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Voltage Regulator for a Small Motorbike, 6V AC Current

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by sign216, Oct 7, 2014.

  1. sign216

    sign216 Member

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    I'd like a diagram for a voltage regulator, 6 volt AC, to go on a small motorbike's taillight-only circuit. With no regulator, the taillight keeps burning out when I'm at max speed. Headlight is ok though.

    Bike is a 1965 Sears/Gilera 106cc, that has AC electrics with no battery and no regulator. No DC current at all.
    There are two coils;
    1) Powers the headlight and the horn.
    2) Powers the ignition coil, and the taillight.

    Powering the tailight off the ignition circuit was common on antique Italian mopeds, for unknown reasons. Whenever the brake light burns out, the ignition goes to ground and stops the engine.

    I found this circuit, but it's too complicated for my small bike. I'd like something simpler, more compact. Any ideas?

    http://home.comcast.net/~loudgpz/GPZweb/RegRec/GPZacRegulator.html


    [​IMG]
     
  2. dr pepper

    dr pepper Well-Known Member

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    Depends on your electronic skills, if your handy making brackets you could try 2 bulbs in series of higher wattage, if its a 6v 5w you could have 2x6v12w lamps in series which might give off enough light for the job and would withsatnd a significant overvoltage.
    Another way would be a transistorized shunt regulator and series resistor.
    I beleive there are such things as dynamo shunt regs, I think someone on this forum posted about one recently, do a search.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That circuit is actually pretty simple - and is probably as simple and basic as you could make it?.

    I would suggest it's exactly what you want.
     
  4. dave

    Dave New Member

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  5. sign216

    sign216 Member

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    Nigel, I'd really like to pare down the size. I looks like R3 is the biggest component, and is huge compared to everything else. Appears that circuit was designed for 10 amps. Since it's only for a 3/15 watt, 6V tail/brake light (2.5 amps, right?) do you think I could make R3 a smaller unit?

    Dr P, I like your creative thinking. That's a good work-around, but I'd rather keep the existing lamp mount. There's no really enough room in the tail light to mount dual tail/brake lights. Nice approach though.

    Joe
     
  6. dr pepper

    dr pepper Well-Known Member

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    Fair enough, I was thinking you might be able to use a dead lamp cap and solder 2 lamps to it without modding your bike, but if your shorta space maybe not.
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Try a 10 ohm 5W resistor - see how that works.

    But for a start I would make a rough (large) version, and test how it works, then worry about making it more compact once you've proved it.

    As it's only for brake lights, how about replacing the brake light with LED's? (as is becoming common now), and using the alternator to charge a small battery to power just the brake light?, with a constant current driver for the LED's. This would ensure the brake light works well at all speeds.
     
  8. sign216

    sign216 Member

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    How about using an LED designed for 12V AC, in this 6V AC system, and dropping the regulator idea? Would I get a reasonable amount of light with only half the designed voltage?

    I thought about changing from AC to DC, which would facilitate LEDs. But that would mean rewiring almost half the bike, and adding a battery, regulator+ rectifier. Also means there's the chance that the ignition or lights wouldn't work. It took me a while to trace and correct a previous ignition problem, so I am loathe to disturb a working system now.

    Switching to just an LED bulb, using the existing unregulated 6V AC current is an idea. Then I could do away the regulator assembly, which appears as large as the whole taillight anyway. There are some LEDs that fit the 1157 socket, but it's hard to find one that's ok with AC current, and ok with 6v current.
     
  9. dr pepper

    dr pepper Well-Known Member

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    Leds themselves are dc, a proprietry ac led is probabyl a led with abridge rectifier inside or a diode, or a pair of leds in inverse parallel.
    A pair of luxeon leds in inverse parallel might do the trick, but you'd have to hack the original wiring.
     
  10. sign216

    sign216 Member

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    I wish they had bridge rectifiers. I think the AC-friendly LEDs for auto lights just use a single diode, dumping half the current. With that loss, and operating at 6V instead of 12V, I don't know if they would be sufficiently bright.

    I could add a bridge rectifier to the taillight, but would that cause a problem? It's grounded to the frame, and so is the headlight and horn, which run on AC.
    Would the DC taillight and AC headlight, sharing the same ground, cause issues?
     
  11. Misterbenn

    Misterbenn Active Member

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    You can use a full wave rectifier to create a DC-link, as long as the bulb can be isolated from the chassis ground.

    basically you use the supply from your alternator and the chassis ground reference as the inputs to your bridge rectifier. and this will give you a floating DC voltage, its very important that this DC voltage (positive or negative) shouldn't connect to the chassis or alternator supply at any point (I'd suggest fusing the input to the rectifier).
    use a big capacitor to smooth the ripple and then a chopper or SMPS or LDO to drop the voltage as required.
     
  12. sign216

    sign216 Member

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    A motorcycle website reports that it's not easy to get LED brake lights that are brighter than incandescent. Once you throw in reduced current/voltage then LEDs aren't viable.

    Best solution is to use a regulator and the existing AC system. Looking at the circuit diagram, it appears that with the taillight current of 2.5 amps, I could replace R3 with a small 1W resistor (or maybe 2 or 3W to be safe), and use smaller transistors too. That could make the regulator small enough to use on this lightly framed bike.

    As outlined in the webpage, the regulator looks large, about as large as the bike's taillight!

    http://home.comcast.net/~loudgpz/GPZweb/RegRec/GPZacRegulator.html
     
  13. Misterbenn

    Misterbenn Active Member

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    use a rectifier as in my post above
     
  14. sign216

    sign216 Member

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    Converting the taillight circuit to DC to use LEDs doesn't offer benefits that outweigh the cost. It would take a batter/capacitor, regulator, rectifier, etc, and re-working the taillight to isolate it.

    Sticking with AC, and working to shrink the 6V regulator is more attractive.
     
  15. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Is it possible to power the taillight from the headlight circuit, which seems to work okay?
     
  16. sign216

    sign216 Member

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    I don't think the headlight circuit is better, I think the headlight is more robust, so it can take the voltage spikes. At high rpms I imagine both unregulated coils are putting high voltages.

    At idle, like stopped for a traffic light, all the lights are very dim. So I'd rather use both circuits and keep as much of the meager electrical capacity as I can.
    Although the Italian idea of tying the taillight into the ignition circuit in a pass/fail way, is a little crazy.
     
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Test this theory: The lamp fails shorted. Remove the bulb and see if the bike still runs.

    ==

    Surface mount parts can make the circuit smaller, but not terribly small. The resistor and the two TO-3 transistors are quite large.

    ==

    The regulator is a SHUNT regulator. It regulates by putting a 0-10 Amp load on the output of the AC generator.
     
  18. sign216

    sign216 Member

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    Already tested: one day after tying for top speed the brake light burnt out, and then whenever I used the rear brake the engine would stop. Fortunately the front brake is separate, so I could still stop and keep the engine running.

    Re: surface mounts, I'm sure this is why the commercially manufactured regulators are so compact.

    I don't think the referenced regulator is totally a shunt design. I quote the webpage = "The type of regulator being used is called a clipper. This is because it clips the top and bottom of the voltage waveform. It is similar to a shunting regulator, but instead of shorting the current completely, so the waveform has 0 volts, the clipper shunts the current until the waveform is limited to 7 volts. This is necessary since there is no battery (or capacitor) to maintain voltage to the load (in this case, lights)."

    I'm very open to other designs, bring them on!
     
  19. Misterbenn

    Misterbenn Active Member

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    I didn't say anything about using an LED and you wouldn't need a battery unless you wanted it to work without the engine on.

    With a DC voltage you would just need a 6V or 12 bulb, you could probably use the existing one. The capacitor and Linear regulator would sort out any peaks.

    So again all you need is a small 4A rectifier, an electrolytic capacitor and some kind of regulator (switch mode with no heatsink or linear with a heatsink)
     
  20. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Burt out and removed are different. Believe me! I had a headlamp short on me and it resulted in blinkling headlamps on the way home. The headlamp was fused by a thermal
    fuse.

    A shunt regulator when the voltage is below it's threshold does nothing. Semantics, of clipping makes sense when you try to describe it. There is clipping say in an audio amplifier that's unwanted. A shunt regulator does not short to 0 V. Here http://www.ti.com/lit/ds/symlink/tl431a.pdf is an example of a shunt regulator and it's clearly not 0 volts. I worked on a shunt regulator that used a tube that regulated 15 KV at 1 Amp.
     
  21. Misterbenn

    Misterbenn Active Member

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    circuit diagram to make it extra clear how simple you could make this.

    KISS
     

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