Pros and cons of ac motorcycle lighting.

[Mark P.Participant @markp]

Hello all, can anyone tell me the advantages or disadvantages of ac over dc lighting for a motorcycle?
Mark P.

[Mark P.Participant @markp]

[Mark RandParticipant @markrand96270]

Lighting or generation?

[Phil PParticipant @philp]

My 1972 Honda XL250 has a sort of rectified AC lighting circuit, and I can tell you from expensive experience that LED bulbs are not happy with it.

Phil

[not done it yetParticipant @notdoneityet]

Is this is for a bike with no battery?

[John OlsenParticipant @johnolsen79199]

If a bike is using AC directly to light bulbs, or even if rectified without a battery, then it is relying on the characteristics of the setup to limit the current. The way it works is that a given permanent magnet and coil will be limited in the maximum current it can provide. So the bulbs are chosen to run with just that current. The open circuit voltage will vary linearly with speed, but with the load from the bulbs on it the voltage should stay about the same giving reasonable bulb life.

There are some snags to this. If the headlight bulb blows, the tail lamp will generally follow almost instantly. This is because the system is trying to force the same current through it that the headlight took. If the dip beam filament is rated at a lower current than the main beam, the current will be wrong for one of them. If the engine stops, so do the lights. Such a system would not lend itself to changing to LED bulbs unless you are a bit into electronics. You would want to rectify the AC and then regulate the voltage.

So most smaller bikes in the seventies were using a somewhat refined version of the same idea. They had a permanent magnet alternator, usually wound with two windings. The AC was rectified and used to charge a battery. One coil was wound to provide about the right current to match the daytime load, the other provided more current and was switched in when the headlight was on. This was better than the straight AC system, since the lights would run off the battery if the engine stopped. However, most had no regulation, so there was often a bit of a tendency to cook up the battery. This tended to be accepted, since the life of a battery with the vibration of a motorcycle tends to be short anyway. However some makes did put various forms of regulator onto this basic system. Lucas used a large Zener diode to shunt excess power, and at least one model of Honda had an SCR based regulator.

Sometimes the lighting coils were included in a flywheel magneto, but the behaviour of such a system is the same as a permanent magnet alternator. (EG Bulb life is likely to be poor!)

Larger bikes at that time, stating I think with the Honda and Kawasaki fours, introduced a much better system, with an excited field alternator like a car.. These still generate AC, but instead of a permanent magnet they have a field coil which has its current controlled by a regulator. This gives much better control of the charge to the battery and adapts itself to the load so it is easy to change the headlight. to something better. There is of course a rectifier in the system. I'm not au courant with modern bikes, but I suspect most of the larger ones would still be using this sort of system.

My advice if you have a bike with the headlight run directly off an AC lighting coil is don't go out at night.

John

[Ex contributorParticipant @mgnbuk]

Some versions of the Indian made Royal Enfield Bullet had a split AC-DC electrical circuit. There was the usual rectifier/regulator arrangement for the DC circuit (everything except the lights) from one set of alternator windings & an extra lighting circuit alternator winding that had an external voltage limiter.

I don't know for certain why this was done, as earlier versions of the same bike had an all DC arrangement – I have had one of each & there was no noticable difference in lighting performance. The all DC bike had a bit of an appetite for rectifier/regulator modules, though, so maybe the change was done to reduce the load on the DC circuit to improve reliability ? This was at the expense of an added winding + another voltage limiter module so, potentially, more bits to wrong !

If the headlight bulb blows, the tail lamp will generally follow almost instantly. This is because the system is trying to force the same current through it that the headlight took.

Not so much extra current, as that the voltage is regulated by the load – if one bulb fails, the load reduces & the voltage rises. At the higher voltage the remaining bulb takes more current, runs much hotter & fails. This also happens with bicycle dynamo lighting arrangements & Shimano make a small voltage limiting module to prevent total loss of lights should one bulb fail. These are only rated at 3W (typical bicycle dynamo output), though, so not suitable for motorcycle use. IIRC use of these modules is a requirement of the German Traffic Regulations for bicycles

I'm not au courant with modern bikes, but I suspect most of the larger ones would still be using this sort of system.

The regulated field alternators have pretty well disappeared now – 3 phase permanent magnet alternators have been the norm for the last 20 years or so. Look at most modern bikes & you will find a large finned module placed in the breeze – this is the rectifier/regulator module & they "regulate" much like the old Zener by shorting the windings to ground. BMW may still use the wound rotor type (my wife's R65LS has one, but that is over 30 years old !), but I have not had a recent BMW so can't say for certain. Some older Moto Guzzis used the same Bosch unit as BMW did at the time (late '70s early '80s), but my Mille GT bought new in 1989 had a permanent magnet type.

Nigel B

[Mark P.Participant @markp]

The thing is at the moment my bike a 1957 650 Thunderbird has no electrical system apart from a suspect lucas magneto. I have a choice of getting the mag rebuilt (about ?350) and buying a 210w alternator and a regulator/rectifier (?200),or I can get an intergrated alternator with a self generating electronic ignition with all the bits for ?275. This uses ac for lighting but dc for battery charging, the drawback is the alternator output is only 65w.
Mark P

[Tim StevensParticipant @timstevens64731]

The problem with AC lighting is that you need to balance the output with the load. in the old days this meant having two windings – one giving low output for the brake lamp, and the other balancing the headlamp & tail. If you have one of these, and the bulbs are a too low wattage, they will blow, and if the wattage is too high they will only get yellow at best. And you need to be sure that the dip switch connects both sides in the mid position, and that the alternator low-output winding connects to the brake light only, and is open circuit when the brake is off.

Problems arise when you want to run flashing indicators etc, or improve the lighting eg with a Halogen bulb. The whole system was in practice there to comply with enduro rules, and if you were lucky, pass the MoT, and if you used it at night you needed to carry spare bulbs, and not to use routes with which you were not 100% familiar …

My suggestion would be to find an alternator which more than balances your headlamp load, and use a Zener Diode (on a big heat sink) to 'use up' the excess output. This is what the later Lucas system did, from about 1970, as the balanced load system was abandoned. You will also need a rectifier as the Zener will only work one way round (the clue is in the name Diode).

If you go with the 65W version, that should be able to cope as long as you don't expect the battery to charge with the lights on, and you use 12v LED bulbs for all the other lights. If you have a modern headlamp unit (taking H4 bulbs for example, not 'prefocus' ) you can find LED bulbs for this fitting, and this should leave some over for the battery.

If you fit a 210W alternator, what are you going to do with all that electricity? Even at night and with boring filament bulbs you will still have about 150W spare!

Come back to me if you need more details, circuit diagrams etc.

Regards

Tim

Edited By Tim Stevens on 05/09/2017 16:00:41

[Mark P.Participant @markp]

Tim, I am still at the planning stage yet and have seen other systems all come complete with regulators and rectifiers. I suppose I don’t need a 210w system as I only intend to have lights and a horn and no indicators(but I would like a good headlight). As for ignition the jury is still out on getting the mag overhauled. If I go the electronic route there are quite a few choices but I may need a small battery just for the initial starting.
Mark P.

[Tim StevensParticipant @timstevens64731]

Mark – You should be OK to run electronic without a battery if you add a big electrolytic capacitor. This is what Lucas did with what they called Energy Transfer Ignition. I still have one or two of the 'right' capacitors, if you would like to send my a PM.

I ran a similar system to yours on a Yamaha 500 Single – which started with a 6 volt alternator. I played with the connectors, disconnecting the earth, and used an ordinary motorcycle recitifier, and a 12v Zener. I tried it without the capacitor and it started OK – so fortunately, the pulses of output happened to be timed right to give good output at the ignition point. If this had not happened, a capacitor (which behaves like a tiny battery but lasts longer before failure) would have put the job right.

One thing I should have said in my earlier post is that running lights directly from an alternator is a very old tradition. The model T ford did it. The term usually applied to it is 'Direct Lighting' – meaning direct from the generator, but confusingly it relies on Alternating (not Direct) current. This has caused confusion over the years.

Hope this helps

Regards, Tim

[john fletcher 1Participant @johnfletcher1]

Hello Tim, what value is the capacitor, I have quite a selection of caps. I think Tiger Cubs used that idea but not sure. Ted

[Mark P.Participant @markp]

Hello Tim,I have a very big electrolytic (32000 uf at 48v) as I said I’m still at the planning and costing stage.
Mark P.

[duncan webster 1Participant @duncanwebster1]

My Velo Venom had a 28W dynamo, and a 25w headlight bulb (not even halogen). It was capable of around the ton downhill on a good day, but you don't need to see where you're going when you're young do you. However, if you can get 210W of regulated DC, can you then just use the points in the mag to operate a conventional coil to provide the sparks? I must admit I'm not sure how the mag copes with 2 cylinders, does it have a distributor? In any case I'd use the wasted spark system, it's simpler. If this works you could then when funds permit ugrade to electronic ignition

[MuzzerParticipant @muzzer]

There will be a slight reduction in life of the filament bulb if you achieve the same light output relative to a given DC current. For similar light output you need an equivalent RMS current – but the lifetime of a filament is inversely proportional to the fifth power of the temperature. So if you achieve the same RMS current from an AC voltage, the peak temperature will inevitably end up being higher which impacts the lifetime

Some years back when I was working at Motorola Automotive, I did some work on running 12V filament headlights from PWM'ed 36V. This allowed the voltage to be regulated precisely at the normal voltage (13.8V typically) but we were able to show clearly that the lifetime was affected by the PWM frequency – and the theory is not hard to understand. It's a very similar effect.

The thermal time constant of a typical filament headlamp bulb is surprisingly short (of the order of milliseconds), so the reduction in lifetime due to operating it at a frequency such as 100Hz compared to say 250Hz is measurably lower (something like 20-30%, I forget the precise numbers).

Probably not a major consideration in the light of what has been reported above in terms of the regulation of some of these systems but might be of interest to some. Conversely, you could improve the lifetime of a domestic halogen bulb by running it from regulated DC. The cost of doing so wouldn't be justified but the result would stand.

Murray

[Mark P.Participant @markp]

Tim, some of the thunderbirds were coil and battery ignition with a distributer and some with a mag (lucas K2F) unfortunatly the mag points revolve with the armature making conections tricky. I do have other options to look into.
Mark P

[Tim StevensParticipant @timstevens64731]

I do not know the size in microfarads- Lucas didn't want us to buy from Radio Spares so they just used their part number. About 30mm dia and 40mm long. I am fairly sure the exact value is not critical.

The dynamo – a Miller – used by Velocette for many years was 50 watts, same as the Vincent. Yes, the World's fastest motorcycle with the world's poorest lights.

Yes, the magneto used on parallel twins includes a distributor (just a slip ring and two pick-ups, no gearing). And yes, the points rotate, but that only makes the conversion a bit more complicated. What is needed to start with is a totally duff magneto, not one that could be rescued. Then using the original bearing system, make up a new spindle carrying a points cam with two lumps. This opens one set of ordinary coil ignition points, on a plate made to fit, and the current from this goes to a pair of 6v coils in series, one feeding each plug. It needs 12v to make the job easier, but if you are fitting a decent alternator that is no problem. And yes, there are other ways of doing the same thing. There are certainly coil-ignition points and distributor assemblies made as a direct replacement for a magneto, fitting onto the same three cornered flange. This would only need one coil, and it could be a 6v coil if you insist. I think from memory there were Triumphs around 1960 with such a system, mainly the cooking models rather than the more sporting.

I have a coil ignition converted magneto on my Lea Francis car (1928) and so far it has worked well. But it is a long winded way to do the job, as all it does is the same as a more modern dizzy but much heavier. But I can get a tick-over at about 300 rpm.

Cheers, Tim

[John OlsenParticipant @johnolsen79199]

I guess they were not thinking in terms of people going fast at night. Mind you, the same sort of thing applies to the early models of Honda 750 four. They had a good alternator, but only came with a 50/35 watt headlight. Not what I would call adequate for that sort of bike. Actually, around that time I converted my Honda 100 to 12 Volts and provided it with a 55W quartz halogen. So my little 100 (About 70mph max) had a better light than the 750 four came with standard. (Maybe 120mph or so max?)

I had one of the Triumph 250 singles with the capacitor system, but never ran it without the battery. I think you would want to go for about a 20 Volt rating, and as many microfarads as you can reasonably fit in the available space. A high ripple rating is desirable, but since this sort of capacitor is intended for power supply smoothing applications, they are likely to have quite a high rating.

John

[thaiguzziParticipant @thaiguzzi]

Posted by Mark P. on 05/09/2017 15:22:29:
The thing is at the moment my bike a 1957 650 Thunderbird has no electrical system apart from a suspect lucas magneto. I have a choice of getting the mag rebuilt (about ?350) and buying a 210w alternator and a regulator/rectifier (?200),or I can get an intergrated alternator with a self generating electronic ignition with all the bits for ?275. This uses ac for lighting but dc for battery charging, the drawback is the alternator output is only 65w.
Mark P

550 quid for new electrics (ignition and charging) sounds quite reasonable in today's world and prices. If you go the Electrix World self generating system, let me know how you get on. This system is fairly new on the market for these bikes and i'm interested in how they fare. I believe originally they came out without the lighting side, aimed primarily at competition bikes not running a battery.

You would not like the price for Joe Hunt Magnetos, mine was over 500 quid, trade, in the mid 90's. But boy, do they spark!!!

[Gordon WParticipant @gordonw]

65 W for a bike light seems to me a lot. Do not go lucas energy transfer type systems, that way madness lies. Had one on a Cub trials and ended up throwing the whole lot into a quarry. Some triumph twins had car-type distributer in place of a mag.. Nowadays I would go for 65W alternator and coil ignition.

[Tim StevensParticipant @timstevens64731]

Gordon was not alone in having trouble with his Energy Transfer system. The problem was that the alternator gives out its output in lumps, three lumps one way round, alternating with three the other way, every engine revolution. These were intended to be timed on assembly at the works so that one of the peaks coincided with the points opening. But of course, the points were on an auto advance unit, so as the revs changed, so did the timing. So, the points opening position was only timed to hit a peak over a short range of revolutions. You could set it up to start easily, or to run well. The answer, of course, was to rectify the output and add a big capacitor. Until about 1965, rectifiers were not reliable (in the types used on motorcycles). (And as they were often fitted under the plywood seat base, and became known as rectum-friers!). It was around this time that J Lucas became known universally as 'Prince of darkness'.

So, while Gordon had a bad experience, things have moved on, and much better kit is now available.

And I have found the drawer with the capacitors in – my guess of size was nearly right. 30 dia and 50 long is more like it.

Cheers, Tim (as shown in MEW)

[Mark P.Participant @markp]

Well I have almost decided to go for the electrex world kit One question though will 12v led headlight bulbs work on ac? I have been playing with a single 12v led and it appears to work ok on ac, so I assume that the headlight bulbs will?

Mark P.

[Mark P.Participant @markp]

As an afterthought there is a circuit diagram of the cdi/alternator in my album under my Triumph.
Mark P.

Edited By Mark P. on 12/09/2017 13:57:54

[richardandtracyParticipant @richardandtracy]

deleted – misread question, so gave irrelevant answer.

 

Edited By richardandtracy on 12/09/2017 14:17:52

[SillyOldDufferModerator @sillyoldduffer]

I wouldn't do it unless the kit says it OK on AC. As the quote below from this website explains:

LED reverse voltage

LEDs are not tolerant to large reverse voltages. They should never be run above their stated maximum reverse voltage, which is normally quite small. If they are then permanent destruction of the device will almost certainly result.

If there is any chance of a reverse voltage appearing across the LED, then it is always best to build in protection into the circuitry to prevent this. Normally simple diode circuits can be introduced and these will adequately protect any LED.

See the last sentence though: it has the answer. If you put an ordinary power diode ( rectifier) in series with LEDs, it will protect them. I know nothing about bike electrics and can't advise on what's available. The circuit is easy enough and parts can be got from Maplins / other websites; what I don't know is how to protect them from the weather, vibration and heat on a motor bike.

I expect one of the bike experts will know.

Dave

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