monitoring a 24v Batt with wc32

[BTCAD]

Hey Guys!
 
I'm looking, to monitor 24v batts with wc32 boards. I already made it with wc8 wich is 0-5V analog input. Now with Wc32 there are 0-10 analog inputs.
 
I want to scale from  24.56v to 25.30v. Wich 25.30 is 100% full and 24.56v is the real 50% value. Under 50% the system will turn off the circuit to prevent batteries degradation.  
 
So the question is:
 
I want to convert 0-10v to 24.56-25.30v, how should I do?
Any idea?
 
If I'm wrong please feel free to correct me !

 

[CAI_Support]

To scale the 25.30V to full scale, that can be done by resistor network to divide. However, for make 24.56 the 50% at the same time, that is not easy. I think you can use resistor network to make 25.30 full scale, then read the 24.56 in PLC to use that in your PLC logic as 50%.

 

[RAL]

I wouldn't bother trying to scale things so that 25.3V is full scale and 24.56V is at 50%.  Just construct a resistor network consisting of 3 equal value resistors (say 1k, 1%) in series to divide the voltage by 3.  That would make 30V full scale and with 10 bit resolution, each step would represent about 0.03V.   That should make it easy enough to detect the two voltages of interest.
 
If you really need to scale the voltages to the range you proposed, you'll need to use a differential amplifier to do that.

 

[BTCAD]

ohhh, i'm wrong wc32 is 0-5v analog input 10 bit resolution.  I will try something and let you know

 

[BraveSirRobbin]

I did this Guide back in 2005, but it is still valid and you may want to give it a read.

http://cocoontech.com/forums/topic/2827-guide-analog-to-digital-converters/?hl=%2Banalog+%2Binput#entry26535

I'm wondering why you need this accuracy? Is your situation with battery cut-off that critical where you need to discern in the tenths of volts range?

RAL's suggestion would work well and should give you the resolution you need though (plus it's simple).

I would do 1K and 4K in series, then read the voltage across the 1K with your analog input since you only have 0-5 volt input scale and not 0-10.

 

[rossw]

If you want an expanded range (to get the most of your 10-bits), you could either use a zener diode (say, 21V) to give you a suppressed-zero, then your 0-5V range will be from 21 to 26V with 0.5mV resolution.
 
Since Zeners are not ideal and will change voltage with temperature, choose two or more (say for example an 18V and a 3.3V) in series to largely balance out the positive and negative coefficients.
 
Or, get more sophisticated and use an opamp as a level-shifter, and use a precision voltage reference.

 

[pittom]

I did this before in my camper it was 12 volts though I just put a resistor in line worked great