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Moped Voltage regulator

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qbone

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Hello guys
I wanna install LED lights on my moped, as it keeps blowing my normal bulbs, probably because it is tuned a little bit :)

At first I figured I could just use a regulator we use at work, you know, rectify the AC and the out comes the DC through our regulator (based on LT1074).. But it turned out not to be that easy.

The power I get from the AC is about 20-70VAC, but havent measured going down hill or real fast, so to be safe I would say up to 100VAC or so.

What I need on the output is 12V and able to draw atleast 225mA, so I'd say 500mA to be safe.

Now, if I can just regulate the AC down to a maximum of 40V that would be great, cause then I can just connect our regulator from work, but if this requires a whole new circuit - no problem :)

I hope you guys can help me out a little here, I have searched high and low all over the internet, and havent quite found what I needed.

So to sum up, I need help with regulating 20-100VAC down to a 12VDC/500mA.
 
You could try an LED driver meant for running off the mains, e.g. **broken link removed** but with some mods to allow it to run at a lower voltage (would require changing the number of turns on the transformer primary winding)

Or if you want to start from the beginning, you could try rectifying the AC, filtering with a large capacitor and then converting to a lower voltage using a SMPS, e.g. https://www.romanblack.com/smps/smps.htm or https://www.romanblack.com/smps/a04.htm but using higher voltage transistors. The latter circuit has current limiting which could make it perfect for directly driving your LEDs
 
Could I limit my power with a real simple zener transistor regulator? Like this one:
**broken link removed**
I was thinking of using a BZX85C13 as Zener, and a BUX48 transistor.
I am however, a bit unsure of how to calculate the resistor value.
 
If you didnt want to mess with the bikes electrics you could use a constant current circuit for your leds, it would compensate for any changes in voltage.

Leds are not that sensitive to changes in power level as far as blowing one up that is, but you would see changes in brightness with rpm if you just used a series resistor.
 
Read Post #10 and #13 in this thread.

In your schematic on Post #13 you mention the NFet should be >30V, Why exactly 30?
Bear in mind that after just rectifying with a single diode I easily reach around 60V and possibly up to a 100 down hill I would guess.

If you didnt want to mess with the bikes electrics you could use a constant current circuit for your leds, it would compensate for any changes in voltage.

Leds are not that sensitive to changes in power level as far as blowing one up that is, but you would see changes in brightness with rpm if you just used a series resistor.

I am unsure of how to design this with varying such high voltages :-/

My moped is a Puch Maxi, it has a static alternator with a light coil and a coil for the sparkplug if that is any help. :)
 
If you can tell me the maximum voltage you get then I might be able to find something, 100v would be a problem if its that high then a shunt reg like Mike built would probably be more practical.
 
In your schematic on Post #13 you mention the NFet should be >30V, Why exactly 30?
Bear in mind that after just rectifying with a single diode I easily reach around 60V and possibly up to a 100 down hill I would guess.

I am unsure of how to design this with varying such high voltages :-/ ...

If the regulator always shunts the output of the rectifier (i.e. it can not be disconnected as with a switch), then the voltage will never climb more than a few percent above the set regulation voltage.

To get to the open-circuit voltages you are measuring, you would have to run with the regulator disconnected, get up to speed, and then suddenly connect the regulator to the spooled-up alternator. Why would you do that?

Also, do you have the specs for the LEDs you would like to light? How many? Can you run them in series?
 
Here is an idea for Moped LED light current regulator. Suppose you have 3 1W (0.4A) high power LEDs. The alternator/rectifier combo puts out ~70V open-circuit (zero current). This simple regulator shunts the excess current (>0.4A through the LEDs) and holds down the voltage at the anode of the top LED in the process:
 

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