Automatic defrost timer. It is a device used in automatic-defrost refrigerators and freezers to take the system into the defrost and then back to the cooling mode in regular intervals.
It consists of an electric motor and a switching mechanism (see fig. 11a). The electric motor is geared to move a cam in such a way that after every six, eight, or twelve hours of compressor operation (depending on the design of the timer), the cam rotates from one contact to another for no longer than thirty minutes. This causes a temporary change in the direction of current flow from the cooling terminal to the defrosting terminal and then back to the cooling terminal again. The cooling terminal of the timer is wired to the compressor and the evaporator fan motor circuits. The defrost terminal is wired to the defrost heater (or a defrost solenoid valve) circuit. (See figs. 54C through 54L for more detail.)
When the timer takes the system into the defrost cycle, the compressor and fan motors remain de-energized, and the defrost heater becomes energized. When it takes the system into the cooling mode, the compressor and evaporator fan motors become energized while the defrost heater remains de-energized.
The four timer terminals are numbered on the back.
When air (which contains moisture) comes in contact with a cold evaporator coil, the moisture condensates and immediately changes to a layer of ice. If the evaporator coil is not heated from time to time, the accumulation of ice will insulate the coil, preventing it from absorbing the heat from its surrounding air inside the unit, and the unit will lose its ability to cool. When the unit is in the defrost mode, the defrost heaters are energized and the evaporator fan(s) de-energized. The water produced by the melted ice flows by gravity through a plastic pipe and collects in a tray on or beside the compressor where it is heated and evaporated by the condenser and the heat from the compressor.
In many models, an electric heating element is placed around the inlet of the drain tube. It is wired into the defrost heater circuit. Every time the timer takes the unit into the defrost mode, this drain heater is also energized to prevent drain water from freezing in the drain line and clogging it.
In side-by-side units, this drain heater can be seen at the bottom of the evaporator around the drain opening when the evaporator panel is removed (see fig. 9). In the case of units with the freezer on top, it is on the bottom of the freezer compartment around the drain opening.
Different evaporator designs have different defrosting requirements for optimum efficiency. Some evaporators must be defrosted every six or eight hours, some require defrosting every twelve hours, and some in commercial systems are designed to be defrosted after every cooling cycle in addition to the periodic defrosting at regular intervals. Timers are usually mounted inside the cabinet next to the cold control or behind the toe plate in the front.
One of the most common problems leading to timer replacement occurs when the timer motor freezes in one position. When it gets stuck in the defrost cycle, the refrigeration unit remains in the defrost cycle, the compressor no longer runs, and the evaporator panel in the freezer compartment feels warm to the touch. When it gets stuck in the cooling cycle, temperature in the freezer and fresh-food compartments goes abnormally high as the compressor runs continuously, causing a thick layer of ice to form on the evaporator coil.
Timer terminals rarely get fused together. This causes the defrost heater to remain on while in the cooling cycle. When this happens, the compressor runs continuously and the unit no longer cools. In this case, the timer must be replaced.
