Continue to Site

# Measuring Resitance using ADC of Micro-Controller??

Status
Not open for further replies.

#### xpress_embedo

I am using a sensor from novalynx corporation which changes it resistance when temperature changes.
The sensor has two terminals and both terminals are having two two wires.

And in datasheet it is written connected to this two bridge.

Do anyone having bridge circuit to measure resistance, i think we will convert resistance to voltage and measure adc value using micro-controller's adc

If it is too low, the method you suggested may not work.
In that case use an op-amp.

ismu

### ismu

Points: 2
My temperature sensor gives 100 ohm at 0 deg c and if temperature increases resistance increase and if temp decrease it decrease
Temperature Coefficient is 0.00385

I think maximum resistance range is 200 and minimum will be 50

I suppose.
How to do so.
Pls help

Hello!

The bridge method should work. However, due to the accuracy of both fixed and
variable resistors, you should have some means to tune one of the other
resistors. Is it for a few samples or even one system, or is it for mass
production? In this latter case, you should have a way to auto-tune your
system. In this case, you may use digital potentiometers controlled by
your MCU, but the accuracy will be bound to the digital potentiometer accuracy.

Dora.

If you use a 100 ohm resistance and the sensor as divider network, for 1 degree change(consider 0 to 1) in temperature the sensor resistance will change from 100 to 100.385. Normally this is very little as I already mentioned. So what you can do is, connect the divider network to an op-amp in differential amplifier configuration. Thus the voltage change which is very low is amplified. Now your adc will work properly. Design the gain of the op-amp in such a way that for the maximum temperature, the output is not saturated for the adc.

If you are talking about the U3 OP AMP that is a constant current generator. A small amount of current is passed through the PT100. If more current is passed through PT100 it will heat up.

Last edited:
xpress_embedo

### xpress_embedo

Points: 2
If you are talking about the U3 OP AMP that is a constant current generator. A small amount of current is passed through the PT100. If more current is passed through PT100 it will heat up.

Hello!! Sir
I am using the circuit of this post

And its working fine in proteus, when i made the hardware it is showing the greater variations.

Can you tell me how to calibrate, means how to identify the voltage output in terms of temperature.
I am confused this sensor is harder to use.
:-(

Post your proteus file and hex file. If possible post the code.

I am using the circuit of this post
RTD PT100 solved almost

And its working fine in proteus, when i made the hardware it is showing the greater variations.
That's a pure simulation circuit e.g. depending on ideal voltage sources. I can imagine many ways to make it fail in a hardware design.

Use this files. It will work fine. Use the code in the attached file.

#### Attachments

• pic18f4523_2lm35_2pt100_1v.rar
95.9 KB · Views: 47
xpress_embedo

### xpress_embedo

Points: 2
Thanks for sharing sir.

I build the circuit on hardware and it is working as per expectations.

I want to know why we have used the instrumentation amplifier not a normal differential amplifier.
Why to use instrumentation amplifier??

Can anyone tell me.. Why we have used Instrumentation Amplifier rather than using a normal differential amplifier.

Pls tell me

Status
Not open for further replies.