Frost on an evaporator coil is normal. Frost that keeps growing is not. Once the ice layer gets thick enough to restrict airflow, a walk-in cooler begins short-cycling, a display case drifts out of its temperature window, and the compressor runs longer to deliver less cooling. The part that decides when the defrost heater starts and when it stops sits in one of the least comfortable places in the circuit: bolted onto the coil it protects.
We make defrost thermostats for supermarket display cases, walk-in coolers and industrial refrigeration rooms. Most of the failures we hear about are not electrical. They are moisture problems.
A refrigeration coil with a bimetal switch on it lives through a repeating pattern. The coil ices while the cabinet holds temperature. The defrost heater warms the coil, the bimetal snaps at the set point and ends the cycle. The melt water drains, and within minutes the coil is back below freezing with condensation on every cold surface.
That last stage is what breaks inexpensive switches. Humid air meets a surface colder than its dew point and turns to liquid on the housing, the terminals and the joint between them. Our answer is a stainless steel housing closed by laser welding rather than a gasket, because a hermetic joint does not care how many condense-and-dry cycles pass over it. Insulation resistance above 100 MΩ at 500V DC is the figure that keeps a wet switch from leaking current across its own body.
Datasheets are read against real cycles, not against a bench. These are the values that decide whether a defrost thermostat survives a walk-in freezer or an industrial refrigeration room:
Defrost is a switching load with an edge to it. The heater pulls full current the moment the contacts close, and they open again while the coil is still warm. Plain silver contacts can weld under that treatment, which is why our defrost thermostats use silver-nickel contacts with arc-quenching behaviour. When a welded contact sticks, the heater keeps running and the cabinet warms up while the control board still reports a normal cycle.
Calibration is the other half. Bimetal elements for defrost duty see faster temperature change than most thermal switches, so every unit is tested at several temperature points that mimic a real defrost cycle, with response time and accuracy checked together. Response time is typically under 10 seconds, and reset is automatic, so the switch returns to its normal position without anyone opening the cabinet.
Over-defrosting and under-defrosting cost money in different ways. A cycle that runs too long spends energy heating a coil that is already clear, and it pushes cabinet temperature up around the food. A cycle that ends too early leaves frost behind, and airflow falls until the compressor cannot recover. Automatic termination closes that gap, because it ends the cycle on coil temperature instead of on a fixed timer.
Display cases, cold storage rooms and industrial refrigeration units all cycle differently, and a switch that suits one will be wrong for another. If you are replacing a part in an existing cabinet, send us the set point and the electrical rating and we will match it. For new designs we build to your drawing, including lead length, bracket shape and terminal style.
The full rating table sits on the defrost thermostat product page. Related parts from the same family are the bimetal thermostat range, the adjustable thermostat and the snap action thermostat. Samples are available for evaluation; tell us the cabinet type and the defrost cycle you run.
