tutorials:learn:sensors:thermistor.html
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tutorials:learn:sensors:thermistor.html [2011/06/07 19:21] daigo |
tutorials:learn:sensors:thermistor.html [2016/01/28 18:05] (current) |
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| *They work at any voltage (digital sensors require 3 or 5V logic). | *They work at any voltage (digital sensors require 3 or 5V logic). | ||
| *Compared to a thermocouple, they don't require an amplifier to read the minute voltages - you can use any microcontroller to read a thermistor. | *Compared to a thermocouple, they don't require an amplifier to read the minute voltages - you can use any microcontroller to read a thermistor. | ||
| - | *They can also be incredibly accurate for the price. For example, the 10K 1% thermistor in the shop is good for measuring with ±0.1°C accuracy! (Assuming you have an accurate enough analog converter) | + | *They can also be incredibly accurate for the price. For example, the 10K 1% thermistor in the shop is good for measuring with ±0.25°C accuracy! (Assuming you have an accurate enough analog converter) |
| *They are difficult to break or damage - they are much simpler and more reliable | *They are difficult to break or damage - they are much simpler and more reliable | ||
| Line 35: | Line 35: | ||
| <class center> | <class center> | ||
| - | [[http://www.adafruit.com/products/372|You can pick up a 10K 1% waterproof thermistor in the Adafruit shop]] | + | **[[http://www.adafruit.com/products/372|You can pick up a 10K 1% waterproof thermistor in the Adafruit shop]]** |
| </class> | </class> | ||
| Line 42: | Line 42: | ||
| [[http://www.adafruit.com/products/372|Here are technical details for the thermistor in our shop]] | [[http://www.adafruit.com/products/372|Here are technical details for the thermistor in our shop]] | ||
| - | * Resistance at 25°C: 10K ±1% | + | * **Resistance at 25°C:** 10K ±1% |
| - | * B25/50: 3950 ±1% | + | * **B25/50:** 3950 ±1% |
| - | * Thermal time constant <= 15 seconds | + | * **Thermal time constant** <= 15 seconds |
| - | * Thermistor temperature range -55°C to 125°C | + | * **Thermistor temperature range** -55°C to 125°C |
| - | * Wire temperature range -55°C to 105°C | + | * **Wire temperature range** -55°C to 105°C |
| - | * 28 AWG PVC Wire | + | * **28 AWG PVC Wire** |
| - | * Diameter: 3.5mm/0.13in | + | * **Diameter: 3.5mm/0.13in** |
| - | * Length: 18in/45cm | + | * **Length: 18in/45cm** |
| - | * Resistance/Temperature table | + | * [[http://www.adafruit.com/datasheets/103_3950_lookuptable.pdf|Resistance/Temperature table]] |
| Note that even though the thermistor can go up to 125°C the cable itself maxes out at 105°C so this thermistor is not good for measuring very very hot liquids | Note that even though the thermistor can go up to 125°C the cable itself maxes out at 105°C so this thermistor is not good for measuring very very hot liquids | ||
| ====Testing your thermistor==== | ====Testing your thermistor==== | ||
| + | |||
| + | |||
| + | Because thermistors are simply resistors, its easy to test it out. Simply measure the resistance using a multimeter: | ||
| + | |||
| [[http://www.ladyada.net/images/sensors/thermistormeasure.jpg|{{ http://www.ladyada.net/images/sensors/thermistormeasure_t.jpg?nolink&500x356 |}}]] | [[http://www.ladyada.net/images/sensors/thermistormeasure.jpg|{{ http://www.ladyada.net/images/sensors/thermistormeasure_t.jpg?nolink&500x356 |}}]] | ||
| Line 80: | Line 84: | ||
| Say the fixed resistor is **10K** and the variable resistor is called **R** - the voltage output (**Vo**) is: | Say the fixed resistor is **10K** and the variable resistor is called **R** - the voltage output (**Vo**) is: | ||
| + | <class center> | ||
| **Vo = R / (R + 10K) * Vcc** | **Vo = R / (R + 10K) * Vcc** | ||
| + | </class> | ||
| Line 89: | Line 94: | ||
| Now we want to connect it up to a microcontroller. Remember that when you measure a voltage (**Vi**) into an Arduino ADC, you'll get a number. | Now we want to connect it up to a microcontroller. Remember that when you measure a voltage (**Vi**) into an Arduino ADC, you'll get a number. | ||
| + | <class center> | ||
| **ADC value = Vi * 1023 / Vcc ** | **ADC value = Vi * 1023 / Vcc ** | ||
| + | </class> | ||
| So now we combine the two (**Vo** = **Vi**) and get: | So now we combine the two (**Vo** = **Vi**) and get: | ||
| + | <class center> | ||
| ADC value = ** R / (R + 10K) * Vcc * 1023 / Vcc** | ADC value = ** R / (R + 10K) * Vcc * 1023 / Vcc** | ||
| + | </class> | ||
| What is nice is that if you notice, the Vcc value cancels out! | What is nice is that if you notice, the Vcc value cancels out! | ||
| + | <class center> | ||
| ADC value = ** R / (R + 10K) * 1023** | ADC value = ** R / (R + 10K) * 1023** | ||
| + | </class> | ||
| It doesn't matter what voltage you're running under. Handy! | It doesn't matter what voltage you're running under. Handy! | ||
| Finally, what we really want to do is get that **R** (the unknown resistance). So we do a little math to move the **R** to one side: | Finally, what we really want to do is get that **R** (the unknown resistance). So we do a little math to move the **R** to one side: | ||
| + | <class center> | ||
| **R = 10K / (1023/ADC - 1)** | **R = 10K / (1023/ADC - 1)** | ||
| + | </class> | ||
| Great, lets try it out. Connect up the thermistor as shown: | Great, lets try it out. Connect up the thermistor as shown: | ||
/home/ladyada/public_html/wiki/data/attic/tutorials/learn/sensors/thermistor.html.1307474490.txt.gz · Last modified: 2016/01/28 18:05 (external edit)
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