January 2006

Rev 2

1/17

AN1353

Application note

ASD

ST62000C software description for cooling thermostat applicatons

1

Introduction

In this document, we explain the software of an Electronic thermostat bread board. The

demonstration kit has been developped by STMicroelectronics and is available under

THERM01EVAL reference.

This board illustrates the operation of a low cost electronic thermostat for 220-240V 50Hz

cold appliances, including STMicroelectronics ACS102-5TA, ACST6-7ST and ST62 devices.

The microcontroller will ensure four functions:

●

Temperature regulation (temperature capture through NTC resistor + Hysteresis

regulation).

●

Compressor monitoring: the motor is controlled depending on fridge temperature. To

start it, the starting triac Ts and the run triac Tr are triggered simultaneously for 500ms.

Then, only the run triac will continue to conduct.

●

Overcurrent detection: this is based on the measure of the peak current using a shunt

resistor. During the one second (500 ms+500 ms) of starting transient, this routine

does not run.

●

Internal light bulb control.

2

Hardware configuration

2.1

General information

The power supply of the microcontroller is a capacitive one. Its particularity is that the VSS

is 5V less than the Neutral. This power supply can be called a “negative supply”. This

generates flowing out current from the ACS/ACST gates (ACS are triggered only with a

negative gate current). This feature must also be kept in mind when the overcurrent

detection is implemented. It will define in which polarity the current can be sensed.

Figure 1 illustrates the board electrical circuit.

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