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Rear brake light

Started by yosemite, 17 Aug 2023 at 18:28

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The rubber insulation on the wiring harness of my T35 has in places disintegrated so started to rewire,  so started to remove old wiring was going ok till I got to rear light which has a single filament 25w bulb, what is confusing me is that I have searched everywhere for a ballast resistor in the tail light circuit, but cannot find one,so the only way I could think of is its using a bulb as a ballast, but having replaced all the bulbs at various times I can not recall any of them affecting the tail light. Is there any other way that anyone can think of that could be used control the light output from the bulb?


Surely it is just a question of someone at some stage has fitted the wrong bulb. It should be a 5 watt bulb or if you fit a brake light it should be 5/21 watt.


Its a single filament bulb, 21w, light it gives out when lights are on is about normal for a rear light depress the rear brake pedal and the light intensity is about right for a brake light. Bulb holder has only one  central contact, so wouldn't work with a normal stop/tail 21/5 bulb as I said the only way I could think of to make it work would be to have a ballast resistor in tail light circuit with the brake light circuit  bypassing it and supplying the full voltage. Have searched everywhere but cannot find any sign of it. The wiring harness appears to be home made as it only has one colour of wiring, so maybe the ballast resistor is covered in the harness tape. Will split the old harness tomorrow, but at moment I am still searching.
I thought about changing rear bulb to a double filament but that would be difficult as the whole bulb holder would then need to be changed so as to accommodate a bulb with offset pins.

My 4th bike

A few small diodes wound into the tail light circuit will lower the volts feeding the 21w bulb.
This will lower the bulbs brightness when running with a tail light.
Brake light circuit has to bypass this string of diodes.
You can adjust the taillight brightness by using more or less diodes in the line.
Diodes can be had from your local electronics shop.


Diodes ??
Ye are thinking of resistors.

Diodes flow voltage in one direction, and not t'other.
With generally minimal voltage drop.

There could well be a biggish (value) resistor in the wiring someplace.
You may need to examine the harness closely to even spot it ...


I would check out the stoplight switch - it may be wired in circuit with the taillight, and have a resistor across the terminals. With the lights on, the voltage passes through the resistance - with the brakes applied, the resistance is bypassed by the stoplight switch.



My 4th bike

"Diodes ??
Ye are thinking of resistors."
No I said DIODES.
There is a voltage drop over a diode and the diodes also stops the reverse current.
As I said adjust the diode number to determine the number of lumens you required at the taillight.
Good luck.


Perhaps you could find the load resistor by looking for a hot spot - after all when the tail lamp alone is running there must be a resistor somewhere dumping 21-5=16 Watts. I like Eddie's idea: a neat solution would be to mount the resistor on the brake light switch. In this case the wire from the lighting switch would run directly to the brake light switch, through the resistor (which could be connected across the terminals of the switch), and on to the tail light. When the brake is applied the resistor is bypassed and the tail light gets the full voltage. Is the brake light switch hot when the tail light is on?

Were bikes ever built like this, or was it just a bodge to modernise old bikes that had just a single filament in the tail light?

If using diodes instead of resistors (a little more complicated, but an ok idea), make sure they are rated at a couple of amps.



Well, we can say with some certainty that prewar bikes are rather unlikely to have had any diodes in them  !
Interesting idea though.

Quote from: cardan on 25 Aug 2023 at 11:45
Were bikes ever built like this, or was it just a bodge to modernise old bikes that had just a single filament in the tail light?
Definitely a bodge.

A prewar bike here (not Douglas) has an electrical system which has an entire output of 24w.
To have a 25w brake light chewing into that (!!!) is a skew-whiff setup, to say the least ?
But understandable ?, from a modern desire to have a visible rear light and brake light.
Presumably the battery also supplies the headlamp, near total loss style, if needed at night.
If ever ridden at night...

Modernization - air cooled electrics ?
Made for the job.
Bolt to any metal surface, toolbox, etc


A 24 watt generator should be easily capable of powering a 25 watt stoplight - it only has intermittent use (as against the constant output of the generator). It is easy for us to establish what the original wiring would have been (there are plenty of wiring diagrams available), but what is uncertain is what modifications have been made over the years, as owners have tried to keep up with more modern lighting requirements (the need for 'running lights', and 'stoplights' in some countries) - hence my earlier suggestion regarding the wiring of a ballast resistor across the stoplight switch.
  We also need to take into consideration that the original posting on this subject mentioned that the bike was a postwar machine, so would have had a Lucas dynamo capable of giving about 60 watts output - my first Douglas (a Mk3) had the same generator and blew every bulb on the bike when the voltage regulator gave up - our local Lucas depot tested it and found the dynamo to be giving off over 20 volts!!



Hi, found the resistor, buried under the rubber cover of the wiring, couldn't tell what resistance it was as it had signs of overheating so hunted through a pile of boxes holding stuff thatwill be useful  one day and found a twin bulb holder that fitted the light unit, albeit with a little work, so back to "normal" wiring and the rewiring has now been. The wiring that was removed was rubber insulated, and not sure wether age has turned it all the same colour or if it all started the same way but it was all black.
If it stops raining here will take it for a run  to check its ok


Good to hear you got it sorted - in a more conventional manner !
You didn't put a multimeter on that resistor, see what ohms it was ?

Wiring diagrams from back then sometimes had a dotted line for the brake light configuration,
brake lights only becoming compulsory from 1954 (?).

Quote from: eddie on 26 Aug 2023 at 08:01
A 24 watt generator should be easily capable of powering a 25 watt stoplight -

I was thinking more along the lines of if the taillight was in use.
It'd still be burning up that 24w, even if half (?) was heating up the wiring loom.


Think I stored the resistor in the  scrap bin , my multimeter is not working so will try to remember to fish it out of the bin and put it somewhere safe till I either get the meter working or replace it
Went out on the bike yesterday afternoon, almost managed to avoid the downpours , about an hours ride, and a stop for fuel where I forgot to put lights off, when I got back home all lights were good, and battery seemed to be well charged, which is an improvement over the previous situation, as the lights always seemed to dim if used  for a while.