[0001] This invention relates to a device for detecting frost formation and/or for eliminating
it by heating, particularly for domestic refrigerator evaporators.
[0002] Devices for detecting frost formation on domestic refrigerator evaporators and devices
for eliminating evaporator frost by heating have both existed for some time in the
most varied forms.
[0003] The former cause the latter to operate when a given frost quantity is reached on
the evaporator. Their drawback is the need to provide two totally separate devices.
[0004] Refrigerators are also known in which thermal defrosting is effected cyclically,
ie independently of the quantity of frost which has formed on the evaporator. This
arrangement has the drawback of consuming electricity even when defrosting is not
necessary.
[0005] An object of the present invention is to provide a single means which performs both
the frost detection and heating functions.
[0006] A further object of the present invention is to provide a device which both monitors
and eliminates frost using the same means.
[0007] These and further objects which will be more apparent from the ensuing detailed description
are attained by the invention as defined by the accompanying claims.
[0008] A preferred embodiment of the invention is described in detail hereinafter by way
of non-limiting example with reference to the accompanying drawing, in which:
Figure 1 is a schematic front view of the means for frost detection and/or heating
to eliminate it from the evaporator;
Figure 2 is a disassembled sectional view showing a possible location of the means
of Figure 1 in relation to the evaporator region in a refrigerator;
Figure 3 is a schematic view of the circuit arrangement in which the means of Figure
1 acts as a heater, ie as a defroster;
Figure 4 is a schematic view of the circuit arrangement in which the means of Figure
1 acts as a capacitor, ie as a frost detector.
[0009] In the figures, the reference numeral 1 indicates a support of flexible insulating
material, such as polyamide. A suitable material is that known commercially as KAPTON
or MYLAR (or the like) manufactured for example by DU PONT and 3M.
[0010] Two physically separate resistive tracks extending mutually parallel in a substantially
castellated arrangement are applied by known methods to said support.
[0011] In the refrigerator (Figure 2), which can be static or of forced air circulation
type, the trackless face of the support 1 carrying the tracks A and B is in contact
with a plastic wall 2 bounding the refrigerator preservation compartment 3, its opposite
face being in contact with a conventional evaporator 4 of the refrigerator refrigeration
circuit.
[0012] The reference numeral 5 indicates an external wall of the refrigerator and 5A indicates
the refrigerator thermal insulation, for example polyurethane expanded in situ, which
joins together the various described components. Before expanding the polyurethane
in situ the support 1 is for example fixed to the evaporator with biadhesive tape
at points on or along its periphery, the ends of the resistive tracks A and B are
electrically connected, as shown in Figures 3 and 4, in the following manner.
[0013] The end A1 is electrically connected to a contactor means 6 in the form of a switching
means operated and controlled by a microprocessor µP. The end B1 is connected to earth.
The ends A2 and B2 are connected to a contactor means 7, the purpose of which is to
connect said ends together or disconnect them from each other, and is operated and
controlled by the microprocessor µP.
[0014] The contactor means 6 connects the end A1 either to a voltage source, for example
at 24V, or to a node 8 to which there are connected a resistor R (connected to a voltage
source at 5V) and a variable frequency oscillator OSC suitably connected to a conventional
ammeter 9 feeding its signal to the microprocessor µP which processes it in known
manner.
[0015] The resistive tracks A and B act as the plates of a capacitor when the contactor
means (6, 7) are in the position shown in Figure 4, ie during normal refrigerator
operation, in which said plates define a frost detection capacitor. This capacitor
has a certain capacitance when frost is absent from the evaporator and a different
capacitance as the layer of frost grows. This means that the ammeter 9 feeds differing
signal values to the microprocessor µP. When the signal assumes a given value the
microprocessor switches the contactor means (6, 7) into the position shown in Figure
3 in which the two tracks A and B are connected in series between the source and earth.
[0016] By this means the capacitor of Figure 4 is transformed into an electrical resistance
heater by virtue of the resistivity of the tracks. The "resistor" configuration of
Figure 3 varies cyclically. In other words, the microprocessor µP switches the contactor
means (6, 7) alternately into the position shown in Figure 4 and that shown in Figure
3, so as to monitor the progress of defrosting via the ammeter 9.
[0017] When defrosting has reached the desired extent (preset as reference data in the processor
µP) the configuration of Figure 4 becomes stabilized, whereas if defrosting is inadequate
the system switches to the configuration of Figure 3 and then of Figure 4, and so
on until the desired extent of defrosting is achieved.
[0018] A particular embodiment has been described, applied to a static refrigerator. Other
embodiments can however be provided (such as one applied by suitable and obvious expedients
to a forced air circulation refrigerator with an evaporator of radiator type) and
are to be considered as falling within the scope of the present document.
1. A device for controlling the frost layer on a domestic refrigerator evaporator, characterised
by comprising, in contact with the evaporator, a switchable means which when in one
state acts as a frost detecting capacitor and when in another state acts as a frost
heater.
2. A device as claimed in claim 1, characterised in that the switchable means comprises
an insulating support and at least two resistive tracks on said support.
3. A device as claimed in the preceding claims, characterised in that the tracks extend
parallel to each other.
4. A device as claimed in one or more of the preceding claims, characterised in that
the switchable means is switched by electrical contactor means controlled by a microprocessor.
5. A device as claimed in one or more of claims 1-4, characterised in that when in its
capacitor state the switchable means is connected to an oscillator.
6. A device as claimed in claim 5, characterised in that the connection to the oscillator
is made via an ammeter connected to the microprocessor.
7. A device as claimed in claim 6, characterised in that, during evaporator defrosting,
the microprocessor cyclically alternates the closure of the contactor means during
actual defrosting, so as to switch the configuration of the switchable means from
heater to frost detector to establish whether the required state of defrosting has
been achieved or not.