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LM317 circuit

The LM317 is a very useful device for producing specific voltages for your layout.
 
You can buy it as a 3 pin device and by adding a few capacitors and two resistors you can set the output voltage to between 1.25 volts below the incoming supply voltage and 1.25 volts higher than ground.

The ratio of the resistors determine the output voltage based on the following formula.

 V out = 1.25V x { 1 + R2 / R1 }

The Output voltage is also affected to a small extent by the current flow in the adjust line.
Resistors are normally 5% tolerance so producing an exact fixed voltage can be problematic. It's for this  reason that one of the resistors is normally made a variable one to enable fine tuning.

You can make these yourself on a piece of strip-board,  but given the costs involved it is much cheaper to obtain them as a  built module.



I bought some warm white LED's which theoretically operate in the 3.0 Volt to 3.2 Volt range. Using the LM317 module I could set the voltage to 2.9 Volts to achieve a very pleasing almost gaslight effect. 

A word of CAUTION.  I used my Bench Supply with current limiting to establish the desired voltage on the LED. I then adjusted the LM317 module to provide the required output voltage before connecting things up on the layout.

Andy R







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