Re: LED\'s in auto interiors
You might also try using a 3-terminal regulator like LM317 or LM7805 as a constant current source so the LEDs will stay the same brightness regardless of what the car's charging system is doing. This will allow you to adjust the current to the LED's max and not have to settle for a little less so that the LED won't blow when the alternator pushes the voltage up to 13.6 volts or more.
With the LM317, you can use one resistor to set the current. The IC puts out 1.25 +/- .05 volts between "out" and "adj"; select a resistor to put between them to draw the amperage you want (Where ohms=1.25/amps). For 20mA that would be about 62 ohms (1/4 watt is ample). For a luxeon star, about 3.6 ohms (at least 1/2 watt) will give about 350mA. With the resistor in place between "out" and "adj", connect the car's + to "in", the load's + to "adj", and the load's "-" to the car's ground. (Take care with the TO-220 package as the tab with the hole is connected internally to the middle pin, so contact with anything would be bad.)
The regultator and resistor will together drop 3 volts minimum, so the load can be up to about 9 volts of LEDs. Figure Red, Orange, Yellow, Yellowgreen at about 2 volts and Green, Blue, Bluegreen, Aqua or White at about 3.3 volts.
LM317s cost about 50 cents to a buck at places like All Electronics or Electronics Goldmine.
Note that the regulator is good up to about 30 volts drop, but dissipation can be a problem. For ordinary LEDs, I use the small TO-92 LM317L version that is good for up to 100mA and a max .625 watt dissipation. At 20mA dissipation is not a problem. At 40mA it would only be good up to 15 volts continuous. For higher currents, up to 1 Amp use the TO-220 cased LM317T. I use an LM317T to drive a pair of Luxeons and only provide a small heatsink, for one luxeon you would need a larger heat sink or another resistor (about 10 to 15 ohms 2 or more watts) in series to drop some of the voltage and keep the regulator cool.
LM317 spec sheets provide more information, try the National Semiconductor website.
With the LM317L held with the pins down and the flat side facing you, the pins are "adj", "out", and "in". With the LM317T held with the mounting hole up and the face with the printing toward you, the pins are also "adj", "out", and "in".
If you don't want a regulator and just want a limiting resistor, figure the resistance needed is:
ohms = ((car voltage)-(sum of voltages of all LEDs in series))/(current desired)
Size of resistor = ((car voltage)-(sum of voltages of all LEDs in series))*(current desired)
Where car voltage is 13.6 volts, a White LED is dropping about 3.3 volts, and you want to drive the LED at 20mA, the result is (13.6-3.3)/.02= 515 ohms and the size of the resistor is (13.6-3.3)*.02= .206 watt or more.
If the car voltage without the alternator running is 12 volts, The LED will be driven at a lower current (12-3.3)/515 = 16.9 mA
If the car voltage with the alternator is actually 14.4 volts, the LED will be driven at (14.4-3.3)/515 = 21.5 mA
If you have a regultor, the current is always 20mA.
You never want to run an LED or a series connected string of LEDs without a regulator or SOME resistance in series. LEDs have a knee in their curve and will draw a lot more current when the voltage goes just a little above the knee. The knee also moves with temperature, so it may be fine on a Spring morning and self destruct on a hot Summer afternoon. Without regulation, I'd drop at least 4 volts with a resistor.
Remember the actual value of most resistors is +/- 5% of the marked value and you usually won't be able to find a resistor of the exact ohms you want, so use the next value up. The closest you'll find to 515 would be 560.
A meter is a lot of help because LED drop voltages can can very a lot (1.8 to 4 volts),
and the LM317 output voltage is actually 1.25+/-.05 volts. With a meter you can use a small trimpot to set the value exactly, or select from a pile of 5% resistors the one that is closest to the value you need.
Most LEDs are nomially 20mA, but some may be made to take 50mA. If you are lucky, the folks who sell it to you will tell you, but mostly you won't be able to find out. To be safe, stay at 20mA or less. If you need the brightness, you can "overdrive" the LED to maybe 40 mA, but it will reduce the life of the LED if it wasn't built for 40 mA, and it will likely fail in a hot car. In any event, most LEDs should not be asked to dissipate more than .12 watt, and the hotter the environment, the less you want the LED to dissapate.
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