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post #1 of 7 Old 05-21-2005, 9:27 AM Thread Starter
 
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Question about LED voltage/mounting

I'm thinking about buying some LEDs to increase my visibility and had a couple questions. What voltage is supplied by the bike to the turn signal wiring and can I just splice into the turn signal wires? Using this supplied voltage I just need to keep the total voltage going to each LED less than its rated forward voltage right? I have recently taken physics and understand parallel/series circuits and resistors. I will end up wiring LED's in series since it looks like most only handle around 3V max. I also know to watch out for LEDs w/ a small "viewing" angle. Also I'm on one of the sites and the sizes are as small as 3mm...that's seems really tiny to me. Should I go w/ 5mm?
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post #2 of 7 Old 05-21-2005, 11:11 AM
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Re: Question about LED voltage/mounting

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What voltage is supplied by the bike to the turn signal wiring and can I just splice into the turn signal wires?
It varies between (roughly) 12-14.5V. Splicing into the signal wires is fine. The LEDs aren't a big enough load (current draw) to be a problem.

Quote:
Using this supplied voltage I just need to keep the total voltage going to each LED less than its rated forward voltage right? I have recently taken physics and understand parallel/series circuits and resistors. I will end up wiring LED's in series since it looks like most only handle around 3V max. I also know to watch out for LEDs w/ a small "viewing" angle.
So you also know Ohm's Law?

It is actually current that makes LEDs work, makes them dimmer or brighter, or blows them up. The usual method is to configure your LEDs in series or parallel such that the specified forward voltages add up to a little less than the total supply voltage. Then add a series resistor that will restrict the current through the LEDs to something less than the max specified forward current for those LEDs. The resistor is selected to drop the remaining supply voltage at the desired series current. This is more important when the supply voltage varies, as on a bike or car. If you configure the LEDs so that the forward voltage adds up to the supply voltage (say 12V), and don't use a resistor, you'll have the LEDs biased at their lowest possible operating current (dim) and no way to control the current when the supply voltage increases (to 14V), which will over drive them and eventually kill them.

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Also I'm on one of the sites and the sizes are as small as 3mm...that's seems really tiny to me. Should I go w/ 5mm?
Often the chip is the same in the 3mm and 5mm LEDs, only the epoxy case is a different size. Pick the size by what looks best to you and use the brightness and viewing angle specs to choose. 3mm is very small on a vehicle, though.

I found some very bright 5mm LEDs with a relatively wide viewing angle of 45deg at Superbrightleds.com. The price was good and they were very responsive.
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post #3 of 7 Old 05-21-2005, 11:38 AM
 
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Re: Question about LED voltage/mounting

I'm not sure about using them for turn signals or other applications, but I have made some indicators (idiot lights) with them. On the RC30 I use LED's in place of the stock indicator bulbs for neutral, oil psi, and high beam. On my XR650, I needed to find some way to put light on my BC800 (bicycle speedometer) so I used a really bright blue LED pointing right at the face. I used a 560 ohm resistor from Radio Shack for each one, and have had no problems at all.
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post #4 of 7 Old 05-21-2005, 3:34 PM Thread Starter
 
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Re: Question about LED voltage/mounting

cool thanks guys. So one question: To attain the proper current and voltage through each LED (around 2 -3V, 20 - 30 mA) I need to use a resistor along w/ Ohm's and 12V? 12V = (20mA)(R), R = 600 Ohms. But what is the resistance of each LED? Since they will be in series I would add the resistances of each LED along w/ the resistor.
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post #5 of 7 Old 05-22-2005, 1:31 PM
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Re: Question about LED voltage/mounting

It's even easier than that. The forward voltage of an LED stays relatively constant for a given operating current, so you don't need to know the equivalent resistance. The starting point is to choose the amount of current you want to run through your LED(s), which should be something slightly less than the maximum specified on the datasheet. Let's say you want to run your LED at 20mA and it has a forward voltage of 3V at that current. You have 12-3=9V left to drop, so you choose your resistor to drop 9V at 20mA.... 9/.02 = 450 Ohms.

If you ran three of these LEDs in series they'd drop 9V. With 3V left to drop, your resistor would be 3V/.02 = 150 Ohm.

You know this already but a reminder:

Series circuit: current is the same through each device
Parallel ciruit: current through each branch is added to the total

Let's say you have three parallel branches of three series LEDs. The voltage drop across all three branches is still 9V (same as one series branch), but you now need 20ma for each of three separate current paths, or 60mA. The series resistor here is 3V/.06 = 50 Ohms.

Last edited by dB; 05-22-2005 at 2:05 PM.
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post #6 of 7 Old 05-22-2005, 3:41 PM Thread Starter
 
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Re: Question about LED voltage/mounting

Thanks!
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post #7 of 7 Old 05-22-2005, 5:08 PM
 
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Re: Question about LED voltage/mounting

Be careful with voltage drops across LED's. Their current consumption is exponential with repsect to forward voltage. Its what is called the "knee" current. They have a nominal operating voltage and after that their current rises through the roof. At 1.7 volts, a typical LED might consume 15mA. At 1.8 volts it could be 40mA. At 1.9 volts it could be 100mA. And with that constant voltage supply the current will still change radically with temperature. Higher the temperature, the less efficient it will be, and it will generate more heat with less light output, causing a slight runaway condition.

Your bike's voltage changes a LOT! It could vary between 13 volts to 15 volts. 6 LED's wired in series with a small resistor and a back protection diode to limit total voltage can barely be visible or painfully bright the next moment.
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