Can LEDs run as tail light on 12VAC bitbike?

[surfsand]

I have a honda crf50 Pit bike that originally had no electrical other than what goes from the stator to the cda to sparkplug/ignition system. I found a tutorial showing how to rewind the stator to get power for lighting. Link
I did the mod and it works great. I actually over wound a few turns on each leg, so my output was a bit high. At low idle I get 10-12vAC+. While revving the rpms mid to high its up in the mid 20's, which is way too high to run on two 35watt 12 volt halogen bulbs(They'll just keep blowing). Instead of buying a voltage regulator like the one in the tutorial, i just used a 12v 2.2 amp Fixed Positive Voltage Regulator (Linear IC)
Yes this is 12v AC since I'm not running a rectifier.

Part No. UA7812KC
Output Voltage Nominal (V)=12
Load Current Max. (A)=2.2
Drop-Out Volt Max.=2.0
P(D) Max.(W) Power Dissipation=15
Supply Voltage Maximum (V)=35
Status=Discontinued
Package=TO-3
Pins=3
Military=N

It works great for the halogen lights super bright.
I was running a 12volt 2" Red LED sealed mini clearace/marker light as tail light.Link

And it worked great, I made sure to leave headlights on at all times until i was sure it could handle AC and the regulator would hold.
I let my nephew ride it forgot to tell him not to turn the lights off. Of coarse thats the first thing he did. That and winding out 2nd gear, Fried the little light.

Do 12V LED fixtures like these, or any LEDs in general, care if its 12V AC or DC? (I know the Low Voltage halogen type bulbs do not.)

Is this regulator going to be ok for this application?
Do i need to add a rectifier inline to convert it to DC?


Thanks for any and all help.

Surfsand

 

[Andy1845c]

Why are you feeding ac into a voltage regulator IC? It won't put out regulated ac, i'm skeptical it survives the ac current.

Why not use a rectifier circuit?

In short, leds can be hooked to ac, but will only light on one half of the ac cycle, so they will flicker. The speed of the flicker depends on the frequency of the AC cycle.

 

[surfsand]

Thanks for responding. The owner of a electrical/electronics supply shop i go to said this would cover it. Since i wasnt adding a battery and just using halogen bulbs it wasnt needed. I had another design using a different stator/magneto, but the stator had issues and i scrapped the design which had a rectifire-regulator i put together. and went with the rewinded stator. My design also had a small 12 volt battery.
Now i think about it im going to add them back into the system just to power the Leds and also tiny turn indicators and brake light im putting together.

Fyi the voltage is AC but not true AC. Its generated by the magneto as AC but grounds to bike chasse like DC. Therefore if rectified with battery in line, The ground for that circuit has to have its ground floated (nothing on the circuit can ground to chasse/motor, ground ends at battery)

Surf

 

[Hero999]

At low idle I get 10-12vAC+. While revving the rpms mid to high its up in the mid 20's

Did you measure this without a load on the alternator?

If so, then you'll probably find that it drops down to something like 14V when you connect your two halogen bulbs to it.

Fyi the voltage is AC but not true AC. Its generated by the magneto as AC but grounds to bike chasse like DC.

Connecting it to the chassis won't convert it from AC to DC.

Therefore if rectified with battery in line, The ground for that circuit has to have its ground floated (nothing on the circuit can ground to chasse/motor, ground ends at battery)

There must be a rectifier somewhere, you can't charge a battery from AC, all it'll do is charge when the current is flowing one direction then discharge when it flows in the other direction, a large current will flow and it'll get very hot.

Your fixed regulator needs DC, if the owner of the bike shop says otherwise then he doesn't know what he's talking about. Also the regulator is only good for 2.2A which can only run a 26.4W light bulb.

 

[mvs sarma]

How the regulator could work when the output of the device is still AC and not DC?
However Surfsand deserves all appreciation for the enormus effort and for detailed depiction.

What would happen if we have two white LEDs opposite parallel so that each half cycle is taken by one LED and other LED is protected, due to low reverse voltage felt by it. Like this many sets could be connected in series and finally a small current limiting resistor. would it not be simpler than such serious mod of re-winding a stator.

 

[Hero999]

That would work well and you could safely run the LEDs at double their rated current as the duty cycle will only be 50%.

As it's AC youd could use a suitable inductor instead of a resistor which would provide a higher impedance at the higher frequency generated at full revs thus providing crude current regulation.

 

[mvs sarma]

That would work well and you could safely run the LEDs at double their rated current as the duty cycle will only be 50%.

As it's AC youd could use a suitable inductor instead of a resistor which would provide a higher impedance at the higher frequency generated at full revs thus providing crude current regulation.

An Inductor could well be used if the frequency is stable - in this case it is frequently varies and also the actual frequency is rather small calling for big size and High value inductor, I feel.

 

[Nigel Goodwin]

An Inductor could well be used if the frequency is stable - in this case it is frequently varies and also the actual frequency is rather small calling for big size and High value inductor, I feel.

The whole point of using an inductor is that the frequency varies, and yes, it would need to be a large inductor.

 

[3v0]

I seen a circuit somewhere that allows a LED to be used with any voltage. What it was I do not recall but it was simple and it was not a SMPS or other common regulator. I think it was all passive components. Does that ring a bell with anyone ?

 

[mneary]

yes, it would need to be a large inductor.

An inductor of 1.0 Henry would have a reactance of about 50 ohms at 8 Hz (500 RPM) and a reactance of 1000 ohms at 10000 rpm. You might be able to steal one from an old (non-electronic) CFL ballast. You might also find a close match from an old interstage transformer (like 750:1500 ohms) from a cheap transistor amplifier.

 

[Hero999]

An inductor of 1.0 Henry would have a reactance of about 50 ohms at 8 Hz (500 RPM) and a reactance of 1000 ohms at 10000 rpm.

That's an incorrect assumption.

For a start most bikes idle at around 1000rpm and the allternator here has six poles.

From the link he orignionally posted:
**broken link removed**

Therefore, the lowest frequency will be 100Hz and the highest will be 1kHz at 10,000rpm which would allow for far more sensible size inductors

 

[mneary]

Thanks for setting that straight. I didn't know from the picture how the number of poles related to frequency. It's right on the crankshaft?

Shame that interstage audio transformers aren't the right value; they're awfully common.