 # Resistor Scaling

Hi,
So I am totally new to learning/using electronics & I have a question regarding resistors. I’m working on a project in which i am trying to take in 0-30v and scale it down to 0-5v so the Arduino can read it. I attached a diagram/picture of my work. I choose my resistors based upon the diagram/formula i calculated. I also double checked my work using: http://www.raltron.com/cust/tools/voltage_divider.asp

My question is about the specific selection of resistors. I have arbitrarily selected R1 = 5000 & R2 = 1000 based off the resistors i have in my kit. But I could also potentially go with R1 = 1000 & R2 = 200 or many other different combinations. What is the potential benefits or using higher or lower scaled pair of resistors if any?

Thanks!

Higher = higher total resistance = less current (and this heat)
lower = lower total resistance = more immune to noise.

Somewhere between 1k and 100k is fine for more cases.

The benefit of using the higher value resistors is that they place less load on your source voltage, but if the value is too high, the input resistance of the measuring device may affect the effective value of the resistor across which you are measuring. Conversely, low value resistors place a greater load on the source but the possibility of error due to the measuring device input resistance is reduced.

For an Arduino you want an input impedance of about 10K, so make that the bottom resistor and calculate what the top one should be.

Just to be clear, by bottom one you mean R2?
Thanks!

RPBOYER: Just to be clear, by bottom one you mean R2? Thanks!

Yes. Typical schematics are from left to right, with power up top and ground at the bottom. It just makes it easier to follow. So the "bottom" resistor would be the grounded resistor, and also corresponds to your picture of R2

That’s quite a neat trick drawing the schematic in the same colours of wires as used on the breadboard

It you want to use less power (for a battery circuit for instance), you can use much higher resistor values, but add a 10nF or 100nF cap across the bottom resistor to stiffen up the voltage going to the analog pin (which would otherwise start to become inaccurate if the impedance it sees goes above the 10k range)

Do not use electrolytic capacitors for this, they have too much leakage current.