Footwell LED project complete
#41
If you look at the datasheets for the higher powered LEDs, including Superflux and the larger Luxeon style LEDs, the manufacturers almost always recommend using a current limiting device. Consistent brightness is acheived with current, not voltage since each LED will have a slightly different brightness at a given forward voltage. It is also for thermal management--the LED forward voltage varies with the current rating and will change with temperature. A typical 20-mA white LED has a forward voltage that varies between 2.6 and 4.2 V over the entire operating temperature range. Constant current also does a better job controlling LED color shift.
#42
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what's the advantage of using the LM317T versus using a voltage regulator like an LM7812(12V 1.5A) and the proper current limiting resistor? For example, using the specs above for the LED's, you have:
Vf = 3.3V
If = 62mA
So if I was using a LM7812 and two LED's in series,
(12V -3.3V -3.3V)/(.062A) = 87 Ohm resistor
Vf = 3.3V
If = 62mA
So if I was using a LM7812 and two LED's in series,
(12V -3.3V -3.3V)/(.062A) = 87 Ohm resistor
Originally Posted by Circuit Designer
The circuit (lower right) illustrates using a LM317 variable voltage regulator as a constant current source. The voltage between the adjustment terminal and the output terminal is always 1.25 volts, so by connecting the adjustment terminal to the load and placing a resistor (R) between the load and the output terminal, a constant current of 1.25/R is established. Thus we need a 12 ohm resistor (R) to get 100mA of charge current and a 1.2 ohm, 2 watt resistor for 1 amp of current. A diode is used in series with the input to prevent the batteries from applying a reverse voltage to the regulator if the power is turned off while the batteries are still connected. It's probably a good idea to remove the batteries before turning off the power.
#43
3.3 + 3.3 + 3.3 + 1.5V = 11.4 volts, and I have seen my battery at 11.8V with the engine off.
Anyways, yes, you've got the right idea.
#44
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The 317 has a dropout voltage of 3v between Vo and Vi, so coupled with the 1.25v across Rg you are talking about 4.5v of voltage required across the current source. There are other voltage regulators with lower dropouts, but since our input voltage ranges from ~10-14v they still don't quite allow you to run 3 leds per chain.
This is partyly what leads to the dismal efficiency of this circuit (~50%), but since it only draws an amp or two it's not a big deal.
- Matt
This is partyly what leads to the dismal efficiency of this circuit (~50%), but since it only draws an amp or two it's not a big deal.
- Matt
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From the graph it looks like at 50C and .35A the dropout is roughly 1.75. The 3v dropout is their maximum or worse case rating.
http://www.national.com/pf/LM/LM317.html
The LM1086 has a 1.3v rated dropout which might be interesting, but 3 Luxeons in series still require a forward voltage of about 10.5v, so won't work if we assume a minimum 11v power source, bummer.
- Matt
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What if you wanted to run 4 LED's off one LM317T? could you do it like the the picture below but by changing the value of the resistor R to:
1.25/R = 2 * .062A
R=10 ohms
1.25/R = 2 * .062A
R=10 ohms
#48
No, you had it right a couple of posts up. A 20 ohm resistor will keep the circuit at 62 mA as long as you have enough voltage to drive the LEDs. You won't be able to four Superflux LEDs at 3.3V a peice because they would pull 13.2 volts (3.3 x 4) before you figure in the voltage drop of the LM317T itself.
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No, you had it right a couple of posts up. A 20 ohm resistor will keep the circuit at 62 mA as long as you have enough voltage to drive the LEDs. You won't be able to four Superflux LEDs at 3.3V a peice because they would pull 13.2 volts (3.3 x 4) before you figure in the voltage drop of the LM317T itself.
#50
Sorry, the diagram you posted did not show up at the time I responded. With 2 arrays, yes, a 10 ohm resistor will give you an output current of around 124 mA, divided into 2 arrays = 62 mA per array.
It is not recommended to use series-parallel arrays behind a constant-current source. This is taken straight from the data sheet:
"When controlling multiple LEDs from a single constant current source, the LEDs should be connected in series to ensure the current in each LED is identical. If LEDs are connected in parallel the varation in Vf will again lead to different currents and therefore different brightness in each branch." In addition, if one LED burns out, the full 124 mA will go to the other array and all of the LEDs will eventually fail. This is why I used one LM317 per array, and at 85 cents a piece, it seemed worthwhile.
I will refer you here for a full explanation.
Ken
It is not recommended to use series-parallel arrays behind a constant-current source. This is taken straight from the data sheet:
"When controlling multiple LEDs from a single constant current source, the LEDs should be connected in series to ensure the current in each LED is identical. If LEDs are connected in parallel the varation in Vf will again lead to different currents and therefore different brightness in each branch." In addition, if one LED burns out, the full 124 mA will go to the other array and all of the LEDs will eventually fail. This is why I used one LM317 per array, and at 85 cents a piece, it seemed worthwhile.
I will refer you here for a full explanation.
Ken
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That is so dope
I've been wanting to put lighting in the foot wells for awhile now but don't really know how i'm going to wire everything up. This looks really good and definitely bright!
#53
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if ur not sure how to work with leds that u have to solder urself, get some led bars or get some cathode light bars (not as efficient but work) and wire them up to the dome light. pretty simple if u have the soldering part done for ya...
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