# 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. 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 spread the inrush current (in transformers and such) over more time:

• place in series with the primarily coil
• 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.

### Diodes

The voltage across a diode will decrease by approx 2 mV per °C in a fairly linear way.

You need amplification and probably a DAC, accuracy isn't great, but it's very convenient inside FPGAs, processors, and such.

### Unsorted

#### DHT11, DHT22

Measures:

• Temperature (NTC thermistor)
0-50 ℃
output reported to 1℃ but don't assume more than ±2℃ accuracy
• Relative Humidity
Range: 20 to 90 ％RH (note: range and accuracy vary somewhat with temperature)
output reported to 1% but don't assume more than ±5％RH accuracy

In a quick test with two DHT11 modules next to each other, temperature was reported 0.0 to 1.0 degrees apart on average, but humidity was ~11 %RH apart on average (fairly consistently so you can probably improve this a little via calibration against a known value).

Pins: 3.5-5.5V, data, NC, Gnd.

You will want a pullup resistor on the data pin, mostly because it's bidirectional onewireish(verify).

Power: ~0.5mA while measuring, 0.1mA idle

DHT22:

RH: 0 to 100% range, 2 to 5% accuracy
Temp: -40 to 80°C range, ±0.5°C accuracy