Defrost Timer, refrigerator

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Joined Nov 30, 2010
Computer aided excess quality removal strikes again.
There is a defrost timer in all refrigerators and a very popular style is the 4 wire mechanical timer. In some refrigerators, the external wiring only uses three of the terminals.
The timer is made of a 2.5 watt synchronous AC motor driving a gear train which flops an SPDT switch.
The switch chooses between Run and Defrost. A typical time sequence is 8 hours of compressor run time and then 21 minutes (+/-3 minutes) of defrost time.
The 460 watt defrost heater usually gets the evaporator coil up to 70F in 9 minutes, then a temperature sensor opens that circuit. The other 12 minutes is a safety margin for the heater time (in case of high humidity) and a drip time allowance.

What I find for sale now is a white, all plastic timer, with a 12 volt motor out of a wall clock, trying to use 2 watts to drive the gear train. Not good enough. The defrost timer I bought failed in 5 months because of the underpowered motor. Of course, the warranty is 30 days on a timer which historically runs for 30 years and is still good when the refrigerator goes to the dump.

The cure, for now, is to demand the, "Universal" timer with the correct timing sequence for your refrigerator. If you can't find the specs after diligent searching, just guess. They range from 6 hours to 12 hours and the 8 hour timer fits my refrigerator. Because it runs only during the compressor cycle, it is about the same as a 12 hour timer that runs continuously. Whether it runs off the power cord or the thermostat is negotiable if you're an electronics nerd. It's all in how you connect the second motor wire when you assemble it. Instructions are in the kit. Having the original schematic is very reassuring.

The thing to look for is the 2.5 watt synchronous motor which is about 2 inches in diameter and comprises about half the volume of the entire timer. If you see a transparent plastic cover protecting what looks like the guts out of a 1.5V battery operated wall clock, that's the bad one.

Edited for correcting the values of the 2.5 watt motor and the defrost heater watts.
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