I see several types of LED string lights now around the holidays. I saw some that operate on 3 AA batteries, which would be 4.5 volts. I cannot see any resistors in the string. Does anyone have any input on using these string lights in passenger cars. They are warm white in color and just the right brightness. I'm wondering if I could just cut maybe 3 or 4 from the string and use them in passenger cars, using a resistor for DCC track power?
When you get down to the individual elements themselves, namely the LEDs - they are all the same. White LEDs typicaly drop 3.1 to 2.4 volts. Doesn;t matter the application, it's the semiconductor physics needed to create white LEDs. Unless there is some new exotic material, but you won;t find such a thing in a $1.99 string of lights, for sure. If the whole string runs on 3 AA batteries, then the string must be wired in parallel. There has to be a resistor somewhere, if not on each LED then in the battery box. 4.5 volts is too much, plus if the LEDs ran near their maximum current of 20-25mA, a string of 20 of them would draw 400 to 500 mA, and those batteries wouldn;t last long. Cut the current inhalf with a resistor, they will still be plenty bright, but by drawing half the current, the batteries will last tiwce as long.
Just be careful cutting individual LEDs out of the string, they MAY have embedded a resistor per LED. AFter taking the string apart, you now will have a pile of LEDs just as if you bought a bag full.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Thanks.
How would you tell if each led has its own resistor?
Joe Staten Island West
Measure it with a meter, though not all meters put out enough voltage on the diode test to light an LED. Or look closely, with a magnifier if needed, a resistor or other component in the circuit will be fairly obvious. Or, at the low cost per LED, sacrifice one. If you feed the LED less than the forward voltage, it will light but not blow up even with no resistor. As you add voltage, if you exceed the forward voltage and there is no resistor, the LED will pop. If there is a resistor, the LED won't pop until the voltage is high enough to exceed the forward voltage plus pass too much current through whatever value resistor is used. Or carefully tear one down by cutting the LED out of whatever plastic it might be coated in and test resistance on the wires.
Or open the battery box and see how the wires go. WIht 3 batteries, of they are all wired in series, that's 4.5 volts and there MUST be a resistor somewhere. If there's one for the whole string, it most likely would be attached right to the switch inside the battery box. It's also possible that there are 3 wires in the string going to the LEDs - a common and then one wire which is two of the batteries in series, and another wire which is 2 other batteries in series.
Or they actually have a cosntant current source in the battery box.
No mattery how it's done - it's 99% sure there is NOT an individual resistor on each LED.
Another LED question. I have a kit that says I should add an enclosed resistor. Does it matter if I use the short stem or the long stem and does it matter which direction. Just at the beginning of LED knowledge.
iwander Does it matter if I use the short stem or the long stem and does it matter which direction.
No and no.
Henry
COB Potomac & Northern
Shenandoah Valley