I have a “light” with a 5-led array powered by 2 AA batteries (3V). I want to run it off vehicle current (12V). How do I figure out what I need to do that? Do I need a choke? A resistor? How do I calculate how much resistance to add?
Simple Solution - Use a 150-180 Ohm 1 watt (2-3 watt resistance preferable) in series. Assuming each LED consumes 10mA, 180 ohm will give you 50mA * 180ohm = 9 V voltage drop.
Or buy a DC-DC converter from Walmart used to run CD players.
andy_fl
Thanks, guy. Just what I needed.
(The DC converter isn’t practicable for what I’m doing)
andy_fl is correct, assuming (of course) the five LEDs are connected in parallel.
Here’s a first-order analysis:
Definitions
V[sub]S[/sub] = Maximum battery voltage (V)
R = value of resistor (ohms)
V[sub]R[/sub] = Maximum voltage across resistor (V)
I[sub]R[/sub] = Maximum current through resistor (A)
P[sub]R[/sub] = Maximum power dissipated by resistor (W)
I[sub]LED[/sub] = Desired current through each LED (A)
V[sub]LED[/sub] = Voltage across each LED @ I[sub]LED[/sub] (V)
N = number of LED’s connected in parallel
Given
V[sub]S[/sub] = 13.5 V
I[sub]LED[/sub] = 10 mA = 0.01 A
V[sub]LED[/sub] @ 10 mA = 3 V (you can get this from the LED’s data sheet)
N = 5
Calculations
I[sub]R[/sub] = N * I[sub]LED[/sub] = 5 * 0.01 = 0.05 A
V[sub]R[/sub] = V[sub]S[/sub] - V[sub]LED[/sub] = 13.5 – 3 = 10.5 V
R = V[sub]R[/sub] / I[sub]R[/sub] = 10.5/0.05 = 210 ohms
P[sub]R[/sub] = I[sub]R[/sub] * V[sub]R[/sub] = 0.05 * 10.5 = 0.53 W
I would use a 220 ohm, 1 to 3 watt resistor.
Of course, this assumes all five LEDs are connected in parallel. A more efficient arrangement is to connect LEDs in series. With LEDs that have a 3V voltage drop, and assuming a supply voltage of around 13V, it is most efficient to connect four LEDs in series; this would constitute one “string.” Multiple strings may be connected in parallel for greater light output. Pick the right LEDs, and you may not even have to use a dropping resistor in each string…
Would a voltage regulator (like a LM317) be impractical for this situation?
Brian
I think I got the part # right
I’d use a voltage regulator. A voltage regulator is a 3-legged component that looks like a big transistor. It shouldn’t be hard to find a 3-volt regulator that can handle 13 volt input. You connect one terminal to the 13 volt supply and one to ground, and you get a constant 3 volt output from the third terminal.
There are advantages and disadvantages to using a voltage regulator vs. a resistor.
A resistor is definitely more reliable. And cheaper. (The latter is only important for mass-produced items.) The disadvantage to using a resistor is that the brightness of each LED may vary a little depending on supply voltage. In other words, they may be slightly dimmer when the car is off (V = 12.6) verses on (V = 13.2 to 13.5). This won’t happen with a voltage regulator. And in case you’re wondering, there is no efficiency difference between the two.
No matter what you do, I would highly suggest sticking a silicon diode in the circuit somewhere.
connecting any kind of diodes in parallel is a bad idea. Better use independent limiting resistors for each LED or connect them in series.
switched mode power supply for max efficiency.
If you’re using white, blue or green LED’s, be careful not to reverse the polarity. They are awfully sensitive to reverse voltage.
Also, most modern LEDs are rated for 20mA ea and 30mA is common.
Here’s an auction (all I could find quickly) with some common LED specs. These are the specs for the individual diodes, not the flashlight being sold.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1790188687
Quote:
“Directivity(viewing angle); 20 degrees
DC Forward Voltage; Typical: 3.6 Max: 4.0 volts
DC Forward Current; Typical: 20 Max: 30 milliamps
Pulse Forward Current ( Pulse width 10ms); 100 milliamps”
Another helpful page:
http://www.plasma-ireland.com/lp/applications.html
If you have many questions, check out :
My second favorite forum page 
All about portable lighting and full on bona fide experts.
Personally, I would use te LM317 regulator. Constant output from 5-25 volts in.
Another helpful site.
http://www.geocities.com/SiliconValley/Bay/4397/led/led.htm
One question I haven’t resovled is the series or parallel concern. I would have thought that parallel would be the way, but this assumes a regulated circuit and same-chemistry diodes. It seems to me that eitherway, you would need some regulation for constant, high output… hmmmm.
Sorry, haven’t been checking in lately. Thanks for all the good links and info.