# Want to measure 0-24V value with 0-5V A/d

Discussion in 'General Electronics Chat' started by fastwalker, Jun 24, 2009.

1. ### fastwalker Thread Starter Active Member

Jun 24, 2009
38
0
Hi There,

I want to measure the voltage on a 24V battery using a microcontroller 0-5V analog input. What would be the best way of scaling the voltage down for measurement?

Thanks.

Apr 5, 2008
15,809
2,390
Hello,

You could make a voltage divider with two resistors.
You need a division of 24 / 5 = 4.8.
The nearest two resistor will be 3k9 / 1 K , this will give a factor of 4.9.
This way you will be able to measure upto 24.5 Volts.
The next possibility is 4K7 / 1K , this will give a factor of 5.7.
This way you can measure upto 28.5 ( this will give some headroom for a very full battery).

The used factor must be used for calculating teh "real" voltage.

Greetings,
Bertus

3. ### fastwalker Thread Starter Active Member

Jun 24, 2009
38
0
Thanks for the response, I was thinking of using a voltage divider, wanted to see if anyone had any other ideas.

Do you think going straight from the voltage divider to the uC input would be OK?

Apr 5, 2008
15,809
2,390
Hello,

How accurate a voltage divider is is also dependend on the tolerance of the resistors and the input impedance of the controller.
If you want to be less dependend on the input impedance you can use a rail-to-rail opamp as a voltage follower between divider and controller input.

Greetings,
Bertus

5. ### fastwalker Thread Starter Active Member

Jun 24, 2009
38
0
Thanks Bertus, what op-amp would you recommend if I wanted to use one, LM324 be OK or need something more fancy?

Fastwalker

Apr 5, 2008
15,809
2,390
Hello,

I think you can use the LM324.
You will probably not measure near 0 Volts.

Greetings,
Bertus

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Which microcontroller are you using?

8. ### fastwalker Thread Starter Active Member

Jun 24, 2009
38
0
Haven't chosen the exact hardware yet, was thinking of using one of the Rabbit single board PCs.