Adjustable Voltage Regulator

Originally, I wanted to make an adjustable voltage regulator with an LM317, 1k ohm potentiometer (variable resistor... same thing), and a 10k ohm potentiometer. Unfortunately, we didn't have any 1k ohm pots at work, but I had found some old 100 ohm pots - exactly nine of them. I thought it was great, since I can put them all in series with a 100 ohm resistor and be able to adjust the resistance between the adjustment leg and voltage out of the LM317. The 10k ohm pot I found was enough for the resistance between the adjustment leg LM317 and ground, since I don't really work with anything over 12 volts. I also used some old prototype boards to make it easier to assemble, but even with that it took some time to figure out and make certain that the paths were correct.

For the main board, I used some salvaged 0 ohm jumpers to make a couple jump to be able to use a couple of the through holes, but was quite pleased that I was able to fit everything that I needed to fit on it. And for the secondary board, it was nice taht I only had to use half of the salvaged board, as the hole placement allowed two pots to be fitted per section.

The finished "super" adjustable regulator.
Unfortunately, the actual size of the image is blurry and unfixable at this point.

It worked quite well and gave me good results, but then I found that all the 100 ohm pots weren't really doing much in terms of adjustment, so then I decided to remove the nine 100 ohm pots and make the necessary changes. Since eight of the pots were on the secondary board, I just de-soldered the jumper wires to/from the main board to save time.

The second version adjustable regulator.
You'll be able to read all the notes I've written on the board to make it easier for myself.

After testing it, it worked okay, but was a bit wonky when I turned the 10k pot down too far (since the 100 ohm resistor actually measured below 100 ohms). At this point, I decided to try one of the extra 470 ohm resistors I had in place of it, and it worked much better. Since the only thing that changed was the resistor, I didn't take a picture of it.

I slightly figured out that a larger resistor between the adjustment leg and output voltage legs made it easier to fine tune the voltage than with a smaller resistor, and once I was able to get hold of a 1k ohm resistor, I swapped the 470 ohm for it.

The finished adjustable regulator.

Once I tested it, I found that I was indeed correct, as it was even easier to fine tune the voltage. After using an LM317 voltage calculator I had found sometime before this project (so that I could understand how to use an LM317), I found that 1k and 10k were all I really needed, since it would (theoretically) give me 13.75 voltage output maximum (assuming I have an input of roughly 15 volts or greater). With a 12 volt source, I get an output range between 1.255 and 11.33 volts (according to the multimeter I have anyway).

About a day or two after putting the 1k ohm resistor in, I took a look at the equation for calculating the output voltage from the resistors used, and the resistor(s) between the adjustment and output voltage legs of the LM317 (aka R1) is the divisor, so it would make sense that using a larger resistance for "R1" would give a lower variance for the output voltage when the pot (R2) is changed.

Along with gaining a voltage regulator to make it much easier to test designs and whatnot (I was limited to using a AC/DC transformer with an output voltage close to whatever I needed it at), I learned that I should just use a 1k ohm resistor for "R1" if I'm going to be using a pot for "R2", because an adjustable R1 doesn't do much good.

I do want to also say thank you to the folks at the reuk site for making such a nice page for the LM317/LM338, as it was quite handy on several occasions along with being simple.

I don't remember when off the top of my head, but I plan on buying some LM350 to allow me to properly utilise my breadboard once I have an adjustable power supply. I'll probably be making a fixed voltage output for 9, 5, and 3.3 volts (the adjustable power supply will be outputting 12 volts), but I'll probably the last "channel" as an adjustable voltage (probably using the above project) for anything arbitrary.