I've got a 1980 Honda XL 185s that has a 6 volt electrical system. The headlight uses a 6V AC, 30 watt bulb powered from the generator. The other lights (tail, blinkers) operate off of the 6 volt battery on 6 volt DC. I'm attempting to increase the intensity of the headlight in order to make it safe enough to ride at night. Presently the headlight is very dim anywhere near idle speed and just slightly dim at any other rpm.

I'm assuming that changing my 6V AC to 12V DC and then installing a manufactured 12V LED bulb will be easier than changing 6V AC to 6V DC and building my own 6V LED light.

I am also assuming that LED is my only option for a bulb, as I believe that I lose wattage in the step up or conversion of the power. Electricity is not my strongest subject, as you can see.

I'm open to suggestions if I'm going about this all wrong.

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    \$\begingroup\$ What is the current draw on the 12v led bulb you want to use? \$\endgroup\$ – Passerby Aug 11 '13 at 0:46
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    \$\begingroup\$ Is it really AC? If it is, it's presumably synchronous to the engine RPM and not of a steady frequency. \$\endgroup\$ – Chris Stratton Aug 11 '13 at 4:02
  • \$\begingroup\$ @ChrisStratton It's an individual 6v AC transformer off the alternator. Nothing else tied to it. Like any other car system, it does vary based on load and engine speed. xrv.org.uk/forums/attachments/xl/… \$\endgroup\$ – Passerby Aug 11 '13 at 4:25
  • \$\begingroup\$ @ChrisStratton It is linked to engine RPM as Passerby stated. There is also a DC leg that comes off the generator, but it can barely support the 6V system and keep the battery charged. \$\endgroup\$ – James Aug 11 '13 at 5:16
  • \$\begingroup\$ @Passerby I hadn't picked out a bulb yet. I was looking to identify what wattage I'd likely end up with to even see if it is feasible. \$\endgroup\$ – James Aug 11 '13 at 5:16

First when you rectify 6 V AC using full bridge rectifier, you'll get 8.4 V DC.

If you use doubling rectifier like those on images, you'll get about 16.8 V DC http://www.electronic-circuits-for-hobbyists.com/images/voltage-doubler-circuit-2-diode.JPG

If 16.8 V is too much then you can drop is somewhat using voltage regulator or so.

I even suggest you to use regulator to have constant brightness.


It is also good to use boost regulator like MC34063. Just follow its datasheet, while keeping in mind you'll need external transistor, diode and inductor rated not less than 10A. And do not forget rectifier before boost.


There may be found that your generator cannot deliver enough power for normal 12 V bulb. In this case you'll have to get more powerful generator or try xenon or LED light.


In order to do this, what you will first need to do is use a transformer. Since you want 12 VDC, this would require a primary to secondary ratio of 2 for the transformer turns.

After the transformer, you can use a full wave voltage rectifier to get the 12 VDC. The reason why Passerby is asking for the current details is you will need to choose parts that can handle the current drawn.


simulate this circuit – Schematic created using CircuitLab

Above schematic should give you a rough idea of the circuit you will need. You may want to add a regulator in case you have variations in your 6 (VAC) supply. The nodes labeled A and B are places you can tap the 12 (VDC). Tap A can be used if you have a steady supply from the 6(VAC), or you can add in the regulator and use tap B to get a regulated DC supply.

All the best.

  • \$\begingroup\$ Sorry, but this is just bad advice. You have obviously neither tried nor simulated this. It will not work as expected. Nor is LED lighting done with series resistors. \$\endgroup\$ – Chris Stratton Aug 11 '13 at 3:55
  • \$\begingroup\$ @ChrisStratton any reason why it wouldn't work? \$\endgroup\$ – Passerby Aug 11 '13 at 3:58
  • \$\begingroup\$ All of the obvious problems with power supplies that are missing parts and understanding; plus the likelihood that this whole approach is not suited to the application. \$\endgroup\$ – Chris Stratton Aug 11 '13 at 3:59
  • \$\begingroup\$ Hi Chris. I am not familiar with how the system on the car works. Would you mind correcting any issues you see with schematic just for the sake of correctness. Thanks \$\endgroup\$ – Edwin Aug 11 '13 at 4:14
  • \$\begingroup\$ As you'd know if you had ever tried to build something like this, we'd need to know more information than given about the source and load to fix it. But even with the information, this is not a good solution for the application - at best, it's going to be a time consuming and expensive divergence from the problem stated in the question. \$\endgroup\$ – Chris Stratton Aug 11 '13 at 4:18

I have a 1979 XL185s. I changed the headlight to a 12V halogen bulb and added a MSR Voltage Regulator 610-203 that I mounted just below the headlight. I am using a H4 60/55W halogen bulb that you can get for less than $3 from ebay.com. I had to use a hole-saw to drill out the old 6v bulb and glue in the new halogen. Instructions on how to do this are at instructables.com. The title of the instructable is: "Reusing old motorcycle sealed headlights". I changed the turn signals and brake light bulbs to 12V LED and power them using a 6800mAH lithium battery. I wired this separately. It is not connected to the stator. I charge when the bike is parked.


All white leds run on about 3 VDC so why do you need 12VDC anyway. Now for the same performance the LED system will draw 20 to 25% of the power of the halogen so you could run a dcdc converter into your leds in fact you are spoiled with options so I will say a few . Off the shelf 6VDC LED headlamp OR 6 to 12 DCDC to run 12VDC LED headlamp OR you could roll your own by using boost convertor to do a series string of LEDs built into your headlamp and running off 6VDC .Voltage doublers as has been described by Vovanium can work if the electrolytic is properly rated for the current .


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