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Old 08-25-2010, 12:44 PM   #80
aalegado
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Joined: Jun 2007
Location: Lost Wages, Nevahduh
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I like these lamps! They're now on my farkle list.

I have to address this issue...

Quote:
Originally Posted by honda_silver
Would it be possible or would it create any problems, to get full power on high beam and then have a dimmer control on low beam??

Quote:
Originally Posted by puntoMX
That's easy to explain, although it's theoretical:

U=Voltage
I=Current (Amperes)
R=Resistance
P=Power (Watts)


So, we use two formulas: U=I*R and P=U*I
Now fill in the numbers: I=P/U and R=U/I thus R=12/2=6 Ohms

To reduce it to, let's say 6V, you have to add a 6 Ohms resistor in front of the LED unit, giving you 6V over the resistor and 6V over the LED unit.

If you like to have the light at 75%, you will have to set a 2 Ohm resistor in front of it, to get 1/4 part over the resistor and 3/4 parts over the LED unit. It's really simple but make sure the wattage of the resistor exceeds the wattage of the of the LED units. Now, a 50W resistor cooled in some aluminum housing will cost some 10USD, they are sold in most car electronic shops as a lot of people would switch to LED these days.

...Stuff Deleted...
This Wrong Wrong Wrong.


If you follow this advice you will burn-out your LEDs.

For a given LED's forward voltage (the voltage the LED requires to operate) you must always correctly forward bias a LED. That means the current going through it must be constant. This means that the series, current-limiting resistor must be a fixed value.

By changing the resistor value, you are causing more or less current to flow through the LED—more current with a lower value resistor and vica-versa. This is because the voltage across the LED (that's the LED's forward voltage) is always the same and the resulting voltage across the current-limiting resistor is always the same (for a given LED and voltage source, that's Kirchoff's Voltage Law). If you lower the resistance, the current increases by Ohm's Law, I = V/R.

Less is OK since that can't damage the LED but if it's too low the LED will not light-up.

More is not OK because the LED will burn itself out.

The proper way to vary the brightness of a LED and by extension modulate it's power consumption is to flash it on and off at a rate that achieves the brightness/power consumption you are after. This is what a Pulse-Width-Modulator ("PWM") is doing: It's flashing the LED on and off at a particular rate as well as controlling how long the "on" time is relative to the "off" time.

A PWM flashing an LED at 1KHz (1000 times on/off per second) at a 50% duty-cycle (50% of the time "on" and 50% of the time "off") will produce a 50% brightness. It also results in 50% of the normal power consumption because half the time it is 100% on (full power) and the other half it is 0% on (zero power). That averages out to 50%. Vary the proportion of "on" to "off" and you get different, average brightness/power.

The power controllers for heated gear work the same way—"on/off" at a given frequency and a variable duty-cycle results in power consumption ("heat") somewhere between 0% and 100%.


Quote:
Originally Posted by ellivnad
If you aren't afraid of a bit of soldering, here is a nice kit for under $10 : http://www.moreleds.com/drivers.htm (half way down the page)

PWM (Pulse Width Modulation) is the way to go for dimming, as you won't be wasting a lot energy to heat. You'll probably even find that you can "dim" them quite a ways before you visually can tell they are dimming, yet they will be drawing less power (read about apparent brightness on that link).

Update: I like the looks of this dimmer too, search for KICK-KR6 on ebay. Looks like this single unit would control two lights.
Look here:

http://my2wheels.com/forum/showpost....6&postcount=26

Here:

http://my2wheels.com/forum/showpost....0&postcount=30

and here:

http://my2wheels.com/forum/showpost....9&postcount=38

These are PWM LED drivers I made. They fit in a Altoids tin and can handle up to 3A of current.

They are completely programmable so you can have an particular flash/dimmer pattern. I made these as brake light modulators but they can also be used to create a high/low-brightness effect in a LED high-beam/low-beam application.

I'll post the circuit if anyone's interested and wants to BYO.
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aalegado screwed with this post 08-25-2010 at 01:14 PM
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