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FUNCTION BLOCK
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DESCRIPTION
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- A common temperature sensor is a Thermistor. This is a type of resistor that changes its resistance depending on the temperature.
- There are 2 types
- PTC = Positive Temperature Coefficient
(Resistance increases as temperature rises)
- NTC = Negative Temperature Coefficient
(Resistance decreases as temperature rises).
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CIRCUIT DETAILS
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The most common type is NTC thermistor - where the resistance decreases as the temperature rises.
Like most sensors, a Thermistor must be used as part of a potential divider, this converts a change in resistance into a change in voltage.
There are 2 ways to connect a thermistor into a potential divider. These are shown below and work exactly opposite from one another.
Supply voltage from 3 to 15v.
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Output
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Circuit Diagrams |
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As the temperature changes, so does the output voltage.
VR1 can be used to adjust the amount of voltage for any particular temperature.
The output current is very low and anything connected to it must have a high input resistance.
It is common to connect the output to a comparator or Schmitt Trigger.
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With the thermister at the top of the potential divider, when the temperature rises, the output voltage increases.
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With the thermister at the bottom of the potential divider, when the temperature rises, the output voltage decreases. |
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THERMISTOR DEVICES
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There are many types of thermistor, the only difference is their resistance range and speed of response. |
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General Purpose
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Glass Bead - Fast Reacting
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High Precision Type
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VARIABLE RESISTOR (POTENTIOMETERS)
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 (preset)
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If you want to frequently adjust the output voltage level, use a chassis mount potentiometer. These are commonly used as volume controls and for light dimmers etc.
Otherwise use a preset which you can adjust with a screwdriver and is mounted on the PCB.
Recommended value: 10K
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DESIGN NOTES
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- If possible mount the thermistor on the PCB on the longest legs possible, if not you will need to provide 2 pads on the circuit board to connect wires to.
- Attach the thermistor as close to the material / air space you want to monitor - if it's free air then allow air flow around the sensor.
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| Written by Phil Townshend - 2008 |
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www.edutek.ltd.uk - Working Electronics For Students & Teachers
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