Electrical power seems low...

[JimF]

The headlight is 25 watts (both hi beam and low beam) and I suspect the brake light is on the order of 15 watts. (The caveat here is that after 40 plus years there is not telling what brake light bulb has been placed in there by now.)

So I get your point , that a combined 40 watts of load on one coil is too much. More so in light of the published specification that entire alternator (both coils combined) was only specified when new to be 40 watts.

Given 25 watts of headlight power and a negligible taillight filament on one coil, sharing the approximately 15 watt brake light filament with the ignition coil is a sensible division of power loads on a system that is by default stressed to its limits.

One good thing I did was to make the headlight bucket "wiring central." The brake light circuit attaches to the "lighting coil" in my headlight bucket. The ignition winding makes an appearance in the headlight bucket as well for purposes of connecting to the ignition switch and then out to the clip-on mounted "kill" switch.

So my next move then is to move the brake light circuit wire off the lighting coil and onto the ignition coil winding.

I will report back again.

Jim

[DewCatTea-Bob]

____ ALRIGHT Jim, this last post of yours is extra productively informative to help bring this particular (extra!) poor lighting issue (of yours) to become all cleared-up !
As you've included a summarization of most all the involved pertinent facts, (thus saving myself from doing such, in-the-dark).
__ This particular load-system set-up that you expected ought work, was a understandable idea to try-out,, however now we both have independently learned that this particular light-wiring set-up only works-out well IF all three lighting-loads never happen to be put in use all at the same time. _ Cuz when the lights are running and the brake-light gets activated, then all three lights are effectively shorted-out to uselessness !
So then, it seems that ANOTHER alternative (to the original/stock brake-light wiring-setup), has to be determined & employed (for your project).


" So my next move then is to move the brake light circuit wire off the lighting coil and onto the ignition coil winding. "

____ That of course is what Ducati decided had to be done, back when the original all AC-powered electrical-system had to be modified so as to also power a brake-light !
And Ducati did the same thing that you now intend to try next, however THEY chose to wire-up the brake-light in SERIES with the ign.circuit, but I'm getting the impression that YOU are going to next try wiring the brake-light in 'parallel' with the ign.circuit. ...
Now if THAT wiring-setup would work-out okay, it seems that Ducati would've went that route rather than employ their second-thought -("crazy method") series-arrangement which they came-up with. _ So thus-then one ought guess that the parallel-method idea won't work either, (as I-myself had ALSO learned way-back when I was a teenager).
So to spare you from learning THIS lesson all on your own, as well,, here's why that parallel-method won't work...
When the brake-light filament is yet unheated, it's then pretty-much the same as a piece of conducting-wire,, so therefore the brake-light circuit would have the very-same effect as a kill-switch ! _ Thus when a (normal-type) closing-circuit brake-light switch is activated, the engine will then become without any ign.spark !

____ So then what's the next ALTERNATIVE brake-light setup idea for your project (as it currently is) ? ...
Well I think I have a great answer for you to try-out NEXT !
Leave your 25w.headlight and current wiring-setup as-is, and replace your old-type 6v.filament tail-bulb with a new-type 12v.LED tail-bulb ! \
Cuz THEN, the brake & tail lights ought to be able to light-up well enough while drawing next to no current, plus your headlight can then have (available to consume) most-all of the power-juice (produced by the lighting alt.power-coil).
__I don't suggest a 6-volt LED.tail-bulb because it would likely get over-powered by such an unregulated power-source, once the RPMs provided the headlight with it's 6v-25w power-rating.
I really think that this suggestion IS your next easiest step to try-out.

[JimF]

And Ducati did the same thing that you now intend to try next, however THEY chose to wire-up the brake-light in SERIES with the ign.circuit, but I'm getting the impression that YOU are going to next try wiring the brake-light in 'parallel' with the ign.circuit. ...
Now if THAT wiring-setup would work-out okay, it seems that Ducati would've went that route rather than employ the second-thought series-method which they came-up with. _ So one ought guess that the parallel idea won't work either, (as I-myself had also learned way-back when I was a teenager).
So to spare you from learning THIS lesson all on your own, as well,, here's why the parallel-method won't work...

And I will learn my lesson the hard way because I respectfully disagree.

In the case of the original wiring, the ignition coil was grounded UNTIL you stepped on the brake pedal. The brake switches employed for the 'magneto' bike were Single-Pole Single-Throw (SPST) Normally Closed switches that connected the ignition winding to frame ground in the default position. So for the majority of the riding time the brake light bulb was not in series with the ignition winding.

When the rider applied the brake you are correct, the ignition lost it's ground reference through the switch (the path of least resistance) and the ignition current discovered another path, albeit reactive, through the light bulb filament.

At this point the bulb and and ignition coil are dividing up the available voltage, so neither is getting the full benefit of the ignition winding's total voltage output.

I believe that this was not done to limit the power being drawn from the ignition coil, but was probably done as some kind of stupid exercise to signal the rider that his brake light bulb had died. After all if the brake light filament failed, the ignition circuit would kill when the brake pedal was depressed.

I think this is the crux of the matter on which we disagree. You suggest that Ducati had wanted to limit the power draw on the ignition winding so they placed the brake light filament in series (but only when braking) with the ignition coil.

I suggest that this in-series string of the ignition coil and brake light filament was just a misguided attempt to create a safety feature.

I acknowledge that I could be wrong, but I am willing to try to run the brake filament in parallel with the ignition coil. The benefit, if this works, is that the brake filament will ride the full peak-to-peak voltage from the coil and will be brighter for doing so. A small secondary benefit is that the ignition coil will not suffer a voltage drop either when the brake pedal is depressed.

I can't say what the impedance of the coil will be or the impedance of the brake light filament, but I have to think all those windings in the ignition coil are going to present a high impedance and thus not demand heavy current from the 'ignition' coil leaving enough power to run the brake light filament.

But you say you tired this as a teenager and it didn't work? What happened that it didn't work? What was the indication? Had you changed the brake switch from a Normally Closed (NC) type to the Normally Open (NO) type as you would have needed to?

Once again, we could both find me logging another message here tomorrow wherein I might bow to your wisdom and say "you told me so."

Jim

[DewCatTea-Bob]

____ Well Jim, again today you've once-again placed a reply-post while I'm still yet unfinished with my-own (previous) posting.
Doing-so further clogs-up our wheels from turning-forth & onward in a more straight-forward & more timely manor.
__ After just-now reading your latest-post, I see that we are indeed currently in some disagreement, but unlike yourself, I DO know for sure which one of us is CORRECT on this stuff !



" And I will learn my lesson the hard way because I respectfully disagree. "

____ That's fine Jim, cuz I was going to suggest that if it's easy enough for you to go-ahead & try-out, you can 'prove' that the parallel brake-light circuit will act same as a (normal-type) kill-switch circuit.
I'm quite fine with that being confirmed !
__ As I recall though, I think,, if you try your brake-light circuit while at very high RPM, you may then get the engine to remain running (as the brake-light filament lights-up), but also then get some disturbing back-firing. _ Let us know.



" In the case of the original wiring, the ignition coil was grounded UNTIL you stepped on the brake pedal. The brake switches employed for the 'magneto' bike were Single-Pole Single-Throw (SPST) Normally Closed switches that connected the ignition winding to frame ground in the default position. So for the majority of the riding time the brake light bulb was not in series with the ignition winding. "

____ All certainly quite true of course, except that your very-last sentence (directly above) is not EXACTLY 'correct'...
Effectively 'electrically', the brake-light filament is not in 'series' with the ign.circuit whenever the original-type brake-light switch is in it's default-setting, (just as you contend),, however ACTUALLY, it in-fact remains in series PHYSICALLY at all times. _ It's just that when the (original/odd-type) brake-light switch is put in activated-mode, it THEN removes the brake-light circuit from remaining in parallel with the (default) direct-ground circuit !



" When the rider applied the brake
, the ignition lost it's ground reference through the switch (the path of least resistance) and the ignition current discovered another path,
through the light bulb filament.
At this point the bulb and and ignition coil are dividing up the available voltage, so neither is getting the full benefit of the ignition winding's total voltage output. "

____ All this (directly above) is certainly quite true.



" I believe that this was not done to limit the power being drawn from the ignition coil, but was probably done as some kind of stupid exercise to signal the rider that his brake light bulb had died. "

____ You're kidding, right ? _ Whenever I've heard this tail before, I'd assumed it was expected to inspire a little chuckle.



" After all if the brake light filament failed, the ignition circuit would kill when the brake pedal was depressed. "

____ Yep, and that's why Ducati began putting bypass toggle-switches in the taillight-shell/bodies.



" I think this is the matter on which we disagree. You suggest that Ducati had wanted to limit the power draw on the ignition winding so they placed the brake light filament in series
with the ignition coil. "

____ Well actually, I don't think they were particularly concerned with exactly the amount of 'load' to be placed on the ignition-system's alt.power-coil, but rather simply coming-up with a way to avoid totally robbing-away the ign.system's power-source, (like a kill-switch does), while using the taillight-bulbs available back in those days. _ (Too bad they didn't have LED.type TL.bulbs, as THOSE could've worked without stealing the power-juice from the ignition.)



" I suggest that this in-series string of the ignition coil and brake light filament was just a misguided attempt to create a safety feature. "

____ That thought has never occurred to me as serious. _ I believe that if it ever actually occurred to Ducati, I'm sure it was just an afterthought.
__ It should be realized that the original AC-powered only electrical-system was conceived for the early 4-speed 250-Motocross model (and later duplicated-over to the original Mark-III), and so that system was very simplified...
It had it's two separate power-coils feed two separate load-systems,, one power-coil for just the ign.system, and the other for just the tail & head lights,, (and NO brake-light or anything else!).
Then later, (around the time when the 5-speeders came-out), I believe Berliner had asked Ducati to include a brake-light for the models with this bare-basics AC-system,, and-so Ducati very basically concocted their only means to make additional use of their existing bare-basics AC-system, (thus giving us their notorious brake-light circuit fiasco).



" but I am willing to try to run the brake filament in parallel with the ignition coil. The benefit, if this works, is that the brake filament will ride the full peak-to-peak voltage from the coil and will be brighter for doing so. "

____ If you had fully taken-in all I've stated on this AC-supplied ign.circuit, you'd then wonder how you could think that there could possibly be any sufficient power-juice left to power-up a light-filament,, cuz as I've stated (many times before), the ign.system WASTES directly to ground the major-majority of the power produced by the alt.power-coil which powers it !



" I can't say what the impedance of the coil will be or the impedance of the brake light filament, but I have to think all those windings in the ignition coil are going to present a high impedance and thus not demand heavy current from the 'ignition' coil leaving enough power to run the brake light filament. "

____ But what about the ign.points ? _ Your parallel circuit-setup would have the brake-light bulb's filament competing with (depending on the moment in time), either the ign.coil (which doesn't really have a very-high impedance for rejecting current-flow thought itself), or a complete direct short-circuit (through the points) ! _ So that ign.circuit leaves very darn little power-juice left for the filament to make any use of ! _ (Not to mention again that what juice it accepts will rob the ign.coil of the full inducted-pulse of DC which it must depend upon.)



" But you say you tired this as a teenager and it didn't work? "

____ Right... It was spring of my 2nd.year with DUCATIs, and my brother's 5-speed Scrambler had been going-through tail-bulbs at such a rate that more often than not, he had no brake-light and didn't like the motor dying when he used the rear-brake.
At that time I knew a good-bit about electricity, (but little about how DUCATIs worked), and thought I could wire-up the brake-light to work another way.



" What happened that it didn't work?
What was the indication? "

____ The parallel circuit setup ended-up acting the same as the kill-switch I had installed, (of course) !



" Had you changed the brake switch from a Normally Closed (NC) type to the Normally Open (NO) type as you would have needed to? "

____ Yes, of course... My Monza had had a normal-type universal brake-switch installed when I first got it, (which I had replaced with a stock-switch), so I installed that spare non-stock switch onto the Scrambler.
__ I later still found a way to make use of that non-stock type switch on the Scrambler's system, with the use of a couple diodes (to divide-up the negative & positive pulses of power-juice between the ign.circuit & the brake-light circuit).



Dukaddy-DUKEs,
-Bob

[JimF]

Bob....

You were right.


The bike runs fine but when I step on the brake the brake light filament demand so much power that the voltage drops below a level that can sustain the ignition coil and the engine dies.

Clearly Ducati did not implement the brake light filament in series with the ignition coil as a safety feature but rather as the only viable way to put the brake light on the ignition coil without killing the ignition itself.


You were right. You were right.

You told me so.

Jim

[DewCatTea-Bob]

" The bike runs fine but when I step on the brake the brake light filament demand so much power that the voltage drops below a level that can sustain the ignition coil and the engine dies. "

____ If you had thought it out fully Jim, you then would've realized that the circuit you tested is basically the same as a standard kill-switch circuit.


" Clearly Ducati did not implement the brake light filament in series with the ignition coil as a safety feature but rather as the only viable way to put the brake light on the ignition coil without killing the ignition itself. "

____ You've taken the words out of my mouth (before I got-around to posting them myself) !


" You were right. "

____ I'm always glad to have my claims confirmed !


____ Jim, have you read-through my completed posts on this page ?
There are still a number of alternatives (to the stock-Ducati brake-light circuit) to consider & try-out !
Are you against installing a battery ?
Or what about a LED.tail-bulb ? _ I believe the LED-option on the tail is a better choice than a LED.headlight, cuz then the load-system will better cope without any voltage-regulation circuit, and you could then keep YOUR wiring-setup as you had intended it, (providing I'm right that the LED-circuit won't also offer a kill-circuit around the ign.circuit).
In any case, an added diode or two will solve your project's issue !



Dukaddy-DUKEs,
-Bob