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 (redking56@aol.com), February 14, 2005

Answers

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 (wire4dcc_admin@comcast.net), February 16, 2005.