What potentiometer should I use?

Question

I want to connect 10 to 15 leds of 3 volts, 20 mA. I am trying to reproduce this diagram http://www.reuk.co.uk/Using-The-LM317T-To-Regulate-Voltage.htm but I have no idea what ratings should the potentiometer have to replace R2 (resistor 2).

Answer 1

Using a constant current source with parallel LED's is risky. You could add the currents together and use the formula in the link. To use this, all the LED's must have the same voltage rating. Still, if one fails, the regulator will increase voltage on the rest. This can cause a chain of failures.

I think that making a regulated voltage supply is a better idea.

If you post your whole problem at once, rather than disconnected bits and pieces, you are likely to get a better answer.

What kind of supply are you using? What is your application - is energy consumption important (battery supplied flashlight), or unimportant (lab setup for occaisional use)? Are the LED's all identical?

Answer 2

Very simple! Get any 1/2 watt potienital meter  of 1k ohms. Choose linear type to let the adjustment more smoothly.

Calculation is not 100% accuracy,why not test it directly.

The regulator circuit is for voltage control.

Connect DC amp meter in series with the output and a  load rate at 30 ohms ,10 to 20 watts resistor. That shall let you test the output to supply 300ma at 3 volts.

You need a volt meter connects parellel to the load to see the output voltage. Now adjust the potiential meter until you see 3 volts shows on volt meter. Here is the correct output setting at 3 volts 300ma. Later on,as you like,you may replace the potential meter with 2 resistors by measure the resistance on potential meter setting.