Pros and cons of ac motorcycle lighting.

[Mark P.Participant @markp]

Thanks for the reply Dave, the power for them is regulated ac, putting a rectifier in the circuit presents no problems I’m guessing that a 1n4004 would do the trick?
Mark P.

Edited By Mark P. on 12/09/2017 14:40:20

[Tim StevensParticipant @timstevens64731]

AC or DC? It can matter, or it might not. Some, but not all, of the LEDs made for vehicles, include a fancy circuit to ensure that the device is not ac-dc sensitive. Many of the more expensive and powerful LEDs, as you might use in a headlamp, for instance, include a circuit so they will run on anything from 9 volts to 30 or more. This allows them to cope with commercial trucks (at 24 volts) and cars will badly adjusted voltage, etc.

A comparison with mains bulbs is not helpful in this instance, as all the mains is AC, now, (in the UK at least) so all mains LED bulbs include the necessary circuitry.

You might find with smaller 12v vehicle bulbs (wedge-type sidelights, instrument lights, etc) that the bulb is polarity sensitive, so 50% of the time you fit one it won't work. All you need do is take the bulb out, rotate it 180 degrees, and replace it. This shows that these bulbs are not sophisticated – not least because the price is affected, and there is very little room for the extra gubbins. In general, it does no harm to connect an LED the 'wrong' way round, as they are (as it says on the tin) Diodes. This only applies if you are trying the right voltage the wrong way round – no diodes will stand a reverse connection if the voltage is much higher than it was designed for.

Actually, any diode WILL work 'backwards' in this way, but only very briefly. Micro seconds, perhaps, followed by smoke … This is the Zener effect, and the Zener diodes which are designed take advantage of this effect are made with big chunks of semi-conductor welded to big lumps of light alloy (etc) to dissipate the heat produced when working in this way.

So, it SHOULD be OK to use an LED headlight on a no-battery, 'direct lighting' system. But it may be that the rear bulb, and any others, don't work. Or even if they do, it's not for long. This is because with AC the 'voltage' is actually the average over the full cycle of output. This varies from nothing to maximum, and then through zero to max the other way, and the max can easily be twice the average. The LED will struggle with this, even if there is a clever circuit. The alternator will be pumping out AMPs and if the demand does not match the supply, the voltage will go up until it does. This can happen with a dip switch, for example, as they normally have a momentary off-spot as the mechanism flicks from one side to the other.

Another effect which can be a problem, is the spikes of voltage in the whole circuit caused by the ignition set-up. With a mag you may be OK (not guaranteed, though) but any coil ignition, with points of electronic, will produce spikes of voltage much higher than the LED can cope with. Only a few microseconds, perhaps, but lethal all the same. In a more conventional set-up, the battery helps to cut the spikes down to size.

Hope this helps

Regards, Tim

[Mark P.Participant @markp]

Thanks Tim, my alternator has anoutput of 12v 65w of regulated ac, so I was hoping to use leds for head and stop an tail. The headlight will be a 6w led with a high and dip. I assume ohms law works with leds?

[Tim StevensParticipant @timstevens64731]

A further comment about the rectifier:

The diodes in a rectifier are just as sensitive as LEDs to excess voltage and particularly spikes. This means that you need a rectifier which will stand two or three times your maximum current, and perhaps 500 volts. Not difficult, and if you look around, no more expensive that a couple of beers, but you do need to be careful at the very cheap end of the market. I suggest that if you look for a 'motorcycle rectifier' you should be OK – or use a firm like RS or Farnell where every last detail of the spec is given. In my experience, most e-bay suppliers don't have a clue what they are actually selling, even if you ask a specific question.

An example – I recently saw an advert from a UK supplier for a Flat File. The dimensions given were 32 x 35 x 200, so I asked them to check. No, that what it says on my sheet, came the response.

Cheers, Tim

[Tim StevensParticipant @timstevens64731]

Oh dear, Ohms law definitely does not work with LEDs, Or with most other clever electronic stuff. It depends on how your regulator works. It may be capable of quenching the spikes, or it may be designed on the assumption that most riders are not enduro riders, and therefore they have batteries.

If you use an LED taking 6 watts for the headlight, and say 2W for the tail etc, what else is going to happen to the remaining 50 plus watts? The voltage will rise until the regulator copes with it (whatever that proves to be) – and this extra voltage will go to every LED in the system.

Sorry. Tim

[Mark P.Participant @markp]

Thanks Tim, well that’s food for thought then. I will be fitting a battery so I may be better off running my leds from the dc side.
Mark P.

[Tim StevensParticipant @timstevens64731]

Mark P says: I will be fitting a battery so I may be better off running my leds from the dc side.

Yes, excellent. I have had a close look at some new LED headlamp bulbs here, and it seems that they work by having the dip LED on all the time and the mainbeam LED on as well when you click the switch. So, there may not be any 'no-connection' problem as the dip-switch moves across. But I cannot say that they all do this, just the examples I have.

Do keep us informed how the rest of this adventure works out.

Regards, Tim

[Speedy Builder5Participant @speedybuilder5]

From another site , LEDS can only be fitted to vehicles with approved lights (Vehicle construction and use), so technically what you are doing is illegal and could be 'spotted' by your MOT inspector/ police etc. So keep your old tungsten bulbs handy perhaps. I don't think that I would worry too much though.
BobH

[SillyOldDufferModerator @sillyoldduffer]

Posted by Mark P. on 12/09/2017 14:37:35:
Thanks for the reply Dave, the power for them is regulated ac, putting a rectifier in the circuit presents no problems I'm guessing that a 1n4004 would do the trick?
Mark P.

Edited By Mark P. on 12/09/2017 14:40:20

Yes, provided that the LEDs take rather less than an amp. I'd be tempted to use something a beefier like a 1N5408 (9p from Maplins.) It's up to 3A at 700V (max 1000V reverse). They also do the P600D which are 6A at 140V (max reverse 200V), 39p each.

You also need to regulate the supply. If you don't mind a bit more wiring up after the diode, Maplin carry the L78S12CV (59p) which is a 12V 2A regulator. They need to be heat-sinked, but you could have several – one per light cluster perhaps.

But, if you have a 12V battery available on the bike, better to run the lights off that I would have thought. To charge from the alternator the battery must have a regulator and rectifier already. And I prefer lights that stay on even if the engine stops.

Dave

[Tim StevensParticipant @timstevens64731]

The other site is not correct, in every respect, I submit. The law for older vehicles requires a light, which allows you to use acetylene, or paraffin, etc, too, and there are additional rules which may cover this old Triumph, but which apply to filament lamps. LEDs are not filament lamps. But I would be interested to learn what law is actually being quoted, here. The Vehicle Lighting Regs are the most relevant (not C&U, I suggest).

And if you are going to use a rectifier, please bear in mind my comments about spikes.

Regards, Tim

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