# Difference between revisions of "Electronics notes/Temperature sensing"

 This is for beginners and very much by a beginner. It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book. And some more applied stuff: Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer See also Category:Electronics.
 This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

## Contents

### Thermocouple

Inexpensive High range of temperature Not highly accurate - assume you won't get better than 1 degree Celcius of resolution, less if you don't calibrate well

Age with time and high-temperature use, so will need occasional recalibration and/or replacement.

They produce voltage due to the thermoelectric effect - on the order of millivolts so most uses, and accurate use, requires amplification (with high input impedance to avoid the measurement affecting the thermocouple itself).

### Thermistor

Most resistors vary their resistance with temperature. A thermistor (thermal resistor) does it intentionally, and more pronounced.

NTC: negative temperature coefficient, resistance drops (logarithmically) as its body temperature increases

PTC: positive temperature coefficient,

The 'at-rest' resistance varies with intent

They are frequently used in temperature sensing, temperature regulation, and (over)current protection.

Perhaps the he simplest way to get a voltage from a thermistor (think ADC, comparator) is to have it be one leg of a voltage divider.

#### Power thermistor

A power thermistor is a very low-resistance (NTC) thermistor in series with your main current, as a current limiter and/or (self-resetting) overcurrent protector.

One use is to lessen the sudden inrush current in transformers and such:

• place in series with the primarily coild
• when cold (just switched on) it typically has a few hundred ohm resistance
• and once it warms it (few seconds later) goes to under an ohm.

This lessens the magnitude of the sudden current that can happen right after you switch something on.