I'm a member of a formula student team and we are developing a vehicle control unit. One of the functions I'm working on is reading temperature in the water cooling system using a 10k NTC thermistor. I'm torn between using a resistor in series with a thermistor and measuring voltage across the thermistor, and using a thermistor in a bridge configuration.

We are aiming for accuracy and from what I can find the bridge configuration is more accurate, but I don't understand where that accuracy comes from? From the bit of math I did the accuracy comes out the same.

One benefit I found is that the heating of the whole circuit is offset when using a bridge configuration, but my resistors would be on a PCB while the thermistor is in the water cooling system, so it is only the thermistor that would be heating up meaning this would not apply.

The only benefit I can think of that would apply is when using same 3.3V we use to power ADCs to power the bridge, it would be easier to amplify the output signal to use the full ADC range.

Also, I know a capacitor is added to simple voltage dividers so the output is consistent across all frequencies, is that something I would need to implement for measuring something like temperature, and if so, how would I go about doing that to a bridge?

 

Hi S,
Welcome to AAC.
What is the thermistor type you plan to use.
I assume it is a glass-beaded type suitable for water immersion.?
E
Some additional points.
What is the Coolant temperature range, taking into consideration a possible a pressured coolant system and also may contain antifreeze for less than 0C deg operation.
Also, what temperature accuracy and resolution are you requiring when using a Thermistor with a non-linear Temp versus Resistance curve.

 

After reading your description, I feel that your task will be much easier if you use a ds18B20.

https://www.theengineeringprojects.com/2019/01/introduction-to-ds18b20.html

It has an underwater version. The sensor comes with a long cable. The sensor can be fully immersed underwater, while the cable can be connected to the controller at a distance. You can directly connect the sensor to Arduino or whatever controller you're using. No added circuitry is needed.

 

We are aiming for accuracy

While accuracy is admirable, how accurate does it really need to be for vehicle control, and how fast does it need to respond to temperature changes (I suspect not very fast)?

 

A resistor in series with the thermistor creates a voltage divider.
A bridge is two voltage dividers.
The difference is a bridge configuration is often used because the output is less dependent on common environmental and circuit variations, for example, the effect of temperature on the series resistor. The bridge circuit output is less affected by changes in supply voltage.

Voltage measurement from a voltage divider is usually the first consideration that comes to mind when trying to measure changes in thermistor resistance. There are other techniques for creating an analog to digital converter.

R x C = time constant

R and C combination in a frequency or period measurement technique creates a simple and effective analog to digital converter.

 

Hi S,
Welcome to AAC.
What is the thermistor type you plan to use.
I assume it is a glass-beaded type suitable for water immersion.?
E
Some additional points.
What is the Coolant temperature range, taking into consideration a possible a pressured coolant system and also may contain antifreeze for less than 0C deg operation.
Also, what temperature accuracy and resolution are you requiring when using a Thermistor with a non-linear Temp versus Resistance curve.

I don't know the specific thermistor we'll use, they have already used them on our previous vehicle for the same purpose so they should work fine. I haven't asked about specifics but I know they used 10k NTC thermistors.
Expected temperature range is not known yet, once our mechanical guys run analysis on the thermals we'll know, but for now I'm assuming range between 0C and 100C
Our goal for accuracy is not strictly set, but for now I'm aiming for ±2C. Any non-linearities will be taken care of in software.

 

After reading your description, I feel that your task will be much easier if you use a ds18B20.

https://www.theengineeringprojects.com/2019/01/introduction-to-ds18b20.html

It has an underwater version. The sensor comes with a long cable. The sensor can be fully immersed underwater, while the cable can be connected to the controller at a distance. You can directly connect the sensor to Arduino or whatever controller you're using. No added circuitry is needed.

Thanks for the suggestion, I'll definitely keep it in mind.

 

hi S,
I would use a resistor and thermistor is series for a +/-2C resolution,

Lets know when you have more information.
E

 

hi S,
This is typical response for a 10K NTC thermistor with this Beta factor of 4300, 0C thru +100C.
E