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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Refrigerator control unit. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Home, household, hobby The author was forced to start improving the STINOL-104 refrigerator by a domestic nuisance - for the second time in five years of operation, the thermostat failed. It was not possible to purchase a new one to install it yourself - the device was sold at a completely unacceptable price, which included the cost of installation. The homemade device brought to the attention of readers does not simply replace the standard thermostat. Additional functions are provided to protect the refrigerator in many emergency situations that occur during operation. The weak point of all compressor refrigerators is the overload of the electric motor driving the compressor when it is turned on again a short time after stopping. The cause of the overload is the high refrigerant pressure remaining in the condenser of the refrigeration unit for quite a long time. The operating instructions for the STINOL refrigerator require that the delay between turning off and turning on the compressor be at least 3 minutes. But with the unexpected power outages and restarts that are typical today, it is not possible to fulfill this requirement without “calling for help” from electronics. To protect the electric motor, refrigerators have a thermal relay. Usually it is combined with a starting relay and is called a start-protective relay [1]. However, practice shows the ineffectiveness of such protection. Like any other electrical appliance, it is useful to protect a refrigerator from significant deviations of the network voltage from the nominal 220 V. A large number of publications on this topic (for example, [2, 3]) indicate the relevance of the problem both in rural areas and in big cities. The proposed control unit performs the following functions:
The status of the control unit is indicated by LEDs "Operation" (the compressor is on), "Pause" (the compressor is off), "Blocking" (the five-minute turn-on prohibition has not expired), "<" (mains voltage is below the minimum permissible), ">" (voltage in the network above the maximum allowable). The block diagram is shown in Fig. 1. It consists of a thermostat unit on the DA2 chip, a turn-on delay timer on the VT1 transistor and elements DD1.1, DD1.2, a network voltage control unit on the DD1.3, DD1.4 elements and the DD2 chip, an actuator on the VT2 transistors, VT3.
The contacts of relay K1, connected in parallel, are included in the compressor motor circuit instead of the contacts of the standard refrigerator thermostat. The power supply unit of the unit consists of transformer T1, a rectifier (diode bridge VD1) and an integrated stabilizer DA1 for a voltage of 9 V. To prevent changes in the load on the rectifier when relay K1 is activated and released from affecting the operation of the voltage control unit, resistor R27 is provided, connected by transistor VT3 to the rectifier when the relay winding is de-energized. The resistance of the resistor is equal to the resistance of the relay winding, so the current consumed from the rectifier remains unchanged. Let's say the unit is connected to the network at a rated voltage of 220 V and the voltage control unit does not affect its operation. Transistor VT1 is closed, capacitor C2 is discharged, the logic level at the output of element DD1.2 is low, diode VD3 is open, therefore the thermostat on op-amp DA2 is locked in a state corresponding to the low temperature in the refrigeration chamber, therefore, the compressor is turned off. Transistor VT2 is closed, relay K1 is de-energized. LEDs HL1 “Lock” and HL5 “Pause” are lit. 5 minutes after charging capacitor C2 through resistor R2 to the switching threshold of the Schmitt trigger on elements DD1.1, DD1.2, the level at the output of the latter will become high, diode VD3 will be closed and the thermostat will be able to work. The HL1 LED will go off. As the temperature in the refrigerator compartment increases, the resistance of thermistor RK1 and the voltage drop across it decrease. If the temperature is such that the voltage at the inverting input of the op-amp DA2 is less than at the non-inverting one, the level at the output of the op-amp is high, which leads to the opening of the transistor VT2 and the operation of relay K1, which turns on the compressor. The HL4 LED is lit, the HL5 LED is not. As the temperature in the refrigerating chamber decreases, the voltage at the inverting input of the op-amp increases, which leads to a change in the state of the op-amp and the compressor is turned off. LED HL4 goes out, LED HL5 lights up. The voltage drop across the collector of transistor VT2 at the moment the relay is released causes capacitor C6 to charge and transistor VT20 to open briefly (for 1 ms) with a charging current pulse. Discharged through the opened transistor, capacitor C2 again, as after connecting the unit to the network, begins to slowly charge, which leads to a five-minute prohibition of turning on the compressor. Diode VD2 protects the emitter junction of transistor VT1 from a negative pulse when capacitor C6 is discharged through transistor VT1, which opens when relay K2 is turned on. The required temperature in the refrigerator compartment is set using a variable resistor R16. The width of the thermostat hysteresis loop is controlled by variable resistor R20. The need to change the hysteresis during operation is debatable, but during the initial adjustment this cannot be avoided. The hysteresis should be sufficient so that the compressor does not turn on too often, and during breaks in its operation, the temperature of the walls of the refrigeration chamber reaches a positive value and the frost formed on them melts without accumulating. Let's consider the operation of the mains voltage monitoring unit. If it is within acceptable limits, the voltage at the inputs of element DD1.3 is lower, and at the inputs of element DD2.1 is higher than their switching threshold. The levels at both inputs of the DD2.3 element are high, and at its output - low, allowing all other nodes of the block to work in the manner described above. When the network voltage is less than permissible, element DD2.1 will change state. The logical level at its output will become high, and the same will be at the outputs of elements DD2.3, DD2.4. LED HL3 will light up, and transistor VT1, opened by the voltage supplied to its base through resistor R19, will discharge capacitor C2, thereby blocking the compressor. With the restoration of normal voltage, the HL3 LED will go out, the VT1 transistor will be closed, and after the time necessary for charging the capacitor C2, the thermostat will be allowed to operate. If the network voltage exceeds the permissible level, a low level at the output of element DD1.3 will lead to a high level at the outputs of elements DD1.4 and DD2.3. Then everything happens in the same way as when the voltage is reduced, only instead of the HL3 LED, HL2 lights up. It is recommended to set the mains voltage values at which the protection is triggered equal to 242 (with trimming resistor R5) and 187 V (with trimming resistor R6). The unit will perceive an interruption in the power supply as an unacceptable drop in voltage. It is important that restarting the compressor is prohibited if the duration of the break exceeds that required to stop it. However, the reaction should not be too fast - the likelihood of false alarms will increase (for example, caused by the inclusion of powerful electrical appliances in the same network). The response time of the described device during an abrupt decrease in the voltage in the network - approximately 65 ms - is the sum of the required discharge of capacitor C1 to a voltage corresponding to the permissible minimum, and the time of discharge of capacitor C2 through the opened transistor VT1. The reaction time to a sudden increase in voltage in the network is less - 25...40 ms. It is spent on recharging capacitor C1 to the set threshold and discharging capacitor C2. All elements of the control unit, with the exception of relay K1, variable resistors R16 and R20, thermistor RK1 and fuse link FU1, are placed on a single-sided printed circuit board (Fig. 2).
Capacitors 04, C5 - KM-6 or other ceramic, the rest are imported oxide, and capacitor C2 is of the LL series (with low leakage current). The permissible voltage of capacitors C1 and C6 (25 V) was selected with a reserve in case of an emergency increase in network voltage. Trimmer resistors R5 and R6 - SP4-1, constant resistors - MLT. Variable resistors R16 and R20 - SPZ-12 with a linear (A) dependence of resistance on the angle of rotation of the shaft. The main criterion in favor of choosing these particular resistors was that the thread on their mounting sleeve is the same as that of the standard refrigerator thermostat. LEDs HL1-HL3 are red, and HL4 and HL5 are green. In addition to those indicated in the diagram, other LEDs are also suitable, including domestic ones, of suitable sizes and glow colors. The KR140UD608A microcircuit can be replaced with KR140UD608B or KR140UD708. Transformer T1 should be chosen to be of small height so that it can be placed in the instrument compartment of the refrigerator (see below). The author used a ready-made transformer with a diameter of 40 and a height of 28 mm on a toroidal magnetic core with a secondary winding of 12 V at a current of 0,3 A. Commercially produced transformers, for example, TP-321-5 and TPK2-22 are suitable. It should be taken into account that in emergency mode the network voltage sometimes increases to 380 V. This happens, for example, when the neutral wire of the main cable breaks. If transformer T1, unable to withstand such voltage, fails, this will not lead to the inclusion of an expensive compressor, which is undesirable in this situation. The fuse link FU1 (VP1-1) is designed to protect the transformer from fire. Special attention should be paid to its quality and under no circumstances should it be replaced with a surrogate one. Thermistor - MMT-1 or MMT-4. If its nominal resistance differs from that indicated in the diagram, it is necessary to change the value of resistor R12 by the same amount. However, you should not use a thermistor with a resistance of more than 3...4 kOhm, this will worsen the noise immunity of the thermostat. Relay K1 - RP-21-004 with a 24 V DC winding. The test showed that 12 V is enough for it to operate, and at a voltage of 16 V the relay works quite reliably. You can use another relay, for example, RENZZ. When selecting a replacement, special attention should be paid to the ability of the relay contacts to withstand the compressor starting current, reaching several amperes. The mounted printed circuit board and relay K1 are placed inside the service compartment at the top of the refrigerator. The relay contacts connected in parallel are connected instead of the main contact group of the standard thermostat. Its second contact group, designed to turn off the refrigerator for a long time, is replaced with a jumper. Now the refrigerator can be disconnected from the network in only one way - by removing the power plug from the socket. According to the author, this ensures the greatest electrical safety during preventive and repair work. The unified front panel of the compartment has holes for two thermostats. However, the second one is available only in two-compressor refrigerators; in a regular single-compressor refrigerator, it is convenient to install a variable resistor R20 here. Variable resistor R16 is installed in place of the remote standard thermostat. In the front panel of the service compartment, you will have to drill five more holes into which the LEDs mounted on the control unit board will fit. Explanatory notes can be placed on the panel next to them. The terminals of the primary winding of transformer T1 (one of them is through a fuse-link FU1 soldered into the wire gap) are connected to the network wires running in the refrigerator to the power-on indicator lamp. The shielded wire connecting the temperature sensor - thermistor RK1 - to the control unit board is placed in an insulating, for example, polyvinyl chloride tube and laid along the route of the remote metal bellows tube of the standard thermostat. The thermistor itself is installed inside the refrigeration chamber where the bellows tube ends. It must be well insulated and protected from moisture and frost. Setting up the control unit begins with adjusting the mains voltage control unit. To do this, using an adjustable autotransformer (LATR), they lower the voltage to 187 V. By rotating the slider of the trimming resistor R6, they achieve an unstable glow (“blinking”) of the HL3 LED. Then the voltage is increased to 242 V and the trimming resistor R5 is adjusted in the same way, focusing on the state of the LED HL2. After adjustment, the trimmer resistor sliders should be sealed with nitro paint. Next, having disconnected the unit from the network, move the variable resistor R16 to the minimum position, and R20 to the maximum resistance. Set (using LATR) the mains voltage to 220 V and turn on the unit. LEDs HL1 and HL5 should light up, after approximately 5 minutes LED HL1 should go out. The duration of its glow and blocking the start of the compressor, if necessary, is changed by selecting resistor R2. To facilitate further adjustment, the inputs of the DD1.1 element (pins 8, 9) are temporarily connected by a jumper to the +9 V circuit, for example, to pin 14 of the DD1 microcircuit. Thermistor RK1 is immersed in melting ice. After its temperature has stabilized, the resistance of the variable resistor R16 is gradually increased, ensuring that relay K1 is activated, LED HL4 lights up and HL5 goes out. The reverse switching should occur with a slight decrease in the resistance of resistor R16. Hysteresis (the difference in the positions of the variable resistor R16 motor when the relay is activated and released) should increase with decreasing resistance of the variable resistor R20. At the end of the test, the previously installed temporary jumper is removed. Before turning on the refrigerator with the new control unit, the sliders of the variable resistors R16 and R20 are set to the middle positions. After allowing the refrigerator to operate for a sufficient time to stabilize the temperature regime, you should make sure that the frost that forms on the back wall of the refrigerator compartment during operation of the compressor thaws during the pause. If this does not happen, you need to increase the hysteresis with variable resistor R20. The average temperature in the chamber is changed by variable resistor R16. If it is not possible to achieve the desired temperature regime using variable resistors, you should select resistors R14 and R15. Some refrigerators provide automatic defrosting of the freezer compartment - every 8...10 hours of operation, the automation forcibly turns off the compressor for a while, during which specially installed heating elements operate. In this mode, the compressor does not operate even when relay K1 is activated and LED HL4 is on. This situation should not be confused with the one that occurs when the compressor motor protection thermal relay is activated, which is accompanied by the same symptoms. It is quite simple to distinguish a “planned” compressor shutdown from an emergency one. In the latter case, the fan installed in the freezer continues to operate (with the door closed). The unit can also be installed in compressor refrigerators of other models, taking into account their characteristics, changing the placement of the temperature sensor, adjustment and indication controls, and, if necessary, the dimensions of the printed circuit board. By removing the elements of the thermostat - thermistor RK1, microcircuit DA2, diode VD3, resistors R12-R16, R20, R21, capacitors C4, C5 - and connecting the left terminal of resistor R23 in the diagram with the output of element DD1.2, the block can be used to protect any electrical appliances from mains voltage fluctuations. Literature
Author: A. Moskvin, Yekaterinburg
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