Discussion in 'General Electronics Chat' started by shankar M, Apr 8, 2014.
How read thermister temp from microcontroller? i want help to design ckt
A thermistor acts a bit like a variable resistor so you need set up a suitable voltage, say through a potential divider, and then use analog to digital conversion to convert the voltage across the thermistor to a value the MCU can read.
It is always good to give as much detail as you can in your questions, you will then get more helpful answers.
This link may help to get you started, its a tool I designed some time ago.
Enter your know thermistor values and temperature ranges
Thermistors are not linear, so they are not as accurate as an LM35 sensor,also they come in different values depending on the temperature range,which micro are you using?
The problem with making such broad generalizations is that eventually someone is going to come along and prove you wrong.
A quick search of digikey identifies about 40 thermistors that are more accurate than the best lm35.
Not only are thermistors non-linear, they're non-simple-exponential-ly non-linear. That thermistor equation is such a bear that I always wind up going with a lookup table.
An LM35 certainly is linear and well behaved, but to get real accuracy you need two calibrated temperature wells and a lot of time.
OTOH - With a DC95F thermistor I can get 0.1degC absolute interchangeability with *no* calibration for around $3. Luv the little puppies.
my input voltage is 3.3v for MC9S08JM32CLH (64-PIN LQFP) micro controller using...im trying interface with thermal printer mechanism ...... 9v input for thermal printer mechanism.. how to read temperature from thermal printer mechanism FTP-637MCL401
LM35 is designed to give a voltage output that increases linearly with temperature, 10mV/°C.
A thermistor resistance is highly nonlinear with temperature but can be used to measure temperatures that exceed the accuracy obtained with LM35.
The first circuit which someone might consider is a voltage divider which outputs a voltage to be digitized with an ADC.
However, there are other solutions.
A thermistor can be used in an RC circuit and the time-constant can be measured very accurately. Another circuit might use the RC combination in a frequency measurement method. The advantage of these methods is that measuring time or frequency is easily accomplished on an MCU and no ADC is required.
The simplest design uses a thermistor and capacitor connected to two pins of the MCU.
An important consideration is the self-heating effect in the thermistor. By using pulse frequency or period the current through the thermistor can be reduced to zero in between measurements thus reducing the self-heating effect.
The simplest design uses a thermistor and capacitor connected to one pin of the MCU.
The US Sensor PS502J2 gives the same performance at about 1/2 the price.
An advantage of a thermistor vs. LM35:
You don't need a precision voltage reference to achieve the stated accuracy when converting to digital.
Edit: see http://forum.allaboutcircuits.com/showthread.php?t=64095
What is the output of the temperature sensor?
What is the range of the output?
The majority of common uses are just to switch on/off at a setpoint.
Assuming that is the case he just needs a thermistor, resistor (to make a voltage divider) and the regulated 3.3v Vdd. Then the micro's ADC just needs to compare to one number for the setpoint.
If you are looking for a single set point then a voltage divider and a digital input or analog comparator will work.
The analogue comparator should work, but there is no software control over the setpoint. It has to be done by resistors and voltages.
Using the ADC and a setpoint number means the micro can just change the number to chnage the regulated temperature.
And the digital input won't work because they have an extremely large hysteresis, from 40-60% of Vdd (depending if a TTL or ST input).