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Some of you may recall that I was working on repairing a smoked taillight. I bought one off e-bay before I read all the horror stories about failures. I don't know who made it. It could be a D2Moto or not.

After reading about the failures I decided to test mine before installing it. I set it on the bench and ran it on 15 volts (a realistic voltage that the system can supply) and after about 15 minutes one side of the red LEDs stopped working. I determined that it wasn't the LEDs that had failed but a switching transistor.

Also, all of the resistors were smoking hot after only a few seconds of operation. This told me that they were likely undersized.

After doing some voltage drop readings across the resistors and LEDs I determined that everything was pretty much underdesigned. The individual LED resistors were dropping .8 watts on a 1/2 watt resistor. The switching transistors were only rated to 600 milliamps when the load on them was several times that.

I could have waited for the next generation light but I decided I could fix this one.

The first thing was to resolve the switching transistor issue. I replace the 600 milliamp unit with one that can do 5 amps. Yeah, it's major overkill but they only cost $.68 ea. The pic below shows the tiny original transistor alongside the new ones. I had to do some creative lead bending on the new one as the pins are not the same as the original (basically, you have to swap the middle and one end pin to get the base, emitter and collector in the right holes in the PC board).

The next issue was to resolve the high current through the individual LED dropping resistors.

I decided to drop most of the voltage/current through some larger capacity resistors on the wires that feed each bank of LEDs. There are 5 banks of LEDs. Left red, right red, center red, left amber and right amber. I installed a 100 ohm 2 watt resistor in each of the supply lines to the LEDs. You can only see 4 of them in the pic below as one if hidden behind them.

The next issue was to replace all of the individual LEDs dropping resistors with smaller value ones. I replaced all of the 177 ohm with 33 ohm. Since most of the voltage/current was being dropped by the new 100 ohm resistors, I only needed 33 ohms. This dropped the current that those resistors had to dissipate from .8 watts to about .1 watts. These are all 1/2 watt resistors so there is virtually no heating. There are 27 of them.

The last issue was to restore the taillight mode brightness. Since installing the 100 ohm resistors dropped the voltage to the red LED banks, I installed a piggyback 100 ohm resistor to the 75 ohm resistor that is responsible for the brightness change from taillight to brakelight mode. This restored voltage back to the previous level.

I tested the light at 15 volts for about 12 hours and there is virtually no resistor heating as before and the transistors run cool to the touch.

I put $8.52 in parts in the light and a few hours of time. Was it worth it? Probably not. The next generation smoked light is coming out fairly soon and I hear the price isn't going to be too bad so I may still by one. But, I have the satisfaction that I could fix it. And that's worth something.

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