Wednesday, November 24, 2010

How does a thermostat in a refrigerator work?

Detailed please, but not too complex.

It is for biology class- we have to talk about negative feedback

So if you could please tell me how the thermostat works and how it controls the overall temp that would be amazing

Thanks :)How does a thermostat in a refrigerator work?It most probably uses a bimetallic strip to sense temperature, and closes a contact when the temperature goes above a set point. The closed contact turns on the compressor, which starts cooling the refrigerator. After a bit, the temperature goes low enough and the contact opens, turning off the compressor. This cycle continues.



negative feedback, from the top of my head, is a process where an error signal is amplified and used to control a process that decreases the error.



The error signal is the difference between the desired temperature and the actual temperature, which is sensed by the bimetallic strip and (amplified) used to control the compressor, which will decrease the error, ie, cool the frig.





A bimetallic strip is a long thin strip consisting of two dissimilar metals soldered or riveted together, and wound into a coil. Since the two metals have different thermal expansion properties, as the temperature changes the coil winds tighter or unwinds. This causes to contacts to close or open, like a switch or relay. You can see one of these in most room thermostats.How does a thermostat in a refrigerator work?My understanding is that it monitors the temperature inside the refrigerator, then when the temperature reaches a point higher than it is set at, it releases cold air into the fridge.



Anything more in depth is beyond me. Hope it helped!How does a thermostat in a refrigerator work?yeah what john said...doubleHow does a thermostat in a refrigerator work?Largely, bimetal strips are not used in refrigerators.



Usually, the thermostat is a capillary (very small) tube filled with a gas. This tube is then connected to a diaphragm. As the temperature in the fridge increases, the gas in the tube expands (PV=NKT) %26amp; pushes on the diaphragm, which in turn operates a set of contacts. These contacts will in turn, operate the compressor.How does a thermostat in a refrigerator work?A refrigerator thermostat is usually just an electrical switch connected to a mechanical device that responds to changes in temperature. As billrussell42 said, in his excellent answer to your question, this can be a bi-metallic strip that bends or winds or un-winds (depending on construction) as a function of temperature. The motion is opposed by a spring whose force is controlled by an attached knob that sets the desired temperature. A capillary tube connected to a diaphragm is another type of mechanical temperature transducer but the principle is the same. (Electronic sensors such as thermistors and thermocouples are also used, especially in laboratory refrigerators, but their use is considerably more complicated)



For mechanical thermostats, depending on the temperature setting, the force produced by a spring will determine when the switch opens or closes. When the actual temperature is below the “set-point” temperature, the switch remains open. When the actual temperature rises above the “set-point” temperature the switch closes.



Negative feedback occurs when the switch is wired to turn the refrigerator compressor ON if the actual temperature is ABOVE the “set-point” temperature and to turn the compressor OFF if the actual temperature is BELOW the “set-point” temperature. Note that turning the compressor ON causes the temperature to decrease. And vice versa: decreasing the actual temperature BELOW the “set-point” temperature will cause the compressor to turn OFF, thereby allowing the refrigerator temperature to gradually increase as outside heat seeps in past the insulation in the walls.



In other words, two opposite and competing effects determine whether negative feedback is in play. Increasing temperature causes a response (compressor turns on) resulting in a decreasing temperature. Decreasing temperature causes a response (compressor turns off) resulting in an increasing temperature. In a nutshell that is negative feedback.



For this type of ON/OFF negative feedback, an important concept called hysteresis must always be present. It is important that the temperature that causes the compressor to turn ON always be a few degrees higher than the temperature that causes the compressor to turn OFF. The difference between these two temperatures is called the “dead band” or hysteresis band. A dead band is very important to prevent “short cycling” where the compressor comes on in response to increasing temperature, but then almost immediately turns off because the temperature quickly dropped.



This means, of course, that the temperature in the refrigerator is never constant. It cycles between the dead-band limits. Industrial refrigeration gets around this problem by using a type of negative feedback called proportional control with integral and derivative feedback, but that is beyond the scope of this answer.



Imagine now what would happen if positive feedback were used. That is, wire the temperature-sensing switch in such a manner that increasing the actual temperature above the set-point temperature will turn the compressor OFF and decreasing the actual temperature below the set-point temperature will turn the compressor ON. Easy to do if it’s a single-pole, double-throw, switch, which is often common on bi-metallic thermostats.



There are two cases: initial temperature above the set-point temperature and initial temperature below the set-point temperature.



If the actual temperature is above the set-point temperature, and the compressor is already ON, it will immediately turn OFF. And it will remain OFF because we assume the temperature rises when the compressor is off.



If the actual temperature is below the set-point temperature, and the compressor is OFF, it will immediately turn ON. And it will remain ON as the compressor continues to make the actual temperature lower and lower than the set-point temperature.



Obviously neither condition is desirable, but then positive feedback in a control system is seldom desirable.



I hope this answer was detailed enough without being too complex.

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