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Johnson A421 Digital Temperature Controller

Johnson A421 Digital Temperature Controller


The A421 Series controls are single stage, electronic temperature controls with a single-pole, double-throw (SPDT) output relay. The controls feature an adjustable backlit LCD for viewing the temperature and status of other functions, and a three-button touchpad for setup and adjustment. An LED indicates the On/Off status of the output relay. The A421 control is 120 VAC.

The A421 controls provide heating or cooling control, sensor offset, temperature setback, adjustable anti-short cycle delay, and a restricted user adjustment mode. The temperature units can be displayed in °F or °C. The temperature adjustment range is -40 to 212°F or -40 to 100°C. The A421 automatically changes from heating and cooling. The A421 controls are available in Type 1/IP20 high-impact plastic enclosures suitable for surface or DIN rail mounting and Type 4X/IP66 watertight, corrosion-resistant surface mount enclosures.

Feature Overview:

  • Use to heat or cool a fermentation environment
  • For use with a refrigerator or freezer, it operates compressor when ambient temperature rises above your programmed temperature
  • For use with a heater, it activates when the temperature drops too low
  • Backlit display for even easier-to-read display
  • Faster Programming with three-button interface
  • Internal defrost settings
  • Sensor probe monitors the inside temperature while the easy-to-read LCD display shows it in either Fahrenheit or Celsius
  • Includes 6’ extension cord with plug and 6’ sensor leads
  • No additional wiring for installation is needed
  • 120 VAC line voltage
  • Range: -40° F to 212° F
  • NEMA 1 enclosure, suitable for dry environments


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