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Thermistor and Op Amp Circuit

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ElectroJW

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Hi,
For this assignment we were told to build a circuit that uses a thermistor and will generate two digital signals: one of them will be high if the temperature is above 10 C, the other will only be high if it's below 0 C. I believe we are supposed to use an op amp in this as well. I was thinking about finding some way to turn the two signals into one (a wheatstone bridge, maybe?) and then feeding that into 2 comparators, one for each signal. I am not sure how to figure out what the resistances will need to be and I am having trouble just putting the whole thing together. Thanks!
 
I suggest you start with a read of this circuit. You do not need a bridge circuit to get where you want to go. I would not use the 741 op amp as shown in the link. I would also give this link a good read as to using a comparator. Remember also when using a thermistor you need to avoid things like self heating. Your thermistor becomes part of a voltage divider network. The divider output is passed on to a comparator circuit.

High below 0C and high above 10C so low between 0C and 10C. Think about a single "Window Comparator". :) They are covered in the second link.

Ron
 
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High below 0C and high above 10C so low between 0C and 10C. Think about a single "Window Comparator". :) They are covered in the second link.
I believe he wants two separate digital outputs. He does't need a window comparator for that but just two separate comparators with different trip voltages, both detecting the same thermistor signal.
 
I believe he wants two separate digital outputs. He does't need a window comparator for that but just two separate comparators with different trip voltages, both detecting the same thermistor signal.

Yeah, thinking I read into it too much.

Ron
 
Hi,
For this assignment we were told to build a circuit that uses a thermistor and will generate two digital signals: one of them will be high if the temperature is above 10 C, the other will only be high if it's below 0 C. I believe we are supposed to use an op amp in this as well. I was thinking about finding some way to turn the two signals into one (a wheatstone bridge, maybe?) and then feeding that into 2 comparators, one for each signal. I am not sure how to figure out what the resistances will need to be and I am having trouble just putting the whole thing together. Thanks!


Hi,

The basic section is made by using a voltage divider where one of the resistances is a thermistor, usually a NTC (Negative Temperature Coefficient) device. The resistance of an NTC type thermistor does down as the temperature goes up, so if you use it as the top resistor of a voltage divider then the voltage at the junction of the two resistances goes up with increasing temperature, but if you use it as the bottom resistor then the voltage goes down with increasing temperature.

That's the basic concept and the comparators are used simply to detect particular voltages that you can calculate using formulas for thermistors and for the resistance voltage divider. You might also want to look into hysteresis for the comparators as that prevents the output from constantly switching states when the temperature gets close to one of the set points.
 
Hi,
For this assignment we were told to build a circuit that uses a thermistor and will generate two digital signals: one of them will be high if the temperature is above 10 C, the other will only be high if it's below 0 C. I believe we are supposed to use an op amp in this as well. I was thinking about finding some way to turn the two signals into one (a wheatstone bridge, maybe?) and then feeding that into 2 comparators, one for each signal. I am not sure how to figure out what the resistances will need to be and I am having trouble just putting the whole thing together. Thanks!

easy way is two comparators and an LM35 temp measure IC. You can also use a thermistor, but you need the data sheet curves for it's resistance.

+1 on using some hysteresis (positive feedback) so the comparator outputs won't bang up and down.
 
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