Digital Differential Control Thermostat

November 2009

Farm Electronics has developed a very useful and flexible differential temperature control using the Siemens LOGO PLC.

This unit can provide solutions to many control requirements for more basic systems where a full blown Cropscan type controller is much too expensive and sophisticated to justify.

The basic unit has two PT100 temperature sensors, one for AMBIENT, the other for CROP.  The operator inputs a desired CROP target temperature and a minimum acceptable AMBIENT (Frost) temperature on the LCD keypad.  A cooling OFFSET (Differential) is also set.  If the CROP sensor rises above the target and the AMBIENT temperature is lower than the Crop sensor value minus Offset value, the fan will be started.  (Note:  Up to 4 fans can be started sequentially if required.  Cooling will continue until the CROP target is reached or the differential becomes too small or the AMBIENT falls below the preset FROST level.

The unit has an HOURS RUN input which can record actual fan run times if connected via the fan start contactor.

CROP and AMBIENT temperature values are displayed on the LCD screen.

Customised Solutions

Because of the flexible programming possible on the PLC controller of this unit, special applications can be accommodated, if necessary.  Additional CROP or CONTROL temperature sensors can be added, if required, as can further control relay outputs.  Please tell us what you need to achieve and we will advise a solution.