01-17-2010, 11:06 AM
Lester,
I have found that not all 1.5v bulbs are created equal... some need a larger resistor than others. If you've got an electronics store in the neighborhood, go get a few small packs of 1/4W resistors (1/2 W is overkill, and they're bigger) starting with 1K, and working down through 820, 640 to about 330 or so. Here I can get a pack of 10 for $1, so for less than $10 you'll get an assortment that will do you for 1.5V bulbs and LEDs.
When I'm starting off with a new bulb, I put the 1K in line with it and apply full voltage from a DC power pack (very briefly - just touch the lead to the contact on the power supply). If the bulb lights very brightly (white) then you need a larger resistor. If the colour is a warm white, then you've probably got a good resistor value. If the bulb barely lights, or is an orangey-yellow, the resistor is too large, so swap in one the next size down. Repeat this until you get to a light level that's both bright enough for your tastes but not so bright it's going to burn out in 5 minutes.
The alternative (one I prefer, if I can get away with it) is to use a 16V bulb. That way there's no resistor required, and you don't need to worry about lifespan, either. Unless I'm trying to fit the bulb into a diesel headlight casting (1.2-1.5 mm), these will fit in most steam loco headlights, and work very well inside the shell where you have a plastic lens extending out into the headlight casting.
I have found that not all 1.5v bulbs are created equal... some need a larger resistor than others. If you've got an electronics store in the neighborhood, go get a few small packs of 1/4W resistors (1/2 W is overkill, and they're bigger) starting with 1K, and working down through 820, 640 to about 330 or so. Here I can get a pack of 10 for $1, so for less than $10 you'll get an assortment that will do you for 1.5V bulbs and LEDs.
When I'm starting off with a new bulb, I put the 1K in line with it and apply full voltage from a DC power pack (very briefly - just touch the lead to the contact on the power supply). If the bulb lights very brightly (white) then you need a larger resistor. If the colour is a warm white, then you've probably got a good resistor value. If the bulb barely lights, or is an orangey-yellow, the resistor is too large, so swap in one the next size down. Repeat this until you get to a light level that's both bright enough for your tastes but not so bright it's going to burn out in 5 minutes.
The alternative (one I prefer, if I can get away with it) is to use a 16V bulb. That way there's no resistor required, and you don't need to worry about lifespan, either. Unless I'm trying to fit the bulb into a diesel headlight casting (1.2-1.5 mm), these will fit in most steam loco headlights, and work very well inside the shell where you have a plastic lens extending out into the headlight casting.