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Old 11-24-2010, 11:48 AM   #11 (permalink)
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When you said Navigation lights I was thinking inside the car. Duh. If you are using high wattage LEDs think of using a buckpuck they are current regulators not voltage regulators. You can use one to power several LEDs if wired right. You might be able to power 4 LEDs in series with one or power two strings of 2 or 3 with one. Of course you will have a lot of wiring to do to make it work. Or a lot of expense powering each one with it's own.

Still you might do ok for nighttime use with smaller LEDs and a resistor at each light. Or just use a larger resistor on your one watt and use less power. I doubt if you need that much light for marker lights.

I tried setting up LED taillight/brakelight on my road motorcycle and ended up going back to standard lighting as the quality of lighting was not the best.

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Old 11-24-2010, 12:17 PM   #12 (permalink)
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Radio shack has LED's with built in holder and resistor.

Green LED with Holder - RadioShack.com

I bought one to use as a cooling fan light on my dash.
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Old 12-04-2010, 01:50 PM   #13 (permalink)
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Quote:
Originally Posted by saand View Post
Interesting gizmo... Too bad it's so pricey. Also, it's output voltage is limited to the input voltage less 1.5 to 4 volts. So in a 12 volt automotive system, you could only run 2 or 3 white LEDs in series with an individual driver. I suppose a DC-DC converter could boost the input voltage, but that's an additional cost. A buck-boost driver would be more ideal, but still more expensive I expect.

I'm joining in late but as has been said above, you want constant current to drive LEDs. This is because of the non-linear voltage/current relationship in semiconductors. A simple resistor follows ohms law - the current pushed though a resistor is proportional to the voltage drop across it. If you were to graph current vs. voltage, it would be a straight line with the slope controlled by the resistance value.
Semiconductors (LEDs included) don't generally follow ohms law - they're called non-ohmic devices. If you ever have an adjustable power supply handy, hook up an LED and gradually increase the voltage. You'll see that almost no current flows (a little does, but it's unmeasurably small) up until a certain voltage (which depends primarily on the LED design, but also on manufacturing variations to some degree). Once you hit that point, current will increase gradually at first and then ramp up quickly.
Here's wikipedia's illustration:

It's a more or less exponential function if you're mathematically inclined. It's easy to overdrive and burn out an LED when you reach that steep part of the curve. Power dissipated by the LED is the current flowing through it times the voltage across it: P = I * V, in watts. The voltage doesn't change much but the current skyrockets, taking the power dissipation with it. As you increase the current though an LED, it will get brighter but less and less efficient. That lost power becomes heat. Without adequate thermal management, the LED literally burns up in its case.
Going back to the original point, controlling current rather than voltage turns that whole voltage/current curve on its side, making the previously steep ramp quite flat and manageable. If you actually flip the curve (or just turn your head so current is horizontal), you'll see the low end becomes a steep drop off as you approach zero current. If you want to run the LED at a very low level like that, other methods can be used such as PWM dimming.

In response to using zener diodes, they still dissipate power as heat as was said. They're not well equipped for that, either... Typical zener diodes can generally only conduct a few milliamps of current. You would need quite a few in parallel to run a simple 5mm indicator-style LED. Using them with a 1 watt LED would be ridiculous.
Zener diodes are generally used indirectly in voltage regulation circuits. For instance, a 7805 fixed 5-volt linear regulator uses a 5-volt zener as a reference voltage in a circuit that controls a much heftier power transistor.


Last edited by bobski; 12-04-2010 at 01:59 PM..
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