I'm wanting to illuminate two headlight LED's for an N-scal project. One for forward, and one for reverse. I'm going to be using a Digitrax decoder. For this decoder design there is a common negative and separate positive wires. Here is the question.... Can I attach one resistor to the negative wire, then both of the LED's negitive leads to the resistor...OR....should I attach a separate resistor to each LED on the positive side? Your assistance is appreciated in advance. Resp, John "delljohnb" Baugher
resistor I would use a resistor for each, but I don't use the blue (hot) wire from the decoder. For the front LED I use the right rail(RED) and install a resistor to the LED then hook up the white wire to the other side of the Led. On the reverse LED I use the left rail (BLACK) and install a resistor to the LED then hook up the yellow to the other side. Not sure what scale you are working with so this might be what you are looking for. I model in N scale and use the left and right sides of the frames for my hot leads. Saves one less wire (Blue) going to the LEDs from the decoder. This may or my not work for you. I use a 750 ohm resistor on my installs.
You didn't mention which decoder you are using but the standard is a common + (blue wire) and the other wires, in this case white and yellow are switched to common. Common safe practice is to put a resistor in each of the switched leads, white and yellow. You can use a small 1/4 watt or maybe even 1/8 watt for the LED leads to save space. Depending on the current of the LED you can start with 1K and se if that gives you the brightness you want. PeteC
There is a led and resistor calculator on the internet that I use all the time. It allows you to enter the supply voltage, led voltage and milli-amps, and will advise the proper size resistor and wattage. For signals I use one resistor on the common as only one color is lit at a time anyway. For multiple headlights I like to use one resistor per led. You can wire them in series and run a lessor resistor(led + to the next led -, then to power). I like doing them individually in parallel as a non DCC guy, the lights will light up earlier, usually before the loco starts moving. Watch your leds there are .75V on up. I usually buy some extra's and do a voltage test with a power pack and meter to assure the right voltage. Green led's will light up green, then get more yellow with higher voltage, then go pop.
Actually as the lights are switched , only one lit at a time, one resistor is sufficient, 1kohm, (connected to the blue wire).
Hi Guys The blue wire for the DZ143 decoder is the positive common. Generally use an 820 Ohm 1/6th watt resistor for a T1 size LED. this value resistor will work well for most LEDS from T3 through SMD. The Anode gets connected to the blue wire.
It is generally a bad idea to apply voltage directly to an LED without some form of current limiting (i.e., a resistor), as LEDs are not voltage operated like incandescent bulbs, but run on current. Their forward voltage drop is a result of the current flowing through them. ALWAYS use a series resistor, with at least 470 Ohms when running from 12 volts (1 K recommended) and let the LED determine its own voltage drop. The key parameter for an LED is current. Most LEDs in our applications are rated to 20 ma max current, with 5 - 10 ma being typical operational values. If an LED had zero voltage drop, with a 1K series resistor and 12 V applied it would draw I = E/R = 12/1000 = 12 ma, well within ratings. The fact that real LEDs will have an actual forward voltage drop of from 1-3.5 volts (depending on color and other techie factors) will mean that the LED + 1K combination with 12V applied will actually be drawing from 11 to 8.5 ma respectively, which is virtually the same in practical terms. Also, at 10 ma, power dissipation for a 1 K resistor would be P =I ^2 * R = (.01)(.01)(1000)= .1 Watt, so a 1/8 Watt resistor would do (although I would probably go with a 1/4 Watt resistor for extra margin if space were not an isssue).
