Doc ID 15772 Rev 2

9/14

AN2991

Electronic thermostat versus mechanical thermostat

14

3

Electronic thermostat versus mechanical thermostat

3.1

Temperature regulation

One main advantage of electronic thermostats versus electromechanical ones, is their

adaptability. For mechanical thermostats, it is a gas compression and decompression that

switches the compressor on or off. This does not allow a hysteresis threshold adaptation

during the refrigerator operating cycle. Moreover, this gas effect yields to the fact that the

refrigerator may not work properly depending on the ambient atmospheric pressure.

For electronic thermostats, the temperature information is measured accurately and at all

times, contrary to electromechanical thermostats where the only available information is that

the temperature is over a fixed level or not. This enables the temperature fluctuation inside

the cabinet to be reduced with electronics.

A reduction of the temperature ripple presents three main advantages:



Better food preservation



Thermodynamic efficiency improvement (lower evaporator temperature ripple; higher

evaporator minimum temperature; better compressor use (see Section 3.2.2: Motor

duty cycle)



Compressor and evaporator downsizing

A particularly interesting feature can also be implemented thanks to electronics - the

hysteresis levels can be changed during the freezing process. This allows automatic

defrosting operations to be implemented. For example, Figure 9 shows that the upper

hysteresis level is increased every 8 cycles in order to let the evaporator temperature

become higher than 0°C. This allows the ice deposit on the evaporator to be removed. This

may be very helpful to increase the refrigerator efficiency as this ice layer plays a real

insulation role.

Figure 9. Temperature fluctuation and defrost cycle with electronic thermostat

0

20

40

60

80

100 120

140 160

180 200 220

240 260

-30

-25

-20

-15

-10

-5

0

5

10

15

20

Time (min)

Evaporator

Left side Cabinet

on/off

Temperature (°C)