To regulate the operating cycle of refrigerators, electronic control modules with NTC sensors or thermostats (thermal relays) are used. After all, almost all modern refrigerators operate on the “work and rest” principle, that is, they have work and rest periods in their cycle. The time proportionality of these stages very much depends on various factors, such as frequently opening doors, the number of products located inside and the tightness of the seal adhesion to the refrigerator. We recommend to our clients a scheme of 30 minutes working: 30 minutes resting (+-10 minutes), with the exception of the first start. Of course, this diagram is not entirely correct, but still it explains complex operating parameters to the consumer as simply as possible, with a low error.
NTC (Negative Temperature Coefficient) is a thermistor whose resistance decreases with increasing temperature, in contrast to the PNC sensor (posistors, see the Compressors lesson), whose resistance increases when heated.
The resistance is monitored by the control module, which sees the changes and turns the compressor off or on at the right time, as shown in the bottom graph. Resistance does not have a linear dependence on temperature, but a paraboloidal one, which increases the measurement accuracy in the required range.
In addition to electronic control modules, many manufacturers still use “analog” technology, using thermostats to control the temperature and operating cycle of the compressor, the types and design of which we will now discuss.
Thermostat (temperature regulator) is a device that controls the temperature in the refrigerator due to the expansion of gas in the capillary tube, pressing on the membrane.
The thermostat consists of a tube (capillary) connected to a membrane chamber, which responds to any change in pressure inside the system (compression or expansion). Inside the chamber there is a special gas that gives the greatest thermal expansion, and the tip of the thermostat is fixed in a special place in the refrigerator. And if the refrigerator is single-compressor, then the temperature is monitored in the refrigerator compartment, and the freezer “goes on a trailer” (it is understood that if the temperature in the refrigerator compartment is normal, 3..7 °C, then the temperature in the freezer is normal).
When the refrigeration compartment heats up, the gas inside the thermostat capillary (not to be confused with the system capillary) expands and closes the relay contacts to the compressor. If there are two compressors, then two thermostats are used, essentially obtaining two independent systems. But this technology has not taken root well and is now becoming less and less common.
Most thermostats have three contacts, two act as the compressor switch, and the third is used for lighting. The standard is that contact (6) goes to lighting, contact (4) goes to the compressor, and contact (3) goes to voltage. But it’s better not to remember such an arrangement, and especially not to remember it by color, as this can lead to errors in repairs. Therefore, we recommend that you always have a detailed diagram on hand or keep in mind, as well as know how to use a multimeter.
In domestic refrigeration, there are many types of thermostats that can be used by different brands and in different devices (freezers or refrigerators). Therefore, you cannot change one thermostat to another without making sure that it is compatible, and it is better to always use the exact thermostat that was originally installed.