wiring of LED's

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I am trying to wire three leg bipolar LED's into a Bi Polar DC power supply for my Tortoise switch motors. I'm using a red wire for + a green wire for - and the white wire as center pole of my SPDT panel switch a black wire is my common bus directly to the motor.

I tried wiring one in parallel with the SPDT and only get one color and off. I suspect it's because I was connecting one leg of the LED to the SPDT red post, one to the green and the long leg of the LED to the white post in series with the 470r dropping resistor, thus getting 30 someodd volts for only one side and probably burning out the green side of the LED, yet the red side of the LED still works. Problem is how do I wire it to work? Do I have to bring the black (common) all the way to the LED to make it work. Tortose shows a 2 leg LED in their wiring diagrams. I can buy them from Radio Shack, but they are much more expensive than the one's I've got from Miniatronics. Another question, should I put a dropping resistor (series/parallel??) somewhere?? (value??)on the bipolar power supply to bring the voltage down to 10/12 volts, the power supplies are two Radio Shack 1.75 AMP and 12 Volt transformers. However they actually put out about 17 volts. The motor run very fast and are noisy. Eventually I will have 30 motors connected will that bring down the voltage by itself? Thanks for your help, Chris Dante

-- Chris Dante (m1919@cshore.com), October 25, 1999


1. Yes you probably did burn out the green LED, Light emitting diodes don't like having reverse voltage greater than 3-5 volts or so. Especially The green and blue ones. None like average current of more than 20-30 milliamperes. You cannot put an LED in a circuit with AC unless there is a reverse voltage protective current path.

2. Most 3-wire two color LED assemblies are connected common cathode with the center lead being the cathode. (The cathode is the point of the diode symbol arrow, or the flat line mark on a regular diode package, or the + sign marking. Current flows through the device in the direction of the arrow from + to -.) You can add two external diodes to make it simulate the 2-wire bi-color type. Use practically any small diodes (1N4148, 1N914, 1N4002, etc.) Connect the anodes (opposite end of the cathode) of the two diodes together. Connect that center point of the two diodes to the center leg of the 3-wire R/G LED. Now connect the cathode end of one of the diodes to the Red LED terminal, and the other diode lead to the Green LED terminal. Now use the two outside Red & Green connections as if it were a 2- wire LED. Use it in any normal 2-wire bi-color circuit, such as that suggested in the Tortiose instructions.

3. The Tortiose switch machine has internal auxiliary SPDT switches that can be used to control indicator lights. (pins 2,3 & 4 or 5,6 & 7) Draw this one out on paper. Connect pin 4 on the Tortoise to (+) of a DC power supply. Connect pin 3 to the Red led. Connect pin 2 to the Green LED. Connect the center LED leg (cathode) to a 680 ohm resistor, connect the other end of the resistor to the (-) side of the power supply. Reverse Red & Green LED connections if the color / direction is reversed.

-- Don Vollrath (dvollrath@magnetek.com), October 27, 1999.

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