Block Signals-Lights Too Brightgreenspun.com : LUSENET : Wiring for DCC : One Thread
I have a few Model Power block signals with red and green 12 volt incandescent bulbs wired directly to the AC side of a separate power pack. They are way too bright, so I want to replace them with smaller Miniatronics 1.5 volt incandescent bulbs. To accomplish this without burning out the bulbs, I realize that I must wire resisitors between the bulbs and the power pack. My question is this: how do you decide the number of ohms to use when purchasing resistors and how do you decide when to use 1/4 watt or 1/2 watt resistors. Is there a general rule of thumb?
-- Rich Jurgovan (email@example.com), February 14, 2005
Resistor value = (voltage source - light bulb) divided by bulb current
Example: (14 - 1.5)/0.02 = 625 ohms Use a 620 ohm resistor
Resistor wattage = square(bulb current) * resistance
Example: square(0.02) * 620 = 0.248W Use a 1/4 or 1/2 watt resistor
Any tolerance is fine.
-- Allan Gartner (firstname.lastname@example.org), February 16, 2005.