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I recently wound a stator for my Honda 70. I'm running a 12V 55w automotive fog lamp for a headlight and it works great. I'm wanting to run a LED tail light off of the stator as well. The questions is will a standard 12v LED tail light operate from 12v AC that the stator is producing. I pretty sure that it would if a series of diodes per placed inline before the light, changing the AC to DC, but LED's are diodes themselves so are the additional diodes needed?:banana:

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No absolute need for the additional diode. The LED will pass half the voltage and block the other half. This may/can cause the LED to appear to be flashing in some instances.

Most LEDs have a fairly wide voltage rage before you have issues, so you want to see the voltage at idle and speed to ensure you are within range. If the idle voltage is too high, you'll need a resistor to step it down. You may be able to use a few small diodes to not only rectify the power but reduce it as well.

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Thanks for the quick response!:banana: At idle it is making 12.5v and at 8k it was making 46v. Those readings were before the voltage regulator was installed to verify the stator operation. A regulator is installed now limiting voltage to 13.5v. I'm ok with it flashing...as that should be more visable. Thanks again...I just wanted a second opinion before spending the cash on a tail light.

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I have a Baja Design LED tail/brake light on one of my bikes and run it off the AC circuit, with a 12volt AC regulator. No flicker and it is plenty bright. As others said worst case with AC is the LEDs only fires half the time, which may be OK. You could also build or buy a small bridge rectifier for the tail light but it will have about a 1/2 volt drop.

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Well its all wired up and it works great. The only flicker it has is at low rpm...which is to be expected. Thanks guys for your input, just wanted to let you know how it turned out.:banana:

Edited by JandJ07
update

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Grote LEDs have a full wave rectification circuit (2 diodes and a current limiting resister) built in and can run brightly off AC and are not affected by polarity.

Hook them up any way you want and they will work.

I know from experience not all are built this way, many just have a resistor.

Be careful using battery chargers to function LEDs.

On some the peak to peak voltage is higher than the LEDs will withstand, even if the average voltage is 12v. They will usually work OK with a battery attached but without a battery a charger can fry an LED regardless of what a voltmeter tells you the voltage (RMS) is.

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Thats good to know. I don't really have room for a battery...the tail light that I started with is 5 dollar trailer light from walmart. So i'll try it for a while and see how it goes before installing a better one.

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No absolute need for the additional diode. The LED will pass half the voltage and block the other half. This may/can cause the LED to appear to be flashing in some instances.

Most LEDs have a fairly wide voltage rage before you have issues, so you want to see the voltage at idle and speed to ensure you are within range. If the idle voltage is too high, you'll need a resistor to step it down. You may be able to use a few small diodes to not only rectify the power but reduce it as well.

You told me to google so I did, sorry for necroing this thread.

On ac power the led would take half the wave and block the other, does that make it half as bright?

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You told me to google so I did, sorry for necroing this thread.

On ac power the led would take half the wave and block the other, does that make it half as bright?

Well, yes and no. The problem is power dissipation with an LED. Most are not designed to handle large amounts of reverse current.

So volt wise, to run a LED rated between 10 and 18V DC, you'd need an AC source of at least 20 Volts and no more than 36 volts. But the reverse current flow would cook the led. So what you still need is a real rectifier designed with diodes designed to block the reverse current and better still, a full wave rectifier that redirects the power so it all is used and not just half. Google rectifiers half wave, full wave. Then a regulator to control to maximum voltage.

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Well, yes and no. The problem is power dissipation with an LED. Most are not designed to handle large amounts of reverse current.

So volt wise, to run a LED rated between 10 and 18V DC, you'd need an AC source of at least 20 Volts and no more than 36 volts. But the reverse current flow would cook the led. So what you still need is a real rectifier designed with diodes designed to block the reverse current and better still, a full wave rectifier that redirects the power so it all is used and not just half. Google rectifiers half wave, full wave. Then a regulator to control to maximum voltage.

I am a mechanical engineer so I have a bit of background on EE. If I use a full bridge rectifier then I would get an output that looks like y= Vmax * abs(sin(x)), do I need to add a capacitor to smooth out the output? or it doesn't matter brightness wise? I need to pass inspection I guess I will just cheat and hide a battery in the air box.

 

I need this right? 

http://www.ebay.com/itm/15-Amp-100-Volt-Bridge-Rectifier-BR156-15A-100V-Full-Wave-Diode-Rectifier-/130906175272?hash=item1e7a9db728:g:vaUAAMXQslhRjYIj

Edited by Yap Yap

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LEDs operate on a low voltage so the 12 volt versions have built in voltage regulators and as previously noted not all are created equal regarding tolerance for high voltage. So safest is to have at least an AC regulator.

I run LED tail/brake lights on the  AC headlight circuit on my CRF250X and they seem bright enough. As always YMMV

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LED's aren't designed to block voltage in teh reverse direction - you either need a path for current to flow (like another LED in the opposite polarity), or a regular diode to minimize that reverse voltage.

 

Bridge rect and a filter capacitor will give you a good-enough DC. 

 

Personally I'd just buy an LED taillight that's got the protection/current-limiting components all ready to go.  A lot easier that way if you don't know what you're doing and just want to plug something in.  Can probably find something on ebay for under $10 shipped to your door.

 

I designed my own because i'm an electrical engineer and didn't like the options that existed.

IMG_20130524_141002-M.jpg

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LED's aren't designed to block voltage in teh reverse direction - you either need a path for current to flow (like another LED in the opposite polarity), or a regular diode to minimize that reverse voltage.

 

Bridge rect and a filter capacitor will give you a good-enough DC. 

 

Personally I'd just buy an LED taillight that's got the protection/current-limiting components all ready to go.  A lot easier that way if you don't know what you're doing and just want to plug something in.  Can probably find something on ebay for under $10 shipped to your door.

 

I designed my own because i'm an electrical engineer and didn't like the options that existed.

I have an led tail light and an incandescent headlight both wired in parallel , so the headlight now is keeping a less resistive  current path. Thanks for pointing that out because if I put an led headlight I might have blow up both the headlight and tail.

I am using an ac voltage reg for dc, I think it should work fine.

Order of power conversion + reasoning:

AC > Rectifier > Capacitor/Condensor > Voltage regulator

My reasoning is that if I put the regulator before the capacitor then the wave will get saturated and peaks will be cut by the regulator, but if I smooth it out first then the peaks blend into the valleys.

This setup gets saturated and makes less dc voltage

AC > Voltage regulator > Rectifier > Capacitor/Condensor 

 

 

Untitldded.png

 

I end up drawing with this circuit: 2200uF capacitor I have laying around + AC regulator I have laying around(should work with DC too?)

Uuuntitled.png

Edited by Yap Yap

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On my bike I used a reg/rect combo and then the capacitor as a battery substitute to provide a filter function for the regulator.

The capacitor does more than smooth out the pulses, it absorbs the voltage spikes that confuse the regulator; the symptoms are a bright light at idle that dims as rpm increase.

Edited by Chuck.
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On my bike I used a reg/rect combo and then the capacitor as a battery substitute to provide a filter function for the regulator.

The capacitor does more than smooth out the pulses, it absorbs the voltage spikes that confuse the regulator; the symptoms are a bright light at idle that dims as rpm increase.

I see, I had lights dim when I used a regulator so I threw away the regulator assuming it's faulty.  :facepalm:

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For anyone viewing my earlier schematic, the order of capacitor and regulator doesn't matter, they are connected in parallel. 

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