Question about Heating & Cooling
Most thermostats just shut down power to the compressor when the set temp is reached. Older models used a mercury based thermostat which simply killed power to a circuit. newer models have a brain which continually monitors changes in resistance of a thermister mounted in front of the evaporator.
FYI: A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the voltage drop produced.
The standard reference temperature is the thermistor body temperature at which nominal zero-power resistance is specified, usually 25°C.
The zero-power resistance is the dc resistance value of a thermistor measured at a specified temperature with a power dissipation by the thermistor low enough that any further decrease in power will result in not more than 0.1 percent (or 1/10 of the specified measurement tolerance, whichever is smaller) change in resistance.
The resistance ratio characteristic identifies the ratio of the zero-power resistance of a thermistor measured at 25°C to that resistance measured at 125°C.
The zero-power temperature coefficient of resistance is the ratio at a specified temperature (T), of the rate of change of zero-power resistance with temperature to the zero-power resistance of the thermistor.
A NTC thermistor is one in which the zero-power resistance decreases with an increase in temperature.
A PTC thermistor is one in which the zero-power resistance increases with an increase in temperature.
The maximum operating temperature is the maximum body temperature at which the thermistor will operate for an extended period of time with acceptable stability of its characteristics. This temperature is the result of internal or external heating, or both, and should not exceed the maximum value specified.
The maximum power rating of a thermistor is the maximum power which a thermistor will dissipate for an extended period of time with acceptable stability of its characteristics.
The dissipation constant is the ratio, (in milliwatts per degree C) at a specified ambient temperature, of a change in power dissipation in a thermistor to the resultant body temperature change.
The thermal time constant of a thermistor is the time required for a thermistor to change 63.2 percent of the total difference between its initial and final body temperature when subjected to a step function
The resistance-temperature characteristic of a thermistor is the relationship between the zero-power resistance of a thermistor and its body temperature.
The temperature-wattage characteristic of a thermistor is the relationship at a specified ambient temperature between the thermistor temperature and the applied steady state wattage.
The current-time characteristic of a thermistor is the relationship at a specified ambient temperature between the current through a thermistor and time, upon application or interruption of voltage to it.
The stability of a thermistor is the ability of a thermistor to retain specified characteristics after being subjected to designated environmental or electrical test conditions.
You have asked a generic question this is an attempt at a generic answer.
A mercury based thermostat simply uses ambient tempts to close and open contacts in the switch.
How about letting me know whats going on with the unit? I could give a better answer.
Posted on Aug 04, 2010
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Posted on Jan 02, 2017
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