I have a pressure sensor working at 24V dc whose output is analog 0-10V for 0-10bar pressure. I am using this sensor with Arduino Uno. I know that the pressure value will never go beyond 4bar. So do I have to use a voltage divider in this case or not using it will still do the job?
Electronic – arduino – using pressure sensor with 0-10v output for 0-10bar using arduino but pressure value won’t cross 4bar. so is a voltage divider required in this case
arduinosensorvoltage divider
Related Solutions
In this answer I showed that the optimal series resistor value is
\$ R_S = \sqrt{R_{MIN} \cdot R_{MAX}} \$
where \$R_{MIN}\$ and \$R_{MAX}\$ are the minimum and maximum resistances of your sensor. The Sparkfun page speaks of between infinity and 300 kΩ. If we assume 10 MΩ as maximum value then your series resistor should be 1.8 MΩ (rounded to nearest E12 value).
These are pretty high values, too high for an ADC, which, like Mathieu says, likes a impedance of less than 10 kΩ. So you'll need to buffer the divider with an unity gain buffer opamp:
Note that a common opamp may have an input bias current as high as 1 µA, and this could distort your reading. A CMOS opamp like the MCP600x has a much lower input bias current, 1 pA typical for the MCP600x, which won't deteriorate your reading.
The cynic in me says that everyone uses a 10kΩ resistor because under the technical data section of the FSR above, a chart is given with output voltages specified with 5V supply and a 10k pull-down resistor. I have copied the chart below for reference: \$\begin{array}{} Force (lb)& Force (N)& \begin{matrix}FSR\\ Resistance\end{matrix} &(FSR + R) Ω&\begin{matrix}\text{Current thru}\\ FSR+R\end{matrix} &\begin{matrix}Voltage\\ \text{across }R\end{matrix} \\ \hline None& None&\infty&\infty&0 mA&0V\\ 0.04 lb &0.2 N &30kΩ&40 kΩ&0.13 mA&1.3 V\\ 0.22 lb &1 N &6 kΩ&16 kΩ&0.31 mA&3.1 V\\ 2.2 lb &10 N &1 kΩ&11 kΩ&0.45 mA&4.5 V\\ 22 lb &100 N &250 Ω&10.25 kΩ&0.49 mA&4.9 V \end{array}\$
So we can see that a 10kΩ gives us a reasonable amount of range over the operation of the sensor. If we wanted to have better resolution with the smaller force ranges, we could increase the pull-down resistance. We could also decrease the pull-down resistance to improve the resolution at higher force ranges, although that would also come at a cost of greater current draw.
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Best Answer
If we assume your Arduino Uno is powered from 5v, then what you appear to be asking is 'if my sensor voltage always stays within the range 0 to 4v, will it be OK to put it into and ADC that can accept 0 to 5v input?'
Under normal conditions, the answer is obviously yes.
The experienced designer designs things so that they work under 'normal' conditions, and don't break under possible but 'abnormal' conditions. Will your sensor output ever go above 5v? What about a brief blip when it's first switched on? What about water hammer in the pipes you're measuring?
It would be prudent to check how much current the Uno input can accept without damage when the voltage exceeds the rail, and use at least a series resistor between sensor and Uno to limit the current to less than this under fault conditions.