General => Douglas Motorcycles - General Discussion => Topic started by: ted on 30 Nov 2004 at 15:33

Title: Electronic ignition
Post by: ted on 30 Nov 2004 at 15:33
Title: Electronic ignition
Post by: KiwiJohn on 30 Nov 2004 at 19:52
Hi Ted

These comments apply to all points and coil type ignition systems, not just bikes.

There are a number of electronic ignition upgrades you could do of varying complexity and cost.

The most simple upgrade is a current amplifier, this is a simple transistor circuit that greatly reduces the amount of current that must pass through the points.  Usually called, I think, 'transistor assisted' systems.  There are numerous kits available for this.  They just make the points last a lot longer before they get burned or need adjustment.

The most popular once upon a time were CDI systems,  'capacitor discharge ignition'.  In this system a special circuit produced a rather high voltage (about 400V I think) which is stored in a capacitor.  At the point of ignition this capacitor is discharged through the coil to produce a very high voltage spark of short duration.    I think these are the best upgrade for an engine where plugs are hard to fire,  two strokes,  oil burners etc.

Nowadays the CDIs have gone out of fashion a little and are replaced by 'high energy' systems.  I am not sure how these work.  There are even more complex systems incorporating microprocessor chips to automatically adjust the advance etc.  Used with knock detectors these can keep an engine optimally timed regardless of load, fuel grades etc.

In addition to the above there are a number of ways of dispensing with the troublesome points alltogether.  Magnetic reluctor pickups,  optical pickups and crank position sensors are all available but most I expect would bring challenges in the way of mechanical fitting.

For something like the Douglas engine I would try the simple transistor ignition as the first step but I would also consider converting the bike to 12V which might bring the desired result anyway along with better lights etc.

Title: electronic ignition
Post by: ted on 30 Nov 2004 at 23:27
Hi John,
           Many thanks for the information,
My initial thoughts were to try to dispose of the points to achieve a good strong regular spark.
Your thoughts regarding converting to 12 volts are interesting,this i imagine would involve changing bulbs uprating the battery and coil.
The problem I forsee is the alternator would this have to be uprated to 12 volts ?.
any ideas on this.

Regards Ted
Title: Electronic ignition
Post by: KiwiJohn on 01 Dec 2004 at 02:47
Ted, I think upgrading to 12V is little more than changing the regulator but you should get more information as it is not something I have done.

After the regulator there is the battery and the bulbs to change.
Title: electronic ignition
Post by: ted on 01 Dec 2004 at 08:27
Hi John,
            The conversion to 12 volts is still interesting.
Maybe there is someone out there who has done this and will give the benefit of their experience.
Title: Electronic ignition
Post by: Doug on 02 Dec 2004 at 03:41

I think you will find it simpler than you think to convert the alternator to 12v.  However I would not expect miracles, as the total output of the alternator will still remain at 60 watts, or whatever it was originally.  The benefit would be in converting it to a full output with control by a Zener diode, rather than the original ‘balanced’ system of switching in additional coils of the alternator as the lighting was turned on.  

In the original system, two of the three coil pairs provide current for the ignition, plus a small surplus to charge the battery.  Switching in the lights brought the third coil pair into play to help balance the drain.  If the headlamp bulb were burned out and you ran around at high speed with the switch set to main beam conceivably you could overcharge the battery.  But in most cases it was arranged that it only just put out enough watts for a given switch setting.  They really is no active voltage control per say.  

In the full output, basically it is just the wiring between the alternator and the bridge rectifier that gets changed.  Both the yellow and the green leads are connected to each other.  (Both the green leads on Lucas systems.)  These go directly to terminal 3 of the bridge rectifier, bypassing the lighting switch completely (eliminating the red wire.)  The brown wire remains as before between the alternator and terminal 1 of the bridge rectifier and blue still goes from the center terminal 2 of the bridge rectifier to the lighting switch.  But in parallel to the blue wire you install a Zener diode (Lucas 49345), which is grounded.  

The advantage is that all the coils are active at all speeds and loads.  Under light load condition the Zener takes care of the excess wattage should the alternator output be too high.  It does this by being voltage sensitive.  At about 15 volts it starts to short the alternator output to ground.  The higher the voltage the more of a dead short it becomes.  Hence the need for a good heat sink for the Zener!  So at low to modest speeds you have full charging without worrying about boiling the battery when on the motorway with the lamps turned off.  However with all the lights and accessories on, you still only have the original wattage of the alternator to play with.  So if that was insufficient before, it is not going to get any better.  

Zener diodes are inefficient, but the current they dump you can not utilize anyway.  And also I understand not manufactured anymore.  Now days they use a diode that works in the milliamp range and use it to trigger a transistor that handles the main amperage.  But the re-wiring is very simple to carry out with the Zener, and the modifications are completely reversible.  In the UK, it probably is still possible to find Zener diodes at auto jumbles, so assuming you can find a place to mount the Zener in the cooling breeze; it is probably your simplest solution.  Probably the Miller AC60 alternator is not going to put out enough wattage to tax the Zener all that much!  

I do not know if the modern transistor regulators mentioned just behave like the Zener, or control the wattage some other way.  Nor do I have any advice on electronic ignitions.  

Wiring diagrams for the pre-Zener and Zener diode systems and other useful info can be found in the Lucas Motor Cycle Electrical Equipment Service Manual, which is reprinted.  Obviously for the Lucas systems, but much pertains equally to the Miller systems fitted to the Dragonfly.