electromechanical defrost control system of household refrigerators

Standard circuit without feedback

The circuit is relatively simple due to absence of feedback. We’ll begin studying No Frost systems with it.
This circuit is characterized by non-stop operation of the timer motor in all system operation modes, including in the defrost mode. That’s why the timer must have rather a long defrost time (typically 30...40 minutes), which assuredly exceeds the time required for the heater for complete melting of frost on the evaporator, even when there’s much of it.

T - timer

The electromechanical timer provides cyclic operation of the No Frost system, controlling the alternation and duration of the cooling and defrost modes.

M - timer motor

The electromechanical timer operating principle is based on rotation of the shaped drum sequencer by the micromotor. The drum periodically switches over the timer contacts. Thus, time is counted down only when the micromotor is powered.

C - compressor

The refrigeration unit operating principle is based on physical processes in its pipelines during coolant circulation in them. Circulation is provided by refrigeration unit compressor. Its operation causes intense cooling of the refrigeration unit evaporator.

F - fan

Heat exchange between the refrigeration unit evaporator surface and food products inside the refrigerator is provided by the air flow created by the No Frost system fan.

R - defrost heater

The moisture in the air, blowing down the intensely cooled evaporator of the No Frost system, gradually settles on its surface as frost. The heater, attached on the evaporator, assures its heating for frost melting and surface cleaning. It is needed to prevent formation of an excessively thick frost layer, impairing effectiveness of heat exchange between the evaporator and air flow.

DT - defrost thermostat

Temperature of the No Frost system evaporator must be monitored during automatic defrost, so as to switch off the heater in time. The defrost thermostat is used for temperature monitoring. Its contacts open at a temperature slightly above the frost melting temperature.

Interactive Circuit Diagram

The system is in the mode of initial rapid cooling, the timer supplies power to the compressor and fan. However, the evaporator hasn’t yet cooled down sufficiently, that’s why the defrost thermostat contacts are open.
The system operates in this mode not long, about 10 minutes. Usually this time is enough for sufficient cooling of the evaporator and closing of the defrost thermostat contacts.

The system is in the cooling mode, the timer supplies power to the compressor and fan. The evaporator has already cooled down sufficiently, that’s why the defrost thermostat contacts are closed.
The system operates for most of the time in this mode. Its duration is from 6 to 12 hours. Air and food products inside the refrigerator are cooled in this mode, while the evaporator is gradually covered with a frost layer.

The system has set to the defrost mode. The timer de-energized the compressor and fan and powered up the defrost heater. Evaporator temperature is low as yet, that’s why the defrost thermostat contacts are closed.
But evaporator temperature slowly increases due to heater operation. When evaporator temperature is above the frost melting temperature, the defrost thermostat contacts will open. By this moment, the evaporator will be fully defrosted. Duration of operation in this mode depends on evaporator frost amount and is usually 10...20 minutes.

The heater warmed up the evaporator to above the frost melting temperature and the defrost thermostat contacts opened, de-energizing the heater. The timer is still in the defrost mode, but its motor is powered, therefore it will switch over its contacts after a while and set to the rapid cooling mode.
This operation mode usually lasts for 15..25 minutes.


The only serious disadvantage of the standard circuit is absence of feedback. The system does not trace the end of the active defrost phase, when the defrost thermostat contacts open and the heater is de-energized. As a result, even after heater switch-off, the timer makes too long a pause before switchover to the freezing mode. It has an adverse effect on temperature conditions inside the refrigerator.
Slightly more complex feedback systems don't have this disadvantage. We’ll continue studying No Frost systems with a simple feedback circuit.