load the output to 12-15 vdc.

 

First confirm that my search found the schematic correct for you bike. Then confirm what you are trying to do.  I often read things wrong.

 

Myself, i would try the LED lamps on the existing A/C circuit.  Turn signals will require a different flasher relay, designed for LED current flow .

 

Just wondering, do the lights get brighter when you rev the engine?  That is how my bikes from the 1960’s worked.

 

Or I could have completely miss understood.  🤔

Edited February 24, 2019 by redxxrdr

[sluggo49]

10 hours ago, XXitanium said:

...just pull it straight from the (is there one?) battery?

I'm adding a battery for the DC circuit. It's there mostly to absorb the ripple. A large capacitor would do about the same thing.

[sluggo49]

26 minutes ago, redxxrdr said:

Without thinking too much.

Quick google using I phone to get these schematics.

The stator looks to be a 3 phase, like most bikes of today. But, they used each phase/coil separately.  maybe to allow operation with a dead battery?

 

I assume that your goal is to switch to LED lighting.  If so, the modern r/r is a full wave bridge. Input to the bridge would be the output of one of the coils.  Output of the rectifier would be plus and minus DC.

If the coil, also has a connection to ground, then  half the rectifier could be bypassed by a short to ground.  Lower output.

 

since LED are still diodes, I bet that they will work on the AC circuit. Diode being a built in rectifier.  But.....  you may experience a dimmer output, since you would be using a half wave rectifier instead of a full wave, as is used on newer systems.  Also, a/c output on 3 phase systems are around 60 vac. The regulator helps load the output to 12-15 vdc.

 

First confirm that my search found the schematic correct for you bike. Then confirm what you are trying to do.  I often read things wrong.

 

Myself, i would try the LED lamps on the existing A/C circuit.  Turn signals will require a different flasher relay, designed for LED current flow .

 

Just wondering, do the lights get brighter when you rev the engine?  That is how my bikes from the 1960’s worked.

 

Or I could have completely miss understood.  🤔

Thanks for the response and the effort.

 

The schematic you reference includes an aftermarket or re-wound stator. It is not the stock wiring diagram, but it is useful for folks who buy aftermarket packages to convert the XRR to a dual sport.

 

The stock stator is good for about 80W out of a single winding, which is what I'm using. The aftermarket stators became popular with HID lighting, but I'm running an LED headlight, 2000 Lu at about 18W. Turn signals and tail/brake light are all LED and almost negligible load. Fan draws load is less than 30 W. I don't much care about the horn load.

 

To answer some of your questions:

 

Yes, the LEDs work on AC, and as you imagine, they flicker , particularly at lower RPM. AT higher RPM they become fairly steady, so they appear brighter.

 

On my XRR, I considered just putting a full-wave bridge rectifier that I have lying around downstream from the AC voltage regulator, but the mounting and wiring looked fairly tacky. So I am using a reg/rec from an XR650L that is a spare. It is also a 3 leg AC unit, but I'll just wire my coil to one of the legs. I think the XRL reg/rec is good for somewhere around 150W, so it shouldn't be a problem.

 

As far as bikes from the '60's are concerned, my first was a 305 Superhawk. It ran lights on regulated AC, but they were incandescent. So while they dimmed substantially at lower rpm, they didn't suffer the diode switching effect that LEDs do.

 

Back to the shared ground plane: I'm thinking that there shouldn't be a problem since the voltage sources are isolated from each other. Nothing on the DC side is gonna suffer from an AC ripple, and I believe the DC side of the rec/reg is a pretty high impedence, so there should be no worries there.

 

On the AC circuit that drives the ignition (CDI and coil), the CDI supply peak voltage is somewhere in the 60 to 100V range, so 12VDC should be invisible.

 

I guess I'm most concerned about the ignition pulse generator which has a peak voltage of less than 1 V and shares the frame ground. But I can't see where that would be a problem, either, since using the frame for a DC ground reference should be electrically transparent.

 

I'm in the process of wiring the bike as I build it. It has a few wiring kludges already built in that I'm disentangling, so it may be a few days before I can try out my solution.

 

Any further thoughts would be appreciated.

[mikesail]

Do note that the usual R/R is a shunt regulator, and will increase the current thru the alternator coil depending on RPM. The newer mosfet R/R units I think are a series regulator so you would not have this issue. If this bike still has the coils dry I would be cautious about using a shunt.

 

Are you sure the ignition trigger is grounded? I would expect it to be floating. Still a single point connection between two floating alternator circuits should be fine.

 

I suspect the original coil for the light is designed to be current limited, but as you are using a lower wattage this should be fine.

 

[sluggo49]

Well, I forgot about this thread but I’m happy to report that using the frame for both the ac and dc ground planes works very well.

 

to summarize, the cdi is powered by one coil on the stator (unchanged from stock configuration) and the other stator coil now feeds a shiny regulator/rectifier to provide dc to lights, fan, horn, directional and a little battery. Everything works properly and the battery charges nicely.