Arduino as a Voltmeter / Amp Meter

Hi
I'm designing a variable Power supply capable of delivering up to 30v. The Power brick am using to power it is capable of 40Watts of power according to the label. I'm using an LM338 configured as a variable regulator.

I understand to measure volts I would use a voltage divider. In fact I worked it out to be 100K for R1 and 20K for R2. Do I just connect this voltage divider to the output pin of the lm338? I was planning on using an arduino to
measure the voltage but doesn't the arduino have a maximum current of 20ma per io pin? If I'm not mistaken the lm338 serves upto 5A current

I wouldn't use fixed value resistors for the input divider. I would look at using maybe just a good ten turn 10K pot. The reasoning is the Atmega 328p claims the input resistance to be 100M Ohms but during A/D sampling that drops. The suggested source resistance should be kept below 10K ohms. This is the data sheet for the chip. The A/D section begins around page 252. You would be using an A/D input pin to measure voltage and not an I/O (as in digital input / output) pin.

Also make sure you set up the divider using a DMM before connecting it to the Arduino. Read the data sheet for whatever Arduino chip you have.

Ron

I want to make sure I'm doing this correctly before I put everything together.

I'm using fixed resistors just for testing purposes. I do have a digital pot.
The LM338 uses a voltage divider to set the output voltage. I've configured R1 as 240ohms and R2 as 3.3K which gives me 16v. I measured the voltage between adj and output pin which gave me 1.2v. Am guessing thats the voltage coming out of the voltage divider yes? So I just wire this to a ADC pin on my arduino yes?

This link is pretty good for Arduino. Which Arduino board are you using? Also, can you post a schematic of what you are doing prior to the Arduino?

Ron