Automatic defrosting refrigerator. Scheme, description

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Automatic defrosting refrigerator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Home, household, hobby

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In refrigerators with a mechanical regulator, the temperature is measured at the evaporator. It happens that the evaporator is covered with frost, and the thermostat starts to operate with an error, causing malfunctions in the operation of the entire refrigeration unit. To combat this undesirable phenomenon (including the appearance of frost), the refrigerator has to be turned off periodically. Some designs have a semi-automatic defrosting mode, for which a heating element with a corresponding button is built into the system.

But devices for automatically turning on the refrigerator defroster, including home-made ones, are becoming more common.

The proposed electronic control device is designed for commercial refrigeration units. With no less success, it can also be used in household refrigerators with separate switching on of the compressor and the heating element of the defroster. The device consists of thermostatic and timing parts. The first, by measuring the temperature in the chamber, maintains cooling in the mode determined by the electronic controller. The second one, every 2-3 hours, turns on the heating element for 10-20 minutes to thaw frost, while the mode of operation of the thermostat is blocked.

At the heart of the thermostatic part of the device is a temperature meter made on a DA1 comparator with a measuring bridge R1R2R6R7R8, the right lower arm of which - the thermistor R2 - serves as a temperature sensor. On the logic elements DD3.3 and DD3.4, a blocking assembly is assembled, and on transistors VT1 and VT4, a current amplifier with an electromagnetic relay K1 as a load, which switches on the contacts K1.1 of the electric motor M1 of the refrigerator compressor.

Schematic diagram of the electronic refrigerator control device (click to enlarge)

PCB topology (click to enlarge)

The "heart" of the time-setting part of the device is an electronic unit on the DD1 chip, which includes a master oscillator, as well as frequency dividers by 32 and 768. The DD60 chip is an additional divider with a division factor of 2. On the logic elements DD6 and DD3.1. 3.2, an RS flip-flop is assembled, and on transistors VT3 and VT4 - a current amplifier, the load of which is relay K2. Through contacts K2.1, the heating element RM of the defroster is switched on.

The operation of the thermostat is based on a comparison of the voltages taken from the shoulders of the measuring bridge, which incorporates a sensor - thermistor R2, the signal from which is fed to input 4 of the comparator DA1.

From the output 9 of the comparator, the temperature signal is supplied (through the blocking unit - logic elements DD3.3 and DD3.4) to the input of the current amplifier, made on transistors VT1 and VT2. The load here is the electromagnetic relay K1. At a temperature above the threshold set by the variable resistor R8, a high level voltage is set at the output 9 of the comparator. Transistors (VT1, and then VT2) open, causing the relay K1 to operate, which, with contacts K1.1, connects the compressor motor M1 to the AC mains. The temperature in the refrigerator will decrease, causing an increase in the resistance of the thermistor R2.

With the last threshold value reached, the comparator is triggered, and a low voltage is set at its output 9. Transistors VT1 and VT2 of the current amplifier are closed. Relay K1 releases its armature, thereby opening contacts K1.1 in the power supply circuit of the compressor motor M1.

Resistors R9 and R12, providing hysteresis for DA1, contribute to a more accurate operation of the thermostat. The 9 V supply voltage of the measuring bridge and comparator is stabilized by the DA2 microcircuit.

Capacitors C3 and C5 are anti-jamming. Resistor R14 serves as an open collector load of the comparator, and R15 limits the base current of transistor VT1. The interlock (DD3.3 and DD3.4) disconnects the thermostat from the current amplifier for the duration of the operation of the heating element RH of the defroster. Diode VD2 shunts a surge of self-induction voltage on the winding of relay K1 at the moment the transistor closes.

The basis of the timing part is a timer on DD1 and DD2 microcircuits. When the supply voltage is turned on, the DD1 microcircuit is set - through the reset circuit RЗС1 - to zero (log. 0), and the R6 flip-flop - through the R16С6 circuit - to a single state (log. 1). Then at the output 4 DD3.2 and at the input 2 DD3.1 there will be log.O, and at the output 3 DD3.1, connected to the reset input I of the DD2 chip, - log.1. The divider counter is then cleared to zero.

The master oscillator in (on the DD1 chip, resistors R4, R5, R11 and capacitor C2) generates pulses from 175 to 280 Hz. The frequency is changed by a variable resistor R11. The oscillation period of the generator pulses at the middle position of the R11 engine is about 4,58 ms. Resistor R4 limits the discharge current of capacitor C2.

Through the connections inside the DD1 chip, the pulses of the master oscillator G are transmitted to the CT divider. In this case, the generation period increases by a factor of 32, and a signal with an oscillation period of 768 minutes appears at the output S1. The latter, entering the C2,5 input of the DD2 microcircuit, is divided by another 1. Thus, at the output M of the 60 microcircuit, pulses are obtained with a period of 001 hours.

From the output M of the DD1 microcircuit, the first positive voltage drop, which appears after about 1,5 hours, passes through the differentiating circuit R13C4, resistor R17 and, entering 1 of the DD3.1 logic element, switches this RS flip-flop. At output 3 DD3.1, a low voltage appears, and at output 4 DD3.2, a high voltage appears. The latter through the resistor R19 opens the transistors VT3 and VT4 of the current amplifier; relay K2 is activated and contacts K2.1 connects the heating element Rn to the mains.

The high-level voltage taken from the output 4 DD3.2 is fed to the input 13 of the blocker DD3.4, which, acting on the enable input of the signal, closes the transistor VT1, as a result of which the thermostat is disconnected from the current amplifier.

At the same moment, the low-level voltage supplied from output 3 DD3.2 to input I of the DD2 microcircuit allows the divider to work by 6. The pulse from S1 DD1 is fed to the CP of the DD2 microcircuit. Then, at pin 5 of this microcircuit, a signal is obtained with a period of 15 minutes, which, entering 6 DD3.2, switches the R6 flip-flop, and a low-level voltage appears at output 4 DD3.2. Transistors VT 1 and VT2 close, relay K2 releases the armature and contacts K 2.1 disconnects the heating element Rn of the defroster from the mains.

The signal at the input 13 DD3.4, affects the permission input. The blocker opens, and the thermostat is connected to the current amplifier. The dividers on the DD1 and DD2 chips are set to zero, and the R6 flip-flop is set to a single state.

With the arrival of the next pulse from output 10 DD1, the positive drop of which in the steady state is repeated every 2,5 hours, the defroster will turn on for 15 minutes. To power the device from a 220 V AC mains, there is a built-in adapter with a step-down transformer T1, a rectifier bridge VD3-VD6, a 9-volt voltage regulator DA2 and a capacitive filter C7-C9.

All components of the device (except for the transformer T1, thermistor R2 type MMT-1, as well as variable resistors R8 and R11 type SP4-1) are mounted on a printed circuit board measuring 118x65x1,5 mm from one-sided foil fiberglass. Fixed resistors type MLT-O.125. As capacitors C1-C7, it is recommended to use K73-9, and C8 and C9 - electrolytic K50-16. Semiconductor diodes - silicon: KD102A (VD1, VD2) and KD106A (VD3-VD6).

The transistors are also silicon. In the input stages - KT315G with the possibility of replacement with KT3102A (VT1 and VT3), in the output stages - KT815A or KT817A (VT2 and VT4), installed vertically, without a radiator. Chips: DA1 - K554SAZ, DA2 - KR142EN8G, DD1 - K176IE12, DD2 - K561IE8, DD3-K561LE5.

Automotive electromagnetic relays type 113.3747-10, the powerful contacts of which easily withstand repeated switching on of both the compressor motor M1 and the heating element Dn of the defroster. Transformer T1 with a power of 2-4 W (used in many industrial adapters).

Debugging of the mounted printed circuit board is performed in a state disconnected from the refrigerator. Instead of a load (electric motor M1 and heating element Rn), ordinary table lamps are used.

The thermostatic part of the device should be sensitive to temperature changes in the range from minus 14 to plus 4°C. However, it is difficult to deal with cold when debugging electronics, so it is recommended to replace the standard R8 with a 1,5 kΩ resistor. Then the adjustment of the thermostat can be carried out already within the more accessible limits for this: plus 18-40 ° C. And to speed up the tuning work on the timing part of the device, it is recommended to reduce the capacitance of the capacitor C2 by a hundred times, then the pulse period from the output M of the DD1 microcircuit will be reduced to 90 s.

A checked and adjusted device (after restoring the elements required by the scheme) is mounted in the refrigerator.

Author: G.Skobelev

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