AUTOMATIC DEFROSTING CONTROL SYSTEM

An automatic defrosting control system for a refrigeration system having an evaporator or the like for absorbing heat from a zone to be cooled to a predetermined temperature level and being subject to the accretion of frost thereon. The control system includes means for causing defrosting of the evaporator, a thermostat, a thermal time-delay relay and a heater. The thermostat is positioned in heat-exchange relation with both the evaporator and the defrosting means and is connected in a control circuit. The thermostat has a first switching position for terminating operation of the defrosting means and a second switching position for enabling operation thereof. It switches from its first to its second position in response to its temperature falling to a lower predetermined level and switches to its first position upon its temperature rising to a higher predetermined level. The thermal time-delay relay is positioned in heat-exchange relation with the evaporator and is also connected in the control circuit. It has a first switching position for permitting operation of the evaporator and a second switching position for energizing the defrosting means and preventing operation of the evaporator. The relay switches from its first to its second position in response to its temperature falling to a lower predetermined level and switches back to its first position in response to its temperature rising to a higher predetermined level. The heater is connected in the control circuit for energization during at least a portion of the period that the relay is in its second position thereby to heat the relay to its higher predetermined temperature level. When the temperature of the thermostat falls to its lower predetermined level and the relay cools to its lower predetermined temperature level, the thermostat and relay will switch to their second positions thereby initiating defrosting.