LEDs + Motorcycle

[Dan East]

I have an older 2 cycle motorcycle on which I wish to upgrade various bulbs from incandescent to LED. However I'm not sure that simply regulating them with a resistor will be enough.

The main problem is that the voltage fluctuates with the engine RPM. This is a small displacement bike (125 cc), so the RPM varies greatly, and these bulbs are driven directly off of the magneto (no battery). So voltage ranges from around 4V to 8V.

Currently the neutral light varies in intensity greatly depending on the RPM. At idle it is difficult to see in daylight. So I would like for consistent intensity at any RPM.

A further problem with the existing bulbs is if the headlight burns out then all the smaller bulbs (in the gauges, tail light, etc) burn out within a few seconds because the amperage available to them increases greatly.

Since both the voltage (based on RPM) and amperage (if the headlight burns out) can vary, do I need an IC regulator, or will a resistor suffice?

Any suggestions would be greatly appreciated.

Dan East

 

[Nigel Goodwin]

You need a 'constant current source', there are many ways to do it, one of the simpliest consisting of just a single FET (but the current depends on the FET). Using a couple of BJT's and resistors gives one you can adjust.

Another option is to clamp the supply, to prevent it rising too high, using a high wattage zener diode!.

 

[Russlk]

My Triumph motorcycle just had a 15 volt zener diode (stud mount) as a voltage regulator.

 

[Nigel Goodwin]

My Triumph motorcycle just had a 15 volt zener diode (stud mount) as a voltage regulator.

In between the forks, on a nice heatsink!, I remember them well 8)

 

[Dan East]

I didn't know there were so many different types of transistors. After some googling I still couldn't find anything showing how to apply them for constant current. Could you direct me to some resources that explain in further detail?

Now the indicator lights are in parallel to the headlight, so if I were to use a zener to regulate current, wouldn't the excess current then be wasted? If I go with LEDs in place of bulbs then I will save over 5 watts, which I intend to use for a brighter headlight. Whatever design I use needs to be efficient, because the headlight dims quite a bit at low RPM already.

Dan East

 

[Nigel Goodwin]

The main problem is a lack of availble power from the 'alternator', I've had various trail bikes and all have had the same type of electrics. There are usually two different alternator coils, the first is used to charge the battery, and side lights, indicators and brake light work off that battery. The second coil feeds just the headlight bulb, with the coil essentially being 'self regulating', this results in a dim light at low revs, with the headlight been reasonably good at decent revs.

However, the tiny battery, and only been a 6V system, plus miniscule power from the battery charging circuit, result in poor performance of the main electrical system at low revs as well!.

Unless your's is radically different to all the ones I've had?, reducing the load won't affect the headlight at all?, as it's off a completely seperate coil.

I gave much thought over the years to improving it, but never actually got around to doing anything! - I suspect the first thing needed is a better alternator system to start from? - and changing from 6v to 12V (with the subsequent halving of current and associated voltage drop) would be a good idea as well.