Please check my LED resistor math?

Model Railroader Electronics and DCC
1

I am installing about 50 occupancy zones using seven RR-Cirkits BOD-8 and seven FOB-A boards. The FOB-A boards each have 8 spots where resistors can be soldered for outputs that drive display panel LED lights. I have measured the voltage supplied to these outputs and it is 3.1 volts.

Based upon this resistor calculator using input voltage=3.1, LED voltage drop (red LEDs)=1.7v, and desired LED current=20 milli amp it says to use 82 ohm 1/8 watt resistors.

I have tested with an 82 ohm resistor and it works, but before I solder 50+ resistors onto FOB-A boards I was hoping to get confirmation of the calculator results?

Thanks.

2

3.1-1.7 = .020R 70 = R 3.1-1.7=1.4 volts across resistor 1.41.4/70= .028 Watts So 82 Ohm 1/8th watt is more then safe

3

Those results look good. Note that you can have up to a ±10% resistor tolerance and they will still be over the 70 Ohm that was calculated.

If you are interested in the equations used, they are:

Vs - Vf = IR  and P=IV  
where Vs  is the supply voltage and Vf is the diode forward voltage.

Remember that the LED's long leg is the (+) pin!
4

Thanks for the replies. It makes me feel better before I start soldering all those resistors to the FOB-A boards. :slight_smile: