LED headlight again

[JimF]

I had used a current-driven LED that adapts to varying incoming levels of DC voltage and a simple diode on the AC to give myself DC (albeit pulsating DC) and for the first time ever I had a super bright taillight and running light that was even bright when I was idling at a stop.


http://www.motoscrubs.com/LED_taillight ... rcycle.htm


I should have left well-enough alone, but I didn't.


I figured I could fabricate myself an LED headlight, at least something that would pass any impromptu police inspection and it would be brighter than my DOT 25W headlight which dims to a firefly at idle.

I made the LED headlight and tested it - Perfect!

LED headlight.JPG

I had to do the diode thing again - no problem with that - but then both the headlight and the taillight LEDs both blew up.


I think that the alternator winding, having no real load on it after I took the incandescent bulb off ,ran rampant with regards to peak voltage and blew up the LEDs.

I may have to rectify and regulate one of the two coils in the Ducati to get DC power that has some limits on peak voltage.

I will go through past posts. I am just bemoaning that I have incapacitated my motorcycle in the middle of the riding season.

[JimF]

This image was posted with regards to the 60 watt alternator. I am fairly certain I have the 40 watt one on my bitsa.

download/file.php?id=1980&mode=view

Can I implement the full wave bridge using the winding whose two ends are available on the engine pigtail? I would then use the other winding that is internally grounded to run the ignition system.

On the output of the full-wave bridge I would likely put a regulator.


Jim

[DewCatTea-Bob]

[quote= JimF ...

I made the LED headlight and tested it - Perfect!
but then both the headlight and the taillight LEDs both blew up.
I think that the alternator winding, having no real load on it after I took the incandescent bulb off ,ran rampant with regards to peak voltage and blew up the LEDs. "

____ That's right,, as your relatively high HL.load on the alt.power-coil had acted to relieve power build-up/over-supply (as alt.rotor-revs climbed).



" I may have to rectify and regulate one of the two coils in the Ducati to get DC power that has some limits on peak voltage. "

____ Don't forget that your old/n-c dual power-coil alternator has BOTH types of power-coil/windings... one for ign.system and-also one for the lighting-system !
While either power-coil will well power your set of LED.lights,, the alt.power-coil that's been MEANT-to power the ign.system, should be kept utilized for it's original-purpose.
(Although however, I've found that some [owners of such] who had gone by trial & error connecting each of the three alt.wire-leads -[yellow; white; red] separately to the ign.coil, had discovered that the red/lighting alt.wire-lead could also power the ign.system.)



" I will go through past posts. "

____ Within past threads you've posted before concerning such topic, I've added related-info which it seems you've overlooked or forgotten.



" I am just bemoaning that I have incapacitated my motorcycle in the middle of the riding season. "

____ Seems to have not been such a chancy endeavor, seeings-how you also have your Mach-I to fill-in,, do you not ?




" This image was posted with regards to the 60 watt alternator. I am fairly certain I have the 40 watt one on my bitsa. "

____ That should indeed be correct (for your Mark-III) AND I'm sure that you've already determined that fact for-sure, before !



" Can I implement the full wave bridge using the winding whose two ends are available on the engine pigtail? "

____ I thought you had once replaced the original alt.cable/pigtail with a PC.cable, AND didn't maintain the original wire-connections !? ...
Concerning the original paired yellow & white IGN.SYSTEM-related alt.wire-leads...
yes, that circuit COULD be full-wave rectified, and the resulting power-output of the F-W.bridge could be utilized for running your ENTIRE electrical-system, (thus leaving your alt.power-coil meant for the original-lighting, left untapped).
This circuit-setup would make-use of all the excess power that the stock-ign.system wastes,, and only a little of the power meant for the original/green-coil ign.system would be shared with your LED.lights, (which may slightly weaken the ignition at idle).
This circuit-arrangement/idea would be an-experiment, (as both your LED.lighting-system & ign.system may-not find it to-be totally agreeable) !
__ Full-wave rectification MAY be useful, IF you're planning-on adding a battery. _ But whether you employ a battery or not, your stock/18-degree AAU.type would need to-be changed-over to the 28-degree/battery-sys.type, (so as to prevent excessive dwell-time).
____ I rather propose that instead of full-wave/bridge-rectification, you consider a dual diode distribution-circuit... that way, with just a pair of diodes, you could then divide-up the pos/neg.power-output (of that single alt.power-coil), so that the ign.system would still receive the power-pulse it REQUIRES, and all the opposite/excess power-pulses (which are normally WASTED) could be diverted to power all your LED.lights !
Even-though your LED.system would then only be allowed access to merely-just HALF of that single power-coil's total available power, I suspect that you would still need to make-use of a Zener-diode (to control voltage) anyway.


" I would then use the other winding that is internally grounded to run the ignition system. "

____ As I've already mentioned, others have previously experimented & found that the alt.power-coil originally-meant for the lighting-system, can possibly adequately power the ign.system,, but I would never recommend running THAT-way (without finding a different/non-stock ign.coil that's better matched for running with the lighting-coil).
__ But what's your reasoning/justification for 'full-wave' rectification, (if you don't intend to add a battery) ? _ As your combined LED.system should-not draw enough power to warrant the full-wave power-output. _ And-so merely half-wave rectification of the lighting-coil, should continue to-be more than sufficient, (as you've discovered !),,
you just need a new/alternate-way to limit that-power (which you have ALREADY !).



" On the output of the full-wave bridge I would likely put a regulator. "

____ If you were to allow full-wave power to become available (from either power-coil), THEN your regulator (such-as a Zener-diode) would then have-to work even-more/TWICE-as-hard ! _ So you may as well stick-with merely-just 'half-wave' power-output, (since you've already learned that even just HALF-power, is TOO-much !).
__ I believe that what you may best make use of, is a 'regulator/battery-eliminator unit' (with a built-in capacitor), connected-up to a suitable diode that's fed power from either alt.power-coil,, to properly power-up just your LED.system.

____ I've now touched-on at-least two system-setups for your consideration,
please ask any questions you may have about any of them, or any other power-system/setup you may think of.


Hopeful-Cheers,
-Bob

[JimF]

Let me go slow here and take one item at a time...

If I am not mistaken the ignition winding is the one that presents both ends of the windings as pigtails to the end-user (me.)

The lighting winding is the winding that is internally grounded so only one end of the wiring comes out and its other end is for all intents and purposes the frame. It is connected to the headlamp only through the headlight switch. Hence this singular 20-watt winding is dedicated to one load only; the 25-watt headlight.

I surmised that both the "lighting" and the "ignition" windings were likely relatively equal 20-watt windings (hence the reason we call it a 40-watt alternator), but that the ignition winding was intentionally internally un-grounded such that one pigtail could be grounded by either the brake switch or the brake light bulb thus using the ignition winding to power both the ignition coil and the brake light bulb by use of the same current flowing through both items in a series circuit.

The reason for this I figured is that the 20-watt winding was already under-powered with regards to running a 25-watt headlight bulb. Thus that one "lighting" winding was solely dedicated to the headlamp.

The only place left to get power to run the brake lamp bulb which is a power hog in its 1960s incandescent manifestation is the "ignition" winding, but as we both know placing the brake lamp bulb circuit in parallel with the ignition coil effectively turns the brake lamp switch into a very effective and undesired "ignition kill switch."

To use the "ignition" winding power to run the brake light bulb meant that the current flowing through the ignition coil had to be made to flow through the brake lamp bulb and thus both ends of that winding had to be made available and neither grounded.

If this is anywhere close to accurate, I could then use the internally grounded "lighting" winding to run the ignition system.

The ignition winding is under OEM wiring grounded through the brake switch in its quiescent state or through the brake lamp filament when braking. By using the internally grounded "lighting" winding I am just circumventing the convoluted way in which the ignition system was originally grounded.

That would free up the "ignition" winding that presents both ends of the winding to the pigtails which might then be connected to a rectifier.

Yes, a single diode on the "lighting" winding could rectify that winding's output, but only during the positive half of the duty cycle giving a pulsating DC every half cycle.

Since both end of the "ignition" winding are available, the use of a full-wave bridge is an option as the bottom of the full-wave diode bridge can be grounded to the frame thus rectifying both positive and negative alternating voltage swings into positive voltage. This would provide a more robust power given that its full wave and not that I need it.

In essence I will still be running the ignition system off of AC voltage as I will be using the internally grounded winding to power just the ignition system.

I will be full-wave rectifying the other winding into DC power, though my remaining power requirements which are DC are quite small at about 6-watts.

I believe that running a simple zener-diode to regulate the system overlooks the problem of what to do with the excess power when the Zener begins to conduct. zeners by themselves make poor power regulators.

So perhaps under ideal loss-less conditions I will be creating from the "ignition" winding a DC power source capable of supplying 20-watts. Yes it's over-kill, but if I only pull 6-watts out there is no harm done.


BTW, I don't have a AAU.

Jim

[DewCatTea-Bob]

[quote= JimF ...
" Let me go slow here and take one item at a time... "

____ Sure-thing, no-problem !
Others ought realize that we're discussing the so-called '40-watt' 4-pole alternator (with dual power-coils) that was first employed on the 250Motocross-model.



" the ignition winding is the one that presents both ends of the windings as pigtails "

____ That's correct for the 5-speed batteryless-models, which had the yellow-wire from that (higher tension) power-coil connected directly-to the ign.coil, while a white-wire from the opposite-end of that same power-coil was kept grounded through the (added*) brake-light circuit.
(* The original setup for these alt.models didn't include a white-wire since the two Duke-models that employed them didn't have any brake-light,, so-thus THEN,
BOTH alt.power-coils were internally-grounded.)



" The lighting winding is the winding that is internally grounded so only one end of the wiring comes out and its other end is for all intents and purposes the frame. It is connected to the headlamp only through the headlight switch. "

____ All correct ! _ And that (lower tension) power-coil has a RED alt.wire-lead.



" this singular 20-watt winding is dedicated to one load only; the 25-watt headlight. "

____ You ought-to specify whether you're in reference-to the original/stock system-setup, or something-else that you-yourself have intended.
The stock-setup allowed the red-wire of the lighting-coil to power both the headlight AND the taillight (in parallel).
__ The lighting-coil itself must be rated for more wattage than merely-just half of the 40-watt rating of the entire '40w.alt'.



" I surmised that both the "lighting" and the "ignition" windings were likely relatively equal 20-watt windings (hence the reason we call it a 40-watt alternator), "

____ Some testing would be called-for in order to determine exactly what's-what on this total wattage issue, but such assumption shouldn't be presumed based on the 'rating' that Ducati happened to allow to-be associated with their alternator.
I suspect that each power-coil could-possibly produce 30 to 40 watts each.



" but that the ignition winding was intentionally internally un-grounded such that one pigtail could be grounded by either the brake switch or the brake light bulb thus using the ignition winding to power both the ignition coil and the brake light bulb by use of the same current flowing through both items in a series circuit. "

____ Yes indeed correct,, as that's the quick-fix which Ducati came-up with to economically/simply ADD a (later required) brake-light circuit to their preexisting dual power-coil alt.arrangement.



" The reason for this I figured is that the 20-watt winding was already under-powered with regards to running a 25-watt headlight bulb. Thus that one "lighting" winding was solely dedicated to the headlamp. "

____ The lighting-coil itself could fairly-well handle the power-demands -(about 30-watts) of the head & tail lights but, the added load of the brake-light (in parallel), would lower the total-load's resistance to a level that's below that of the coil-winding itself, thus-then have more voltage & power dropped-across the power-coil itself rather than the resulted load-system,, and that resulted inefficiency would leave too-little power left-over for the intended loads to consume, thus all those lights would become too dimmed-down (whenever having to also power-up a brake-light).



" The only place left to get power to run the brake lamp bulb which is a power hog in its 1960s incandescent manifestation is the "ignition" winding, but as we both know placing the brake lamp bulb circuit in parallel with the ignition coil effectively turns the brake lamp switch into a very effective and undesired "ignition kill switch". "

____ Indeed so, (and well-said),, because before the brakelight-filament can become heated-up & develop more resistance, it's a relative short-circuit that can rob power-juice from the ign.coil (pretty-much the same as a real kill-switch does).



" To use the "ignition" winding power to run the brake light bulb meant that the current flowing through the ignition coil had to be made to flow through the brake lamp bulb and thus both ends of that winding had to be made available and neither grounded. "

____ Right,, but it's rather important to note that the brake-light wasn't fully powered by merely-just the minor-amount of current that's forced to flow-through the ign.coil, but rather far MORE-SO by all the excess current that the ign.points shorts to ground (during the extended dwell-time) ! - (THAT's the current/power that the stock ign.system/magneto-setup normally WASTES.)



" If this is anywhere close to accurate, I could then use the internally grounded "lighting" winding to run the ignition system. "

____ That's been done before, and is known to work well-enough to get started-up,, but such Duke-models that were brought to me with any running-problems always had that incorrect-wiring issue straightened-out by me, so I have no experience with how well that incorrect-wiring could reliably last.
And you have better options for solving your-issue.
__ But if you care to try to find-out, then swap the yellow-wire with the red-wire and be sure to compare how easy getting started is (with both cold & hot engine conditions).



" The ignition winding is under OEM wiring grounded through the brake switch in its quiescent state or through the brake lamp filament when braking. By using the internally grounded "lighting" winding I am just circumventing the convoluted way in which the ignition system was originally grounded. "

____ If both power-coils were otherwise the very-same, then your stated-conception would be totally reasonable. _ However the ign.power-coil's winding is thinner-gauged and has more coil-turns, so it's MEANT-to provide MORE power-TENSION (than the lighting-coil), to the stock GREEN-ign.coil ! _ The lighting-coil may provide adequate wattage, but it's tension-to-current ratio won't be as able to 'shock' the ign.coil as greatly.



" That would free up the "ignition" winding that presents both ends of the winding to the pigtails which might then be connected to a rectifier. "

____ So what worthy advantageous-benefit does that provide for YOUR-case ?
For YOUR particular issue,, you don't NEED to have both ends available, for half-wave rectification.
And as I've tried to point-out, full-wave rectification will only DOUBLE your already overabundant excess-power.



" Yes, a single diode on the "lighting" winding could rectify that winding's output, but only during the positive half of the duty cycle giving a pulsating DC every half cycle. "

____ So isn't that what you've already been doing, (to your past satisfaction) ?
Have you been getting annoyed by a flickering-light at idle, or what ?
__ If you don't wish to include a battery, I think a small/thumb-sized 100uf.cap will cut any flickering-issue.
Do you not understand how such a capacitor will 'filter' the pulsating-DC ?



" Since both end of the "ignition" winding are available, the use of a full-wave bridge is an option as the bottom of the full-wave diode bridge can be grounded to the frame thus rectifying both positive and negative alternating voltage swings into positive voltage. This would provide a more robust power given that its full wave and not that I need it. "

____ Yes that's all correct, (and I've already-before done-so for charging a 12-battery),, and USUALLY I'd advise doing-so for anyone who happens to care to obtain maximum-power from their alternator (for an INCREASED load). _ However in YOUR-case, you have a vastly DECREASED load ! _ And I thought that you had already understood that a 'balanced' system is MOST efficient.



" In essence I will still be running the ignition system off of AC voltage as I will be using the internally grounded winding to power just the ignition system. "

____ I hope that by-now, since I've joined this w.site (nearly 4-years ago),, that you've since come-to understand that the Green-coil ign.system doesn't actually require any 'AC' ! _ But rather only requires merely-just one-single 'pulse' of DC ('timed' from all the offered pos/neg.pulses of the supplied AC).



" I will be full-wave rectifying the other winding into DC power, though my remaining power requirements which are DC are quite small at about 6-watts. "

____ And so then it's wondered WHY exactly !? _ Why is it so important to you to-go digging-up such available power,, and then-after, not make any good-use of it ??
__ It seems that you don't care to abandon a plan which you must've already conceived-through, before you even began this new-thread.
If you end-up going-through with that preordained-route of yours, then I recommend that you handle your miss-balanced system with the regulation-circuit that Hans has suggested (below in his post).



" I believe that running a simple zener-diode to regulate the system overlooks the problem of what to do with the excess power when the Zener begins to conduct. zeners by themselves make poor power regulators. "

____ Yes they aren't the best,, but how complex do you wish to get, anyhow ?
A power-Zener shouldn't have much trouble preforming your particular voltage-regulation needs, providing that you don't unnecessarily overwhelm it with excessive power-output, (as you're seemingly intent on doing).
__ Have you considered adding suitable resistors to your added diode-circuits, so as to separately tailor the power available through each of your LED.lights ?



" So perhaps under ideal loss-less conditions I will be creating from the "ignition" winding a DC power source capable of supplying 20-watts. Yes it's over-kill, but if I only pull 6-watts out there is no harm done. "

____ I'd say that IT's "capable" of up to 30 to 40 watts. _ But on average, "20-watts" probably-is likely within the ball-park.
Half-wave rectification should help keep the wattage from ever exceeding 20-watts, and also then more-possibly allow added-resistors to more-easily completely-tailor the power for your LEDs (without real need for a regulator-circuit). _ (But-ya-see you have-to start-out with a half-way balanced-system, first though.)



" BTW, I don't have a AAU. "

____ Yes, I hadn't forgotten about that. _ But-anyhow, your points-cam should have pretty-much the same dwell-time as the 18-degree AAU.model.
And-besides, weren't you once considering the installation of an AAU ? _ (I had thought-so, and thought that perhaps you may've already done-so.)
__ I now assume that you must've already by-now finally learned how to reliably get kick-started WITHOUT receiving any kick-backs !?


Dukaddy-DUKEs,
-Bob

[ducwiz]

Hi Jim,

you can make a power zener from a few cheap components: http://sound.westhost.com/appnotes/an007.htm. Alternatively, you can build this circuit also with a pnp transistor, which makes it possible to connect the collector tab directly to ground potential without insulation pad. With a pnp, it is possible to replace the zener diode with an adjustable shunt regulation circuit TL431, which improves the precision of the regulation voltage threshold.
These circuits must be placed behind a single diode or bridge rectifier, in parallel to the battery.
In genaral, a zener (i. e. shunt) diode or regulator acts as a power dissipator, burdening the alternator wdg. always with its max. power rating. Power sharing between load (lamp or LED array) and regulator is "automatically" balanced, the excess power ist converted into heat, which has to be sunk somewhere.

A crowbar circuit http://axotron.se/index_en.php?page=26 can act as a short circuit shunt regulator between one or both AC inputs of the rectifier bridge and ground, resulting in a regulation range of 50 or 100%, respectively. Once triggered by overvoltage the SCR gets conductive, steering the AC current into ground for a half cycle until the next AC zero crossing. Power dissipation in the SCR is lower compared to the power zener circuit, because the SCR has a constant forward voltage of 1.5-2V in conductive state. The larger part of the power is dissipated in the alternator winding, but mostly in reactive manner. Of course, the circuits shown above have to be modified for our regulation purpose. All the voltage sensing circuitry has to be connected in parallel to the battery or output of the rectifier bridge, only the SCR or Triac to the bridge's AC input terminal.
Btw, the majority of the regulators used with 3phase alternators in modern japanese bikes are based on this principle.

Hans

[JimF]

Thanks guys.

I'll be verbose tomorrow, but I had been running both the incandescent headlight and an LED taillight in parallel off of the same ac winding.

The LED's a dual-filamet emulating unit so as to work for the running light and the brake light.

Since the bulb's shell is a common ground to both light intensities and because it electrically "floats" in the fiberglass tail, I used a single power diode in the wire connecting the bulb base to the frame.

This gave me pulsating DC but it lacked any regulation.

Still though, the LED worked great.

When I switched out the incandescent headlamp for a similar LED unit with a diode in the line, both front and rear LED'S blew up quickly.

I am assuming the significant loss of load on the winding created a high enough voltage potential on the winding to over-voltage the LED bulbs and destroy them.

Hence I am looking for a fast and easy solution that will limit the maximum voltage.

I don't really want to bread board components together and worry about water, vibration, heat, packaging, connectors between the bike and the bread board etc.

I was hoping to buy a commercial, packaged, potted, thermally encased rectifier regulator unit so as to go from ac to regulated dc in as little time as possible.

Typed on my phone so please forgive auto correct spelling replacing words.

I have a day job, I am working on three different side jobs for clients outside of the day job and I am a one-man fabricator/designer/salesman/tech support/web master/etc for some specialty products.

I'm not complaining, it's just the way things are.

Time to buy component parts and put them together to make and mount and wire up and test and tweak a custom circuit is nonexistent.

[DewCatTea-Bob]

[quote= JimF ...
" Since the bulb's shell is a common ground to both light intensities and because it electrically "floats" in the fiberglass tail, I used a single power diode in the wire connecting the bulb base to the frame. "

____ So then if you ever had-to, you could possibly un-ground that point and rather connect it to your power-source with a power-return wire instead.



" This gave me pulsating DC but it lacked any regulation. "

____ So was that p.DC any cause of concern for you ?



" When I switched out the incandescent headlamp for a similar LED unit with a diode in the line, both front and rear LED'S blew up quickly. "

____ So how high did you get your engine revved-up to, just before that happened ?
(My guess is about 3500.)



" I am assuming the significant loss of load on the winding created a high enough voltage potential on the winding to over-voltage the LED bulbs and destroy them. "

____ No-doubt of that conclusion,, as with such a minor total-load, the power-coil's voltage can then rather easily climb-up to near 35-volts around 5k.RPM.



" Hence I am looking for a fast and easy solution that will limit the maximum voltage.
I was hoping to buy a commercial, packaged, potted, thermally encased rectifier regulator unit so as to go from ac to regulated dc in as little time as possible. "

____ It seems you've definitely missed or forgotten a couple of past thread-posts I've already-before posted (concerning this particular matter), with links to related items which you ought to have been interested in. _ Here's one (in a thread that you hadn't started)... viewtopic.php?f=3&t=1373&p=9816&#p9816 ,
and here's another in one of your-own threads... viewtopic.php?f=3&t=1420#p10088 .
__ And here are a couple more links to other similar items more currently-found on eBay... http://www.ebay.com/itm/310703308638?ss ... 1438.l2649 ; http://www.ebay.com/itm/TYMPANIUM-AC-Vo ... 67&vxp=mtr .
__ If you had PAID-ATTENTION before, back when these type of v.regulator items were previously discussed (within various past threads),, then you would've already realized & been aware that the more common full-wave rect/reg.units are-not the ONLY way to go !
While their prices & details vary,, with your choice-pick of one of those type of AC.voltage-limiters, your-issue ought to-be most-EASILY solved ! _ As you'd then just simply connect-up one such unit between ground & the red-wire of your lighting-coil !
Is THAT not simple enough for you ? _ Or do you still insist on trying-out your-own pre-established idea/setup ?
__ I-myself would-not choose to-go this simplest-route, (and would rather half-wave rectify FIRST, and then-next employ a Zener-diode to cut-off the power-peaks of just the pos.pulses), as the (slight) downside of such AC.regulation cuts-down the peaks of BOTH positive & negative pulses (needlessly in such case as yours).
I don't really know about those AC.regulators,, but perhaps they may still function as expected, even when connected-up AFTER a half-wave rectifier has filtered the feed-circuit.
____ So NOW do you accept an alternate-plan ? _ And if-so, what circuit-details are you considering next/NOW ?


Hopeful-Cheers,
-Bob

[JimF]

Sometimes it's better to go with the devil you know rather than devil you don't know.

The simple zener on a half wave bridge requires a shunt resistor at the top of the zener. Calculating the shunt resistor's ohmic and power values, the zener's power and the bridge diode required is more effort than I really have time to do correctly. I could make a mistake, order the wrong parts, assemble it, test it, realize my mistake, perhaps damage my LED loads again and for all intents and purposes be back to square one two weeks from now. All this requires time which is in short supply for me.

Do you understand why I am adverse to this methodology?

If I came home from work and after eating dinner had the simple choices of what to do with the remainder of the evening; working on this stuff or sitting on the sofa watching TV from "Wheel of Fortune" to the evening news then yes, I would work on Ducatis all night long instead of watching TV.

As it is I will spend five hours working on a computer after dinner, and none of that work will be on my motorcycles which is why one is still in boxes, one has had no compression for over a year and I have found no time to look at it, and one seemingly has a problem with its accelerator pumps.

I am still thinking to buy a packaged motorcycle application specific combination regulator/rectifier so as to get something that works. Expensive, easy, and non-time consuming trumps inexpensive, easy and time consuming owing to my current life style.

So if I can take a water-tight vibration-proof unit and plug AC into it and get regulated DC out simply by throwing money at the problem then that's the tack I will likely take.

[DewCatTea-Bob]

[quote="JimF ...
" The simple zener on a half wave bridge requires a shunt resistor at the top of the zener. Calculating the shunt resistor's ohmic and power values, the zener's power and the bridge diode required is more effort than I really have time to do correctly. "

____ There's no such-thing as a "half wave bridge" rectifier-unit, (as the term 'bridge' comes from the specific arrangement of 2-pairs of diodes [to obtain F-W.rectification] ).
__ The main-purpose of the resistor that's normally in series with the Zener-diode in a common/std.zener-circuit, is for dispersing the discarded-power so that the Zener-itself doesn't have to do so (as heat). _ But in the case of a 'POWER-zener', the Z.diode is well-able to mostly* disperse the power/heat on it's own (providing that it's max.wattage-rating is-not approached). _ (* The rest is deferred to the alt.coil-winding, rather than that included resistor mentioned.)
It was expected that such a 'power-zener' (as used by Brit-bikes *), would be the only type of Zener-diode to be considered for the-job,, and the rather light work it would be expected to perform in such a case as yours, would not overtax such a capable Z.diode.
(* Here are links to three examples... http://www.ebay.com/itm/320712068055?ss ... 1438.l2649 ; http://www.ebay.com/itm/321121077954?ss ... 1438.l2649 ; http://www.ebay.com/itm/321159189880?ss ... 1438.l2649 )
__ But anyhow, if my post-wording had been correctly-understood as expected, then it ought-to have been realized that I was not-RECOMMENDING such a zener-circuit for YOU-yourself,, but rather, merely-just that that's what I'D choose for MYSELF in such case, (as I deplore waste) !
But I did indeed MEAN-to 'recommend' however, that you get yourself one of those AC.regulators that I provided links to ! _ As ya can't get anything simpler to SOLVE your-issue !



" Do you understand why I am adverse to this methodology? "

____ But OF-COURSE ! _ And I had NEVER MEANT to seriously-suggest that you actually consider going-through such an intricate route (as that which you've given good-reason to not pursue).
I AM indeed actually 'on-board' with your-quest for quickly worked-out SIMPLICITY !
(But it seems that even you-yourself are predisposed to go-with a more complex fix than is necessary.)



" I am still thinking to buy a packaged motorcycle application specific combination regulator/rectifier so as to get something that works. "

____ So did you have any such R/R.unit particularly in-mind ? _ Did you notice such in ANY of the links I had posted ? - (The links in my previous-posts within THIS-thread lead to other posts with their-own posted-links.)
Why do you think you need a 'rectifier' as well ?
__ That you haven't changed your mind (about your rather complex fix), leads me to figure that you must-not have looked-through all the links I provided (to those examples of AC.regulators) ! _ Cuz CLEARLY,, going such a route as simply employing one of those AC/r.units, has GOT-to-be YOUR BEST-possible avenue !
Did you not follow-through any of my posted-links to any of those AC.regulators and read anything about them ?
Whether you checked them out or not, let me explain the intended-function of those AC.regulators...
__ For most-all motorcycles with AC.lighting (which have a grounded lighting-coil setup just-like your Mark-III has), all the lighting-filaments need to be protected from burn-out by over-voltage conditions -(such-as when a high-RPM speed-shift is missed),, so 'AC.regulators' are needed to cut-down excess-voltage on the output/feed-line between the alt.power-coil & the lights, (with respect to ground).
This is pretty-much EXACTLY the same condition that you are now faced with (since you've now MISS-matched your lighting-load to it's power-source) !
Of MINOR-concern, is that an AC.regulator works like a Z.diode, except that it cuts excess peak-voltage of BOTH pos & neg power-pulses,, and while that's half-way wasteful for your DC.LEDs, it doesn't make any noteworthy difference anyway.
__ Now while most AC.regulator-units don't include a rectifier, (since std.lights don't require DC),, YOUR particular application should-not require any further additional rectification anyhow, (since you've ALREADY got that arranged & well handled) !
__ So now what's left for you to be concerned with, for you to not prefer-to choose this rather obvious-route, (as opposed to your-own considered [and much-more questionable] fix/solution) ?



" So if I can take a water-tight vibration-proof unit and plug AC into it and get regulated DC out simply by throwing money at the problem then that's the tack I will likely take. "

____ Do you now fully realize what's-what concerning everything ?
__ I'm sure those AC.regulators are at-least just as bullet-proof as whatever R-R.unit you'd choose. _ But if I've been merely addressing the wall behind you, then what R-R.unit are you intending to connect-up to your ign.designated alt.power-coil ? - (Too-bad that you hadn't taken my old-advice and connected wire-leads to EVERY power-coil pole, back when you replaced your alt.cable !)
__ However if good-reason is to prevail... if expense is no-concern, and your LED.creations are quite important to you,, then I'd suggest that you obtain & employ a pair of those AC.regulators, connecting one unit on the line to your rear-light and the other unit on the line to your headlight. _ That-way, not only are you then doubly-protected,, but with the two units working in parallel, the expected voltage cut-off point will become sharpened (as inevitably*, one will likely kick-in soon after the other has begun striving to do the job alone, [* as tolerance-variances prevent identical workings] ).

____ BTW...
When could you possibly have the time to make-sure that your ignition will still preform as well, with it reconnected & powered by the lighting-coil ?
And also, when will you share all the details of your LED.headlight (like you had done with your LED.taillight) ?


Hopeful-Cheers,
-Bob