Instead of a refrigerator relay. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Instead of a refrigerator relay. 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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It is well known that the compressor start-up relay and thermostat are the least reliable elements of domestic domestic refrigeration units. In most cases, the reason for their failure is associated with the burnout of the pads of the protective node. The impossibility of replacing contacts makes it necessary to fix the problem by simply installing a new relay. The combination of low reliability and high cost of these units makes it very expedient to create their electronic analogues. The proposed control unit performs the functions of a start-up relay and a thermostat.

The schematic diagram of the control unit is shown in the figure.

(click to enlarge)

The comparator on the op-amp DA1 is designed to compare two voltages: the incoming temperature sensor RK1 and the reference, the value of which is determined by the position of the variable resistor R4 slider. Resistor R6 is included in the positive feedback circuit that determines the width of the hysteresis. The control voltage from the output of the op-amp DA1 (pin 10) is fed to the base of the transistor VT1, which operates in the key mode and determines the state of the thyristor of the optocoupler U1. and, consequently, the triac of the starting node VS1. When the temperature in the refrigerator compartment rises, a high voltage level appears at the output of the comparator, opening the transistor VT1. As a result, the triac VS1 turns on and a current of several amperes begins to flow through the winding of the relay K1. Contacts K 1.1 of this relay are closed and the starting winding of the compressor is connected to the network through the opened triac VS2. After its normal start, the current flowing through the relay K1 decreases to the operating value and the triac VS2 closes. The use of a triac in the circuit of the starting winding of the compressor reduces the current through the contacts K 1.1 and protects them from burnout.

The base of the transistor VT1 is also connected to the open collector output of the DD1 chip. on which the threshold protective device is assembled. The current sensor in this device is resistor R15, included in the compressor control circuit. The voltage from the resistor R15 is fed to the VD1C7R16 rectifier and then through the resistor R13 to the input of the precision Schmitt trigger on the DD1 chip. The upper trigger threshold is determined by the voltage at its input II (pin 5). which, in turn, depends on the resistance of the resistor R1 1. With the values \u7,5b\u2bspecified on the diagram and the voltage supplying the trigger + 1 V, it fires at a current of about 7 A. When the Schmitt trigger is switched, its output transistor opens, while the transistor VT20 closes and the compressor disconnects from the network. In this state, the control device remains until then. until the capacitor CXNUMX is discharged to a voltage corresponding to the lower switching threshold of the DDI trigger, i.e., approximately within XNUMX s. This is followed by a new attempt to start the compressor.

Since when the refrigerator is normally turned on, due to an increase in temperature in its working volume, the protection device should not respond to a significant excess of the operating current within 1 ... 2 s, it is blocked for this time. It is provided by a cascade on the transistor VT2. When a positive voltage appears at the output of comparator DA1, capacitor C6 begins to charge through circuit R12. R10 and the base-emitter junction of the transistor VT2. The charging current opens the transistor VT2. and the protection device is blocked by connecting the inputs of the trigger DD1 (pins 2. 6) to the common wire for the above time. Resistors R12. R2 provide reliable closing of transistors VT 1. VT2 in the absence of control voltage.

The control unit is powered by a small-sized +9 V direct voltage source. The current consumed by it does not exceed 25 mA.

The device uses fixed resistors S5-16MV (R15) and MLT-0.125 (the rest), variable - SPZ-4a, thermistor - MMT-4 (you can also use any similar resistor with a direct current resistance at a temperature of + 20 ° C about 1 kOhm). Capacitors C2. C7 - K50-29. C5. C6 - K53-4. the rest - any small-sized.

In order to simplify the design, the surviving winding K1 and starting contacts K1.1 from the old refrigerator relay were used.

Establishing a properly assembled device comes down to setting the threshold for the operation of the protection unit by selecting resistor R11 (when working on a dummy load) and simultaneously selecting resistors R1, R3 until the desired temperature control range is reached. The operating current of the control device is about 1 A, the protection current is 2 A, the accuracy of maintaining the temperature inside the refrigerating chamber is ± 0,25 ° C.

The device is directly connected to the network, so you need to be extremely careful when configuring it.

The design of the device should exclude the possibility of touching its elements during operation. It is best to mount it in a completely closed plastic box with a lid at the bottom and, if possible, seal the box, for example with plasticine.

For reliable isolation of the thermistor, we can offer such a technique. Solder flexible conductors about 0,6 m long to the leads of the thermistor and place it in a PVC tube about 1 m long. Bend the tube in half, bring the open ends of the tube inside the device case.

When connecting the device to the network, it is necessary to observe the phasing of the mains plug shown in the diagram. The common wire must not be connected to the refrigerator body.

Author: D. Pankratiev, Tashkent, Uzbekistan

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