UM1542 datasheet(10/43 Pages) STMICROELECTRONICS

10 / 43 page

Functional description

UM1542

10/43

Doc ID 023172 Rev 1

2.2.2

Compressor control

The ACST610-8FP device (referred to as Q1 in Appendix B) is used to turn on and off the

compressor according to the evaporator temperature, sensed through the NTC resistor.

Enough room around this AC switch has been left available to add a heatsink if needed.

2.2.3

Light bulb and buzzer control

The slider SW2 on the thermostat board has been placed on the board in order to simulate

the fridge/freezer door. A light bulb is switched on or off according to the position of the door

switch, open or closed respectively. The embedded switch may be replaced by an external

switch using connector J15, in which case switch SW2 must be set in the “Open” position.

If the fridge door or the door switch remains in the open position more than one minute, the

buzzer PZ1 sounds and the LEDs D4, D5, D7, D8, D9 flashes. The buzzer can be stopped

by pressing the Temp+ or Temp- button once (the LEDs keeps on flashing while the door is

in the open position). In the PC GUI driven mode the buzzer can be stopped with the GUI

software (refer to the “Help” menu of the GUI). When the door is open, the temperature

order can only be changed using the GUI software.

The light bulb is driven by the ACS102-6TA, Q4 on the thermostat board (Appendix B).

It should be noted that an R23 resistor has been added in series with Q4. Indeed, as the

lamp lifetime ends, the filament breaks. The overall filament can be short-circuited by the

flashover, and the load current is no longer limited. This current can exceed the i²t capability

of the ACS and destroy it (cf. AN1172). To avoid destroying the ACS102 at each lamp

flashover, a power resistor is added in series with the light. This resistor is rated in order to

limit the ACS current to its ITSM value (10 A for a 10 ms half sinus conduction). In this case,

a 33

Ω 1/2 W resistor is sufficient.

These resistor pads could be also used to put an inductor to limit the dI/dt if a CFL/LED lamp

is used in dimming mode.

2.2.4

Defrost resistor and fan control

Defrost resistor is controlled only when the thermostat is set to “Defrost” or to “air

circulation” versions; while the fan is controlled only in the “air circulation” version.

The defrost resistor is driven by the ACS110-7SB2, Q2 on the thermostat board (see

Appendix B).

This device is turned on following the “Defrost activation delay” parameter time defined in

the GUI software. This time is compared to the sum of the compressor ACST ON times.

When this sum is higher than the “Defrost activation delay”, defrost resistor ACST is enabled

and then switched on as soon as the evaporator temperature is higher than THIGH_lim

(Figure 4). It remains on during the “Defrost duration” time defined in the GUI software.

The conduction starts just before the conduction cycle of the compressor in order to reduce

power consumption.

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