Temperature Sensor using Thermistor circuit needed

thermistor circuit

Hi, everyone!

I am builiding a power supply and I would like to control the temperature of the transformer. I think the Thermistor would be the cheapest way, right???
Could someone give me a tip about this???
Can someone get me a schematic??

Thanks a lot

This forum is really awesome!!!!

Regards, Helder Silva

temperature sensor circuit using thermistor

Instead of a thermistor you may use the LM35 temperature sensor:
**broken link removed**
It is linear, has 10mV/° output and works in wide range of temperatures.
In the above link you will find a lot of applications with schematics ..

thermistor circuit design

agreed with IanP. i have used that device before, it is very nice. you communicate to it via one-wire i believe. but you need a microprocessor. does your design have one already? if not, then this approach will not be worth your time.

a thermistor is good for 2 reasons, i) they are cheap, ii) they give the fastest response to rate of change of temperature.

for your app, you probably do not care about rate of change of temperature, just the steady state.
here are your design steps for use of thermister:
1) pick a 10kOhm NTC thermister (they are about 10 cents)
2) pick a temperature range that your transformer will operate in (think of power dissipation and ambient temperature. be modest, a large range is fine.
3) calculate the "linearization" resistor required to put in parallel with thermistor
4) schematicially, the thermistor & parallel resistor will show up in the feedback path of an op-amp
5) output of op-amp goes to negative feedback circuit of power supply (i.e. when temperature is high the opamp increases its output voltage and power supply reduces its current to reduce temperature of transformer).
6) also a schmitt trigger could be used to indicate temperature threshold

not particularily accurate method, probably +/- 2deg C steady state. but this is probably sufficient for your needs? it is the easiest and cheapest method (unless you already have a microcontroller in your design with spare I/O pins)

Mr.Cool

pspice thermistor

Thanks Mr. Cool!

I really do not intend to use a microcontroller in this project.
I would rather a simple circuit using a thermistor. The circuit intend only to control if the transformers rise up a certain temperature, I want the circuit of my power supply goes "OFF", I thought maybe I could use a relay to control this task.
Do you understand what I am looking for??

Thanks a lot

Regards, Helder Silva

thermistor circuit op amp

Then you can build a circuit with a temperature sensor and a relay as per attached link:
**broken link removed**
Instead of the NTC temperatue sensor you can still use the LM35 as suggested in my previous post; it is just easier to select the cut-off temperature, as the LM35 gives you direct reading of 10mV/° ..

thermistor temperature sensor circuit

if you want, i could post my circuit of a thermister. it is useful for simulating in PSPICE only.

if you use a thermistor you must make an effort to linearize your thermistor. you should also read "Transducer Interfacing Handbook" a guid to analog signal conditioning by Analog Devices. it is available in the forumn somewhere. see page 79

without more details about your application it is difficult to say how the best way is to implement your shut off scheme. genearlly, low power applications, say 50v 500mA or less the best is solid state switch. for higher power a relay is good. there are pros and cons to both. i like to use solid state relays wherever possible because they are smaller & available via free samples

Mr.Cool

Added after 2 minutes:

oh, and also where you put your thermister is important. probably you will paste it to the outside of the transformer. don't worry about thermal paste, it is not necessary to read steady-state temperatures.

your case temperature is a lot cooler than the temperature of your inner most coils. your core will usually pass its curie temperature after 120 to 135 C. depending on the size of your transformer, you might consider not allowing the outer case to be more than 80C.

Mr.Cool

Added after 2 minutes:

and as i keep thinking of it...

a 10Kohm thermister will not be good for you. because at temperatures of 80C, yoru thermister will be about 1kohm. and the difference in resistance between say 60C and 80C will only be about 5kohm, maybe less.

you should consider using a 100K thermister rated at a temperature near what your application calls for. if it is rated only at room temperature, then this is why you get a 100kohm, so that in the 60 to 80C range you still have some good resistance range to work with