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Monday, May 02, 2011

Niles Project ~ Resistification

Because LED's are diodes and don't behave like a resistor, a current limiting resistor is needed to prevent the current from exceeding the operating limits of the LED's.

Determining the size of resistor to use was simple enough. John McWhirter and I soldered the 4 LED's together in series. Then we attached a best guess resistor in series with them. Then we gave the circuit "the juice". 15 volts DC. We then carefully considered the results. If the lights were too bright for what I wanted, we would then replace the resistor with a" stronger" one. If the lights were too dim, then we replaced the resistor with a "lighter" one.

Actually, it was easier than I'm letting on because John has built a box with an array of resistors in it. A rotary switch switches from one resistor to another. So all we had to do really was connect our LED assembly to it and with a simple flip of the switch, find the the setting that gave me the illumination I was looking for.

Eventually we determined that a 1000 Ohm resistor (brown, black, red, gold bands) provided the right amount of juice to the 4 LED's to achieve reasonable illumination for the interior compartments of the trolley.

For the one headlight, we determined that a 15000 Ohm resistor works best (brown, green, orange, gold bands).

My next objective was to determine the most unobtrusive location to mount the resistors. I can't mount the resistors on the circuit board because they would interfere with the ability of the circuit boards to slip between the bulkheads and the ceiling. It would also be nice if the resistors weren't someplace visible through the windows. So this is where I got clever...

I need a jumper from the blue wire (lighting positive common) bus on the bottom of the circuit board to the LED leads on top of the circuit board. So why not have the resistor fill that bill? The leads to the resistors were bent so that one lead solders on top and the other lead solders on the bottom with the resistor itself on the front edge of the circuit board. Works for me. Yes sir! Works for me just fine!