ThermoWorks 9135-156 HART SCIENTIFIC 3-POINT IR CALIBRATOR - Manual (Page 19)

7 Controller Operation
The 9135 calibrator uses a precise analog proportional-integral controller that
maintains the temperature of the IR target. The set-point is controlled by a spe-
cial digital switching circuit that allows one button to advance through the se-
quence of set-points. It also contains a power on reset circuit that causes the
controller to default to the first set-point immediately after the power is
switched on. The controller senses the temperature of the IR target using a
thermistor sensor imbedded in the plate. It compares the resistance of the
thermistor to preset set-point resistances to determine whether the IR target
should be heated or cooled and how much power should be applied. The con-
troller uses a solid-state relay circuit to apply power in a time-proportioned
manner to a resistance heater attached to the IR target. The ready indicator cir-
cuit uses the difference signal between the temperature sensor and the set-point
to determine whether temperature stability has been achieved or not and
switches the ready LED on or off accordingly.
19
7 Controller Operation