I’ve been messing around with my fridge and was wondering how the thermostat worked in a more scientific perspective. There has to be some way it monitors the temperature?
I’ve been able to find a labeled image(imgur.com/a/sRMrJK2) of a thermostat but how does twisting the screw change the temp range?
Also, what is a “delay timer” and what is the purpose?
In: Engineering
It does “monitor” the temperature, entirely mechanically! It is common to use a bimetallic strip. What this is, is two different metals joined together that have different expansion rates when heated. This causes it to bend as heated, and will have either electricity flowing through it or a contact attached to it.
The idea is as the heat changes, the shape changes until its contact meets another contact that operates the relay to turn the fridge compressor on. The knob adjusts the spacing between the two contacts, requiring more or less bending from the bimetallic strip before the contacts meet. Check out some gifs of bimetallic strips on Google if you wanna see the strip in action reacting to heat!
The delay timer is intended to protect the fridge’s mechanical components. You don’t want it constantly turning off and on in short bursts, and the timer enforces a certain amount of time before it can turn back on. You’ll see the same in say your household yhermostat, if you go from cool to off then back to cool the fan might turn back on but it’ll probably wait a little while to turn the A/C unit on.
One common type of thermostat uses a coil of flat metal that shrinks when it gets colder and expands when it gets warmer – when it expands far enough, the metal closes a contact which turns on the compressor to cool down the fridge. Then once it gets cold enough again, it shrinks again which disconnects the circuit and the compressor turns off. The clever part is that by turning the dial towards “colder” or “hotter”, you’re also twisting the coil to either make it bigger or smaller, so that the compressor will turn on and off at higher or lower temperatures than before.
A delay timer is additional circuitry that prevents the compressor from running too often, because if it does that, frost can build up and then the fridge has to be manually defrosted, which sucks – but hardly any modern fridges require manual defrosting unless it’s malfunctioning.
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