Voltage Divider - Resistor Values

M

MurrayJ

Senior Member
I have trying out a voltage divider setup on a breadboard, and have it working correctly, but I am unsure of the values for the resistors needed to get a good spread of values for the analogue reading.

At the moment the plan is for two lots 6 identical resistor values going to two analogue pins to make up 12 different readings or 12 on one pin, but my experiment with 1.5K resistors had values like - 254, 15, 8. As you can see not what I want, can someone give me an idea on what size resistors to try, and is the pull down resistor value critical?
 
D

demonicpicaxeguy

Senior Member
google somthing called "electronics assistant" and get a copy of that it's got a good little collection of calculators in it including a voltage divider calculator
 
D

Dippy

Moderator
So, you mean:

+V --- Res1---Res2---Res3---Res4---Res5---Res6--- Gnd/0V ?

And you have two strings of resistors?

And you 'tap off' at any of the 'inter-resistor' points to an ADC input on each string?

If the above is correct, then what do you need a 'pull-down' resistor for?

Yes, you can crib off the Internet but you won't learn much.

If the above is correct, this is where something like Excel is handy.
6 cells to input your resistor values.
1 cell for input voltage.

Calculate current going through the resistor string (Ohm's Law).
Then you can easily work out the voltage drop across each resistor (Ohm's Law).

So, then 6 results cells showing voltage at each intermediate point.

20 minutes work, but I regret I can't do it for you.


If my description isn't correct then I have misunderstood the scenarion and you'd better post a drawing.
A drawing can save dozens of words and remove ambiguity.
 
B

BeanieBots

Moderator
It's not clear what you are actually trying to do.
If it's for 12 push-buttons on one input, then have a look in the finished projects section.

"Is the pull-down resistor value critical?".
Yes it is. EVERY resistor in a divider network is critical.

However, what pull-down resistor are you talking about?
The term "pull-down" is more usually associated with digital inputs.
For an analogue input it more useful to refer to it as the upper or lower section of the divider. (which then actually answers your own question).
 
M

MurrayJ

Senior Member
Electronic assistant is only for two resistors, so not for my application. Sorry about the pull down resistor mistake, I meant had the analogue floating, not connected to anything for a while because I was just connecting the adc pin using a loose wire, and so when not connected giving wierd values.

Yes dippy, thats what I had in mind, looks like I will have to study up on Ohms law (should already know it I suppose).
 
M

MurrayJ

Senior Member
Well I am slow....give me a break Dippy!

I found a post here that was helpful, using all 10K and a 1M to stop the floating, just tried it on the breadboard and it works a treat.
 
D

Dippy

Moderator
Just teasing :)
T'ain't rocket science is it.

You've found something? Thats great. So no need to do any calcs then, even better.
 
M

manie

Senior Member
MurrayJ: If you're sensitive to Dippy's remarks you wont learn a lot quickly. Dippy has a ahhhhhhhh-ummmmmm "unique" way of saying things !;) Learn from it and it will help you a lot later... You do need some basic knowledge and you seem to have it already. Heck, if I could divise my own resistor network to read a 12x matrix keyboard on one ADC pin, then anyone can. Let us know how you're going...:)
 
You must log in or register to reply here.
Top