MTC-20/2S User Manual
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version 2.7
page 9
3.4. Electronic
Control
and Protection Circuits
CONTROL LOOP
The sensor / thermometer, PI-controller, output power stage and heating element form a
closed loop control system. The desired temperature signal is compared with the output signal
obtained from the thermometer giving an error signal. This signal is amplified in the PI
controller and transferred to the output stage where it is converted to a high-power output
signal applied to the heating element (see also Figure 4).
ELECTRONIC PROTECTION CIRCUITS
Each system is equipped with two protection systems:
1. Sensor inputs: an electronic shut-off function disconnects the output if the sensor is
disconnected, broken or short-circuited.
2. Output protection: the output current and voltage are limited electronically.
Important
: The sensor used for the control must be in good thermal connection with the
heating wire.
If not overheating and damage may occur.
ALARM circuit
The internal thermometer (R2252 sensor) inputs are protected by the sensor alarm circuit (see
above). If this thermometer is used as input device for the PI controller, the ALARM circuit
disconnects the power output if the sensor is damaged or not connected. This state is indicated
by the red ALARM LED. If the DIRECT mode or EXT mode is used, the ALARM circuit is
disabled.
ACCURACY
Measuring accuracy of the internal thermometer is 0.1 °C, controller accuracy is 0.5 °C or
better (depending on the amount of heated solution).
3.5.
Modes of Operation
The system can be used in the following modes of operation:
DIRECT mode
DIRECT mode (no temperature control): the MODE switch (
14
, Figure 1) is set to the
DIRECT position. The control unit is not active, i.e. the output voltage is preset directly at the
DESIRED TEMP. control (
1
, Figure 1). With the setting from 25 to 50 the output voltage
increases linearly from 5 V to 12 V. The ALARM circuit (see below) is disabled in this mode.
The MTC-20/2S is designed with an unipolar output. If a resistive load is used, the system
can be used for heating purposes in all modes (SENSOR and DIRECT).
