Model Trains are my hobby. I am 68 years old and retired. I am setting up a lighting system on my train layout. I want to use the LM317 to provide an adjustable brightness function for each building on the layout. I am a novice electronics person. I created the circuit shown in http://electronics-diy.com/1.5v--30v-1.5a-lm317-variable-power-supply.php. This circuit only allowed me to adjust the brightness of the one (1) White LED in the circuit. When I connected three (3) LEDs in series with 100 ohm resister the LEDs would not come on at any level of the potentiometer. What changes do I need to make to this circuit, so it will handle three LEDs. Also, how many LEDs can I expect to operate from one (1) of these circuits and how should the LEDs be arranged in the circuit, series, parallel or something else.

http://electronics-diy.com/1.5v--30v-1.5a-lm317-variable-power-supply.php

Thanks
Charles Malone

 

You need to learn about LEDs first. They have a "breakover" voltage below which nothing happens. It is basically wasted voltage until you get to the operating voltage. Then, they "breakover" and burn up if you don't limit the current externally. Your first try with 100 ohms and one LED was a good experience, then you added LEDs until you exceeded the voltage of the supply or you got too high of a current and smoked an LED or two, or you made a simple wiring error.

Can you tell me the voltage and current ratings of the LEDs you are using?

 

You will want to use pwm to vary the brightness. Basically turning the led off and on very fast. You can use a 555 timer or a mcu . A mcu would give you more options. But I would not be surprised if lighting controllers can be purchased from hobby stores that do just this.

 

Damn, so much feedback noise to just get a few LEDs to light?

Do you have a voltmeter to measure what voltage your regulator is putting out? It's probably too low for 3 LEDs. Each LED needs around 3 volts when lit, plus maybe a volt across the 100 ohm, so you need to get to 10V to turn stuff on.

However, if you put each LED in series with it's very own 100 ohm resistor, then this may well run all three when each resistor/LED set is put in parallel to the others on the regulator output.

The LM317 is a pretty durable device and very hard to kill... one of the few ways to kill it is eliminated by the D1 of your circuit. So don't be afraid to try things.

What is the part number of the LEDs you use?
What is the source of the power input?

 

LEDs are off the shelf 3mm, red color from Radio Shack and the power is from a 26 VDC power supply. That hooking up a 100 ohm resistor on each LED and then hooking them up in a parallel worked great. Problem solved. Thanks ErnieM.