LED Headlight conversion on Moped

Thread Starter

J.Woods

Joined Aug 24, 2020
16
Hey

I'm converting an AC headlight on a moped to DC for an LED.
I'm trying to figure out which capacitor and inline fuse to use. I believe the fuse should be 5 amp. ?
I'm adding the capacitor to smooth out the power to the LED.
Here's the info:
The AC generator puts out 12V and 65W at 5,000 rpm.
The old headlight is 25W. So, 2.8 Amps and 5.8 Ohms. Correct?
I have a 12V, 4 pins regulator/rectifier for a scooter already.
The bulb I have is an 18W, 12W bulb. 12,000 LMS, DC 12V-24V bulb. It's an h4.

I've been going in circles trying to figure out which style and size capacitor.
Any help is much appreciated. Thanks
 

SamR

Joined Mar 19, 2019
3,535
AC headlight? Generators put out DC voltage. I think you may find that it is a direct replacement for a filament bulb. Plug and play. Even AC alternators typically have diodes built in to convert to DC.
 

MisterBill2

Joined Jan 23, 2018
9,518
The first thing that we need is the specifications for the rectifier regulator. The usual arrangement is to have a rectifier followed by the capacitor, followed by the regulator. That saves the regulator from being destroyed by the current charging the capacitor. So we need to know about the 4-pin module.
You mention two power levels for "the bulb", which I am guessing is the LED device.
And another question is about the AC generator. Is one side of the generator output connected to the frame, such that it is grounded? Is the generator part of the magneto system, or does the moped have a battery? And in fact, is it really an AC output generator, or does it have brushes and a commutator? Some of them also work as the starter motor. is that the case?
So here we have a lot of questionsthat need to be answered before you can get anything better than guesses. And youcan certainly get a whole lot of guesses here.
 

Thread Starter

J.Woods

Joined Aug 24, 2020
16
The alternator is grounded. Two lines come off the alternator. One is AC that goes to the headlight. The other is for the rest of the lights. I don't have the specs for the rectifier and regulator. Just the eBay link.
 

MisterBill2

Joined Jan 23, 2018
9,518
OK, the circuit posted does provide useful information about the system. And the pictures of the bulb seem very much to intentionally avoid showing how many pins of the base. The ignition circuit implies that the battery is not required for the engine to run. The lighting circuit implies that the headlight will not illuminate if the engine is not running. The lack of specifications about the headlight mean that we will be guessing about the actual voltage limits.
So the best guess is that the headlight power supply should be a half wave, single diode rectifier charging a capacitor, with some sort of regulator to feed the headlight. From the drawing of the bike it looks like the alternator may provide one pulse per revolution of the alternator, which will place a real demand on the capacitor to supply the LED current the rest of the time. The alternative would be a headlight that flickers when the engine is at a lower speed.
Consider that 5000RPM is quite fast, even for as motorcycle engine, it seems that mostly the speed and voltage will be less. It may make more sense to power the headlight from the battery, and possibly adapt the headlight power connection, with an added diode, as an additional battery charging circuit. Of course, that is based on the guess that the published circuit is someplace close to what is actually present. The circuit may possibly be an extreme simplification of what is really there.
 

Thread Starter

J.Woods

Joined Aug 24, 2020
16
Thank you, everyone. I have no experience designing electrical circuits, outside basic info. This is helping. I'll upload more information to try and help.
 

MisterBill2

Joined Jan 23, 2018
9,518
From that wiring diagram it is clear that the circuit drawings are concept drawings and not actual circuits. And it is also clear that the rectifier block change is not trivial, it is a big deal, and quite non-reversable. And after making the change the original module will not work.
Unfortunately the wiring diagram is just fuzzy enough to be a lot less useful than it should be.
And it is still not clear why the headlight is only on when the engine is running. There are times when the light is needed with the engine off and not running.
 

Thread Starter

J.Woods

Joined Aug 24, 2020
16
It may make more sense to power the headlight from the battery, and possibly adapt the headlight power connection, with an added diode, as an additional battery charging circuit.
I think I'm going to go with this option. I will add a small second battery and charge it off the headlight power connection. I'll go alternator; yellow wire out, to a regulator/rectifier, to a diode, to the switches, and then the headlight. Should I float the ground and isolate the circuit? Hook the green wire from the headlight to the green wire on the reg/rec?
 

MisterBill2

Joined Jan 23, 2018
9,518
I think I'm going to go with this option. I will add a small second battery and charge it off the headlight power connection. I'll go alternator; yellow wire out, to a regulator/rectifier, to a diode, to the switches, and then the headlight. Should I float the ground and isolate the circuit? Hook the green wire from the headlight to the green wire on the reg/rec?
Isolating the other side of the coil does allow full wave rectification and the ability to have almost twice as much power. but making that change is not a small project. If it is well within your skill set then it could be a good plan.
 
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