EP0154119A1 - Defrost control device for a refrigeration system - Google Patents

Abstract

1. Defrosting control device for a refrigeration system comprising a temperature sensor to be attached to the evaporator of a refrigerating machine, comprising an input circuit to which the temperature sensor is connected and which contains a first (13) and second (14) switching unit for a lower and an upper temperature switching point, comprising a timing generator (28) for successive defrosting cycles which is connected to a first output circuit (24) for switching the refrigerating machine on and off and simultaneously swithing an additional heater on and off, and to a second output circuit (27) for switching an additional fan on and off, the connections being conducted via the set inputs of a first (33) and second (43) bistable flip-flop circuit so that, when a timing pulse is generated, the refrigerating machine is switched off and simultaneously the additional heater is switched on by means of the first output circuit (24) by setting the bistable flip-flop circuits, and the additional fan is switched off by means of the second output circuit (27), the second switching unit (14) for the upper temperature switching point is connected to the reset input of the first bistable flip-flop circuit (33) so that the refrigerating machine is switched on again and the additional heater is switched off when this temperature switching point is reached, and the first switching unit (13) for the lower temperature switching point is connected to the reset input of the second bistable flip-flop circuit (43) so that the additional fan is switched on again when this temperature switching point is reached, characterized in that an alarm and safety circuit having a further timing generator (29) for monitoring the refrigeration system is provided which generates at least one timing pulse within one defrosting period and which is connected to one input (59) of a NAND circuit (40), that the output (54) of the first switching unit (13) for the lower temperature switching point and the output (34) of the first bistable flip-flop circuit (33) and the output (52) of the second bistable flip-flop stage (43) are connected to the other input (39) of the NAND circuit (40), that an alarm lamp (17) and/or an alarm switching unit (21) are connected to the output of the NAND circuit (40), that a third bistable flip-flop circuit (64) to maintain an alarm state, once generated, is connected between the NAND circuit (40) and the alarm lamp (17) and the alarm switching unit (21), and that the ouput of the third bistable flip-flop circuit (64) is conducted to priority-affected inputs (35, 68) of the first and second bistable flip-flop circuit (33, 43) so that, after an alarm has been signalled, the refrigerating machine and the additional fan are continuously switched on until the alarm has been acknowledged.

Description

The invention relates to a defrost control unit for a refrigeration system according to the preamble of claim 1.

Larger cooling systems usually consist of a larger goods room in which the goods to be cooled are stored and a separate evaporator room of a cooling unit. The thermal connection between the evaporator room and the goods room is established via a switchable additional fan (fan).

When operating such refrigeration systems, cold-carrying parts, especially the evaporator, are frosted. To maintain the cooling performance, these parts must therefore be defrosted regularly at repeating intervals. To do this, the refrigerator is switched off and an additional heater on the evaporator is switched on to accelerate the defrosting process. The fan, which otherwise blows the cold air from the evaporator compartment into the refrigerator compartment, is also switched off during the defrosting process.

To control the defrosting process, mainly time switches are used, which switch off the chiller and the fan at adjustable time intervals and at the same time switch on the additional heating. The cooling machine is switched on again and the additional heating is switched off by the time switch after a period of time which is set in such a way that the defrosting process is likely to have ended. Since the defrosting time depends on a number of boundary conditions, the setting of a fixed and estimated time period for the defrosting process is unsatisfactory. If the chiller is switched back on too early, the defrosting process has not yet ended and the full cooling capacity of the system will not be achieved. If the chiller is switched on too late, the defrosting process is safely completed, but the additional heating runs too long and the system parts become undesirably too warm. At the same time, there is a risk that the fan will blow warm air onto the refrigerated goods.

The fan is also switched on with a time delay from the time switch, this period of time being selected longer than the preceding one, so that the evaporator in particular has probably already reached its cooling temperature when the fan is switched on. Here too, the optimal time for switching on the fan depends on a number of conditions that cannot be satisfactorily taken into account in the fixed and estimated delay time. If the fan is switched on too early, there is a risk that undesired warm air will be blown into the refrigerator; if the fan is switched on too late, this is unfavorable in terms of energy, because the evaporator cools down very much without affecting the refrigerated goods.

In order to remedy the above deficiencies, an improved defrost control unit is already known which continues the sequence of the defrost cycles in a time-controlled manner, the cooling machine and the fan are switched on again in a temperature-controlled manner. For this purpose, a temperature sensor is preferably attached to the evaporator, which is connected to two switching units for an upper and a lower temperature switching point. The upper temperature switching point is set at about 0 C, so that when this switching point is reached the end of the defrosting process is recognized and the chiller is switched on again while the auxiliary heating is switched off. The lower temperature switching point is used to switch the fan on again after the defrosting process and can be set according to the desired cooling temperature. This ensures that the fan neither conveys air that is too cold nor too warm to the cold room.

Despite the improved function of the above defrosting device, a number of shortcomings and disadvantages have been recognized in practice. It was perceived as unsatisfactory that despite the relatively complex defrosting device with temperature sensor, just as with the earlier time switches mentioned at the beginning, there is no possibility of checking the actual defrosting process and the cooling capacity. This means that control is also carried out from outside the refrigerator without monitoring the (valuable) refrigerated goods.

A well-known component is the temperature sensor, which can either break or fall off the evaporator. The temperature sensor cable can also break or be separated from the device housing. In these cases, the known circuit is erroneously signaled that the evaporator temperature is too high, so that the refrigerator remains switched on despite the defrosting pulses, ie the defrosting process is not initiated. However, the fan is switched off and not closed again due to the pretended high temperature switches. In this case, both the refrigerated goods and the refrigeration machine itself are endangered, since in particular these conditions cannot be seen from outside the cold room. In addition, a so-called safety thermostat is installed, which signals an alarm if the temperature in the cold room rises excessively. However, this only reports irregularities in the cooling system indirectly via a rise in temperature on the refrigerated goods themselves, i.e. with a considerable time delay, since its sensor is not installed on the evaporator. Otherwise, the alarm temperature at the sensor would be exceeded during the defrosting process and the alarm would be erroneously given. Switching off the alarm message during the defrosting process would also not lead to a satisfactory solution, since the refrigeration system would be without alarm monitoring until the fan was switched on.

The object of the invention is to provide a defrost control unit which increases the safety when operating a refrigeration system.

This object is achieved with the features of claim 1. According to claim 1, an alarm and safety circuit is provided in a generic defrosting control device, in which a time clock is included, which generates at least one, preferably eight time pulse within a defrosting period (after the defrosting process or after the fan has been switched on). When these monitoring pulses are issued, it is checked whether the lower temperature switching point (fan switch-on point) is still below, ie whether the chiller is working properly. Furthermore, the lines to the switching units for the refrigeration machine or additional heating and the fan are checked whether the necessary switching states are maintained here. In terms of circuitry, this is achieved in such a way that the monitoring pulse is fed to one input of an AND circuit, while in parallel to the other input of the AND circuit, the transmitter for the lower temperature switching point and, depending on the device version, and / or the output of the defrost flip Flops and / or the output of the fan flip-flop are switched. An alarm lamp is connected to the output of the AND circuit for on-site alarm detection and is mounted on the front of the device housing. Furthermore, it is particularly advantageous to additionally connect a further alarm switching unit for remote alarms. This alarm switching unit expediently consists of a relay which is energized in normal operation and drops out when an alarm is given and closes a potential-free contact which can be connected from the outside.

With the features of claim 1 it is achieved that the cooling system is regularly checked for proper functioning between successive defrosting processes; If errors are detected, a visual alarm is given, in addition to which a remote alarm display can be installed.

According to claim 2, the AND circuit from claim 1 is followed by an alarm flip-flop which maintains an alarm state once generated on the alarm devices, regardless of the further operating states on the alarm devices. This is advantageous because it also detects alarm conditions that will resolve themselves over time. The control unit is briefly switched off to acknowledge the alarm.

Claim 3 proposes a reset circuit which consists of a chargeable capacitor with a downstream inverter. During the charging time of the capacitor, the switching state "I" is emitted by this reset circuit and after the charging time the switching state "0". The reset circuit is connected to all reset inputs of the flip-flops used, so that all the bistable switching elements are reset during the charging time of the capacitor. This is a simple and functional circuit.

With the features of claim 4 it is achieved that a mains failure is also detected for remote alarm transmission, since the alarm relay is de-energized when an alarm is given and closes a contact that can be connected from the outside. The voltage another remote source must be made available for remote transmission.

In claim 5, a circuit is proposed which advantageously ensures that both the refrigeration machine and the fan continue to run in the event of an alarm, i. H. the defrosting control device releases itself from responsibility due to a malfunction and tries to maintain the most harmless condition for the refrigerated goods, namely constant cooling. Whether this actually succeeds depends on the type of error. If the fault is due to the defrost control unit itself, for example in the event of a sensor break, the state of constant cooling can usually be maintained.

With the features of claim 6, an undesirable, possible alarm due to the thermal load in the evaporator chamber is prevented when the fan is switched on.

According to claim 7, the relays for controlling the refrigerator or the auxiliary heating and the fan should be switched so that they are not excited when the refrigerator and the fan are switched on. This advantageously leads to the fact that in the event of a power failure on the control lines, for. B. when a fuse blows or there is a power failure in the defrost control unit, the "constant cooling" state is maintained.

In addition to the alarm lamp, a further expedient indicator lamp, which signals the switched-on state of the refrigerator and the fan, including their control circuit, is proposed in claim 8.

Claim 9 specifies an expedient embodiment of the time clock.

The invention is illustrated in more detail, features and advantages on the basis of an exemplary embodiment.

The single figure shows schematically the circuit diagram for a defrost control device according to the invention.

The circuit of the defrost control unit consists of a power supply unit 1, a temperature measuring amplifier 2 and a driver circuit 3 for various light-emitting diodes and relays, these circuit parts being linked by bistable flip-flops to be described in more detail.

The power supply unit 1 is equipped in the usual way with a mains transformer 4, a rectifier 5 behind it, downstream smoothing capacitors 6 and Zener diodes 80. Direct voltage of different polarity is available at the outputs V + and V-. The line frequency is fed via line 7 immediately after the line transformer 4 to an input 8, a counter circuit 9 described in more detail below.

The connection points of a temperature sensor 10 are specified in the temperature measuring amplifier 2. However, the temperature sensor 10 is arranged on the evaporator of the refrigerator via an extended line. The first connected amplifier 11 serves as a constant current source for the temperature sensor 10. The subsequent amplifier 12 amplifies the sensor signal by a factor of approximately 10. The output signal of the amplifier 12 is fed to the inputs of two Schmitt triggers 13, 14. The switching points of the Schmitt triggers 13, 14 can be set accordingly via adjustable resistors 15, 16. These resistors are potentiometers that can be set on the front panel of the device housing. The circuitry on the Schmitt trigger 14 is dimensioned so that hum is suppressed. The wiring of the Schmitt trigger 13 (Fan activation) is selected so that a relatively large switching hysteresis, the meaning of which is explained below, arises.

The driver circuit 3 is designed in a manner known per se. An alarm light-emitting diode 17 and a cooling light-emitting diode 18 light up when "I" potential (high potential) is present on their control lines 19, 20. An alarm relay 21 keeps a "switch contact 22 closed in the deenergized state and opens it when the line 23 is" I. "The contact 22 can be connected from the outside with remote alarm transmission.

A defrost relay 24 contains a changeover contact 25, with which the circuit for the cooling unit is closed in the de-energized state of the defrost relay 24 and the circuit for additional heating in the excited state. When the defrost relay 24 is excited, a defrosting light-emitting diode 26 (yellow) is also lit.

A fan relay 27 closes the circuit (not shown) of a fan when not energized; when excited, the circuit is opened and the fan is switched off. In the wiring of the relays 24 and 27, two transistors are connected in series in order to obtain redundancy and to increase the switching reliability of these relays in the event of an, albeit unlikely, failure of a switching transistor.

The counter circuit 9 consists of two electronic counters (CD 4040) 28, 29 connected in series. At the counter 28, frequencies formed and divided down from the mains frequency are available at different outputs 30. These different frequencies can be tapped via a switch 31 and fed to the downstream counter 29. The switch position of switch 31 (on the front panel of the housing) determines the duration of the successive defrost cycles. A defrost pulse is generated in line 32 at the output of counter 29 to initiate the defrosting process after a certain number of pulses counted in counter 29.

Line 32 leads to a defrost flip-flop (CD 4013) 33, the output line 34 of which leads to the driver circuit for defrost relay 24. The reset input 35 of the defrost flip-flop 33 is connected to the output of the Schmitt trigger 14 via a line 36 and diode 37. Line 34 also leads via a diode 38 to an input 39 of a NAND element 40. Furthermore, the output signal of the defrost flip-flop 33 goes from line 34 via a branch and diode 41 to a set input 42 of a fan flip-flop 43. Another line 44 with the same signal is connected to a further input 79 of the fan flip-flop. An output line 45 of the fan flip-flop 43 leads to the driver circuit for the fan relay 27 and at the same time via a branching line 46 to an input 47 of a NAND element 48 connected to the cooling light-emitting diode 18; the other input 49 is connected to the line 34 via a diode 50 and to a flashing frequency output of the counter 28 via a diode 51. A further line 52 branches off from the output line 45 and also has a connection to the input 39 of the NAND element 40 via a diode 53.

From the output of the Schmitt trigger 13, a line 54 leads via a diode 55 to the reset input 56 of the fan flip-flop 43. The Schmitt trigger 13 also has a connection to the input 39 of the NAND via a branching line 57 and diode 58. Gate 40.

The second input 59 of the NAND gate 40 is connected to an output 61 of the counter 29 via an RC circuit (one shot). At this output 61 are in a Pulse train generates eight pulses per cycle.

With the help of two NAND elements 62, 63, an alarm flip-flop 64 is formed, to the set input of which a line 65 from the NAND element 40 is connected. The outputs of the alarm flip-flop 64 are connected to the lines 19, 23. Furthermore, the output of the alarm flip-flop 64 is branched off from the line 19 once via the diode 66 to the reset input 35 of the defrost flip-flop 33 and additionally via the line 67 to a priority input 68 of the fan flip-flop 43 connected.

A reset circuit (reset) consists essentially of a chargeable capacitor 69, which is followed by an inverting element 70. The connected line 71 is with reset inputs 72, 73 of the counters 28, 29 via an inverting element 74 with the reset input on the alarm flip-flop 64, via a diode 75 with the reset input 35 of the defrost flip-flop and via a diode 76 connected to the set input 42 of the fan flip-flop 43.

A line 78 leads from the counter 28 via a diode 77 to a branching point with the line 54. This square frequency serves to avoid unsafe switching states because of the edge control on the fan flip-flop 43.

• Beim Einschalten des Abtausteuergeräts wird der Kondensator 69 geladen. Während dieser Ladezeit ist wegen der Invertierung durch das Glied 70 "I" auf der Leitung 7l. Dadurch werden über die Rücksetzeingänge 72, 73 die Zähler 28 und 29 auf Null zurückgesetzt. Weiter wird das Alarm-Flip-Flop 64 in seinen Anfangszustand (kein Alarm) gesetzt. Zugleich wird das Abtau-Flip-Flop 33 rückgesetzt mit der Folge, daß die Ausgangsleitung 34 "0" wird; d. h. das Abtaurelais 24 ist sicher abgefallen und damit die Kältemaschine eingeschaltet. über die Leitung 71 und die Diode 76 wird aber zugleich am Setzeingang 42 das Lüfter-Flip-Flop 43 gesetzt. Dadurch wird die Leitung 45 "I", das Lüfterrelais 27 zieht an und schaltet den Lüfter aus. Dies ist der Startzustand für die Schaltung. Sobald der Kondensator 69 aufgeladen ist, wird die Leitung 71 wieder "O", womit die Rücksetzung zu Betriebsbeginn beeendet ist.

The circuit described has the following function:

• When the defrost controller is switched on, the capacitor 69 is charged. During this charging time, because of the inversion by the link 70, "I" is on the line 71. As a result, the counters 28 and 29 are reset to zero via the reset inputs 72, 73. Furthermore, the alarm flip-flop 64 is set to its initial state (no alarm). At the same time, the defrost flip-flop 33 reset with the result that the output line 34 becomes "0"; ie the defrost relay 24 has safely dropped off and the refrigeration machine is thus switched on. Via the line 71 and the diode 76, however, the fan flip-flop 43 is also set at the set input 42. As a result, line 45 becomes "I", fan relay 27 picks up and switches the fan off. This is the starting state for the circuit. As soon as the capacitor 69 is charged, the line 71 becomes "O" again, which ends the reset at the start of operation.

In normal operation without alarm conditions, the circuit works as follows: After cooling is switched on immediately with relay 24, the temperature on the cooling unit decreases. The Schmitt trigger 13 then switches at the set lower switching point, as a result of which the fan flip-flop is reset via the reset input 56. Line 45 becomes "O", relay 27 drops out and the fan is switched on.

In the meantime, a number of pulses have already been counted in the counter 29. When the predetermined number of pulses is reached (as already stated, the time for a defrost cycle depends on the position of the switch 31), a defrost pulse is generated on the line 32. This sets the defrost flip-flop 33, whereby "I" is formed on line 34. Defrost relay 24 is thereby energized, the cooling machine is switched off and, at the same time, the additional heating is switched on. The "I" signal is also supplied to the set input 42 of the fan flip-flop 43 via the diode 41, so that the fan relay 27 is also excited via the line 45 and the fan also switches off. This series connection of the flip-flops 33 and 43 advantageously means that no state can occur in which (from the point of view of the control) the additional heating is switched on and the fan is running.

By switching on the additional heating, the defrosting process is carried out until a suitable temperature slightly above O ^O C is determined on the evaporator with the sensor lO. Then the Schmitt trigger 14 switches and the defrost relay 33 is reset via the reset input 35. As a result, line 34 becomes "O", relay 24 drops out, the additional heating is switched off and the refrigerator is switched on again. After reaching the lower temperature switching point, the Schmitt trigger 13 switches in the manner described above and the fan is switched on again. As long as the defrost relay 24 is energized, the yellow defrost light-emitting diode 26 lights up (signaling for: additional heating ON, fan OFF).

The cooling light-emitting diode 18 is only switched off (line 20 “O”) when “I” is present at both inputs 49, 47 of the NAND element 48. This is the case when both lines 34, 45 from the outputs of flip-flops 33 and 43 are "I", i.e. H. during defrosting with the fan off. If the chiller is already switched on again (line 34 or diode 50 "O"), the cooling light-emitting diode would burn again continuously. In order to identify this state by blinking, however, a blinking frequency is passed from the counter 28 to the input 49 via the line with the diode 51. Each time the input 49 becomes "I", the LED 18 goes out. If the fan is then switched on and “O” is constantly applied to the input 47 via the line 45 or 46, the light-emitting diode 18 lights continuously regardless of the flashing frequency at the input 49.

The monitoring and alarm conditions are described in the following: Every time a monitoring pulse arrives at the input 59 of the NAND gate 40 after one eighth of the time of a defrost cycle, a check is carried out to determine whether "I" is present at the other input 39 (alarm condition). In this case, the alarm flip-flop 64 is set and the lights diode 17 and the relay 21 alarm given. It is checked via line 54, 57 and diode 58 whether the lower temperature switching point is still undershot and whether the refrigerator is working properly. Furthermore, it is checked via line 34 and diode 38 whether the cooling unit and not the auxiliary heating is faulty switched on. In the same way, it is checked via line 52 and diode 53 whether fan 27 is switched on. These conditions must be met after 1/8 of the defrost period. An alarm is therefore given if the lower temperature switching point or the upper temperature switching point, for example if the defrost heater is defective, is set too slowly. On the other hand, the conditions must be maintained during the entire cooling period within the defrost period, otherwise an alarm will occur. Since the relay 21 drops out when an alarm is given, a power failure is also displayed remotely. The LED 17 goes out of course. An alarm acknowledgment is carried out by switching off and on again, whereby the entire circuit is reset in the manner described above.

After an alarm has been given, "I" potential is present at the output of the NAND gate 63 of the alarm flip-flop 64. This is led via the diode 66 to the priority input reset input 35 of the defrost relay. This "I" potential is also fed to the fan flip-flop 43 via the line 67 to the reset input 68 with priority. Neither flip-flop 33, 43 thus remains set until an alarm is acknowledged, as a result of which the output lines 34, 35 remain "0" and the relays 24, 27 are therefore not energized. This is the state "constant cooling and constant fan operation". This condition is most uncritical in the case of an error detected by the device for the refrigerated goods. Because of the priority inputs, other pulses, especially defrost pulses on line 32, have no effect.

On the basis of the monitoring pulse sequence described above after every eighth period, the evaporator also usually detects a misplacement or a drop in the sensor. Since the sensors are usually relatively slow and have a large heat capacity, they only work sufficiently quickly if good thermal contact with the evaporator has been created. If this is no longer the case, for example due to the sensor drop, i.e. the sensor is hanging in the air, the lower temperature switching point is normally not reached by the sensor in the sensor housing within the eighth period. As a result, the defrost control unit is simulated for a cooling temperature that has not been reached (fan activation point), which leads to the alarm being triggered.

The 12.5 Hz alternating voltage from the counter 28 on the line 78 is used for the safe resetting of the edge-controlled fan flip-flop 43. This is because special cases can occur in which the output of the Schmitt trigger 13 goes to zero, but practically comes late, so that another level may have applied to a priority input of the flip-flop 43. Then the zero level from Schmitt trigger 13 would have no effect. The cold room, for example, could be so cold immediately after switching on the device that the edge of the Schmitt trigger no longer comes from "I" to "0", but is already permanently at "O" level. In theory, therefore, no edge would be generated anymore and the flip-flop 43 would not take over the data at the input 79 without the AC voltage.

The larger hysteresis on the Schmitt trigger 13 for the lower temperature switching point, already mentioned above, is provided for the following case. If the fan is switched on to establish the thermal connection between the evaporator compartment and the cooling compartment, the evaporator compartment can be thermally stressed, so that the temperature rises there briefly. If the connection were made just when the verification pulse was already coming, this would lead to an alarm. With the help of the hysteresis provided, the device is allowed to briefly go over this point without triggering an alarm.

In summary, it is found that a cooling system can be operated more safely with the above-described defrost control unit.

Claims (9)

1. Defrost control unit for a refrigeration system with a temperature sensor for attachment to the evaporator of a chiller, with an input circuit to which the temperature sensor is connected and which contains a first and second switching unit for a lower and an upper temperature switching point, with a timer for successive defrost cycles, which is connected to a first output circuit for switching the chiller on and off and simultaneously switching an additional heater on and off, and to a second output circuit for switching an additional fan on and off, the connections being made via the set inputs of a first and second bistable flip-flop (defrost flip-flop and fan flip-flop) are performed, so that when a clock is generated impulses by setting the bistable flip-flops with the first output circuit, the chiller is switched off and at the same time the additional heating is switched on, and with the second output circuit the additional fan is switched off, the second switching unit for the upper temperature switching point is connected to the reset input of the first bistable multivibrator, so that when this temperature switching point is reached the refrigerator is switched on again and the additional heating is switched off, and the first switching unit for the lower temperature switching point is connected to the reset input of the second bistable multivibrator, so that when this temperature switching point is reached the additional fan is switched on again,
characterized, that an alarm and safety circuit with a further clock generator (29, 60, 61) is provided for monitoring, which generates at least one clock pulse within a defrost period and which is connected to an input (59) of an AND circuit (NAND gate 40) is that with the other input (39) of the AND circuit (NAND gate 40), the output (54) of the first switching unit (13) for the lower temperature switching point and / or the output (34) of the first bistable multivibrator (defrost flip-flop) Flop 33) and / or the output (52) of the second bistable flip-flop (fan flip-flop 43) are connected, and that an alarm lamp (light-emitting diode 17) and / or an alarm switching unit (relay 21) are connected to the output of the AND circuit (NAND gate 40). 2. defrost control unit according to claim l, characterized in that between the AND circuit (NAND gate 40) and the alarm lamp (LED 17) or switching unit (relay 21) a third bistable flip-flop (alarm flip-flop 64) to receive of an alarm state once generated is switched. 3. Alarm control device according to claim 1 or 2, characterized in that with the reset input of the third bistable multivibrator (alarm flip-flop 64) and with the reset inputs (72, 73) of the timer (counter 28, 29) and with the reset input a reset circuit is connected to the first and the set input of the second flip-flop flip-flop (defrost flip-flop 33 and fan flip-flop 43), which consists of a chargeable capacitor (69) with an inverting element (70) connected downstream, so that during the Charging time of the capacitor (69) is released by the reset circuit in the switching state "I" and after the charging time "O", so that all the resettable circuit elements are set to an initial state during the charging time. 4. defrost control unit according to one of claims 1 to 3, characterized in that the alarm switching unit is a relay (21) which is de-energized in the event of an alarm, and then closes a potential-free, externally switchable contact (22). 5. defrost control device according to one of claims 1 to 4, characterized in that the output of the third bistable multivibrator (alarm flip-flop 64) to priority inputs (35, 68) of the first and second bistable multivibrator (defrost flip-flop 33 and fan flip-flop 43) is guided, so that after an alarm is issued until the alarm is acknowledged, the refrigerator and the additional fan are constantly switched on. 6. defrost control unit according to one of claims 1 to 5, characterized in that the switching unit (13) for the lower temperature limit is provided with a switching hysteresis, so that no undesired alarm is given when the additional fan is thermally loaded when the additional fan is switched on. 7. defrost control unit according to one of claims l to 6, characterized in that the first output circuit is a changeover relay (24) which is energized during the defrosting process for switching off the cooling machine and simultaneously switching on the additional heating, and in that the second output circuit is a relay (27) that is also energized to switch off the additional fan (fan). 8. defrost control unit according to one of claims 1 to 7, characterized in that a cooling indicator lamp (18) is provided which is controlled via an AND circuit (NAND gate 48), the first input (47) with the output of the second bistable Toggle switch (fan flip-flop 43) is connected, and its second input (49) to the output of the first bistable flip-flop (defrost flip-flop 33) and additionally to a third time clock (28 via diode 51) for a flashing frequency connected is. 9. defrost control unit according to one of claims 1 to 8, characterized in that the second timer (29) is coupled to the first timer (28), different frequencies being tapped on the first timer (28) for different defrost cycles and the second timer (29 ) can be supplied, and the second timer (29) emits a fixed number, in particular eight, monitoring pulses within each set defrost cycle.

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