JPH081457Y2 - Liquid level detector for dishwasher - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to detection of the level of cleaning liquid in a dishwasher,
More specifically, it relates to a device for measuring the electric conductivity of a washing liquid in a dishwasher to detect the liquid level in the washing tank.

(Prior art) To detect the washing liquid in the washing tank of the dishwasher, a conventional float switch is mounted at a desired position in the washing liquid tank, and if there is a liquid level, the switch is turned on by the buoyancy of the float. Is used. When such a device is used for a long time, an ionic component such as calcium contained in the cleaning liquid may be deposited and fixed on the float or the rotating part of the movable arm connected to the float, and the function of the float switch may be lost.

In order to solve such a problem, a device is proposed in which a sensor electrode is provided in the cleaning liquid tank to measure the electrical conductivity of the cleaning liquid, and the presence or absence of the cleaning liquid in the cleaning liquid tank is detected.
An apparatus according to this method is shown in FIG. In the figure, reference numeral 30 indicates a metal cleaning liquid tank. The cleaning liquid 31 is stored in the cleaning liquid tank 30. In addition, a detection electrode 33 is provided at a predetermined position in the cleaning liquid tank 30 via an insulator 32, and the cleaning liquid tank 30 is grounded to the housing of the cleaning machine. The detection electrode 33 is transformed by the electrode W into a transformer 34.
Is connected to one end A of the secondary winding. The other end B of the secondary winding of the transformer 34 is grounded by the diodes D3 and D4 via the resistor R1. The other end B of the secondary winding is a capacitor C1.
Is connected to a voltage doubler rectifier circuit composed of rectifying diodes D1 and D2. The output side of the diode D1 is grounded by the resistor R2 and the capacitor C2, and is also connected to one input terminal of the voltage comparator 35 via the resistor R3 connected in series with the diode D1. The other reference voltage input terminal of the voltage comparator 35 is connected to the variable resistor VR for setting the comparison voltage.

The primary winding of the transformer 34 is connected to an AC voltage generation source, and an AC voltage of 40 to 50 kHz is applied.

Next, the operation of this device will be described. When the cleaning liquid 31 is stored in the cleaning liquid tank 30, the AC voltage of 40 to 50 kHz applied to the primary winding of the transformer 34 is applied to the transformer 34.
Induced on the secondary winding of the same winding, point A of the same winding, detection electrode 33, cleaning liquid 31, cleaning liquid tank 30, diodes D3, D4 and resistance R1
And flows through the circuit composed of point B. Therefore, the resistance R1
A potential difference is generated across the circuit composed of the diodes D3 and D4 by the alternating current flowing through the circuit. This potential difference is voltage-doubled and rectified by a rectifying diode composed of diodes D1 and D2 via a capacitor C1, smoothed by a capacitor C2, and added to an input terminal of a voltage comparator 35 via a resistor R3.
The voltage set by the variable resistance VR for setting the comparison voltage is compared with the reference voltage input terminal of the voltage comparator 35, and if the cleaning liquid tank 30 has a predetermined amount of cleaning liquid 31, the output of the voltage comparator 35. Turns on.

If there is no cleaning liquid 31 in the cleaning liquid tank 30, the detection electrode 33
Since the resistance value between the cleaning liquid tank 30 and the cleaning liquid tank 30 is infinite, no AC current flows through the circuit described above, and the resistance R1 and the diodes D1, D2
There is no potential difference across the circuit. Therefore, the potential of the input terminal of the voltage comparator 35 becomes 0, and the output of the voltage comparator 35 becomes OFF. The reference voltage of the voltage comparator 35 is adjusted by adjusting the variable resistance VR for setting the comparison voltage.
It goes without saying that the cleaning liquid 31 is set to such a value that it can be discriminated whether or not it is present. The resistance
R2 is for discharging capacitor C2 and also diode D3
And D4 compensate the forward voltage drop (VF) of the rectifying diodes D1 and D2.

(Problems to be solved by the invention) However, point A of the secondary winding of the transformer 34 and the detection electrode
When the electric wire W connecting the 33 is long and arranged close to the housing of the dishwasher, the stray capacitance C shown by the dotted line in FIG.
As a result, a small amount of alternating current flows even when there is no cleaning liquid 31 in the cleaning liquid tank 30, and a potential difference due to this flows across the circuit composed of the resistor R1 and the diodes D3 and D4.
The S / N ratio of the input voltage applied to 5 is reduced. In addition, electric noise generated when turning on and off a large-capacity induction motor or the like that pressurizes and sprays the washing liquid of the dishwasher onto the dish to be washed is induced in the electric wire W and the input voltage of the input voltage applied to the voltage comparator 35.
Reduce the S / N ratio. As a result, there is a problem that the liquid level detection device may malfunction.

The purpose of this invention is to solve the above-mentioned problems. The point A of the secondary winding of the transformer 34 and the detection electrode 33 are provided.
Even if the electric wire W connecting the cable is long, the noise signal induced in it is reduced, and the leakage current due to the stray capacitance is eliminated,
It is to provide a highly reliable liquid level detection device.

(Means for Solving the Problems) In order to achieve the above object, the present invention is a transformer in which a secondary winding has a midpoint and one end and the other end thereof are in opposite phase and generate substantially the same voltage. The primary winding of the transformer is excited by an AC voltage, and one end of the secondary winding of the transformer is connected to the electrode provided in the cleaning liquid tank made of metal with the first electric wire. The other end is a second electric wire that is in close contact with the first electric wire and extends near the above electrode to form an open end, and the output generated between the middle point of the secondary winding of the transformer and the cleaning liquid is referenced to the other end. In addition to one input terminal of the voltage comparator to which a voltage is applied, it is characterized in that the presence or absence of the cleaning liquid in the cleaning liquid tank is detected.

(Operation) In the liquid level detection device of the present invention configured as described above, the electric wire connecting the secondary winding of the transformer and the detection electrode is laid with the voltage of opposite phase in close contact with the electric wire, and the transformer is connected. Since the potential difference between the middle point of the secondary winding and the washing liquid tank is detected, the leakage current generated by the stray capacitance of the wire is canceled and the noise voltage due to the external factor induced in the wire is also canceled.

(Embodiment 1) FIG. 1 shows a circuit diagram of a liquid level detecting device according to an embodiment of the present invention. In the figure, 1 is an AC voltage generation source, 2 is a transformer, and the AC voltage generation source 1 is connected to the primary winding of the transformer 2. Further, the secondary winding of the transformer 2 has a midpoint terminal C. Reference numerals 3 and 4 denote parallel electric wires connected to one end A and the other end B of the secondary winding of the transformer 2, or twisted first and second electric wires, which are in close contact with each other. 5
Indicates a cleaning liquid tank for storing a cleaning liquid made of a conductive metal such as a stainless steel plate, which is grounded to a metal casing (not shown) constituting the dishwasher. A cleaning liquid 6 is stored in the cleaning liquid tank 5. Reference numeral 7 is an electrode for detecting the cleaning liquid 6, 8 is an insulator for fixing the electrode 7 to the cleaning liquid tank 5 in an insulating state, and the electrode 7 is fixed at a predetermined position in the cleaning liquid tank 5 via the insulator 8. ing. And this electrode 7
The electric wire 4 having an open end is extended immediately adjacent to.

The midpoint terminal C of the secondary winding is connected to one input terminal of a voltage comparator 18 via a capacitor 12, a diode 14 and a resistor 15 which are connected in series. Further, one end of a resistor 11 is connected between the midpoint terminal C and the capacitor 12, and the other end of the resistor 11 is connected to one end of a parallel circuit composed of diodes 9 and 10 connected in parallel in opposite directions. The other end of this parallel circuit is grounded. Also, the capacitor 12
The anode side of the diode 13 is connected between the diode 14 and the diode 14, and the cathode side of the diode 13 is grounded,
A voltage doubler rectifier circuit is configured. The above diode
Reference numerals 9 and 10 are for compensating the forward voltage drop of the diodes 13 and 14. Further, one ends of capacitors 16 and 17 whose other ends are grounded are connected to both sides of the resistor 15 to form a π-type filter. Furthermore, the resistor 20 for discharging the residual voltage of the capacitors 12 and 16 is connected when the input signal to the voltage comparator 18 disappears.

A reference input terminal, which is the other terminal of the voltage comparator 18, is supplied with a predetermined reference voltage via a variable resistor 19.

Next, the operation of the device of the present invention will be described. The AC voltage generated by the AC voltage generator 1 is supplied to the primary winding of the transformer 2 at a frequency of 40 to 50 kHz in order to reduce wear due to electrolysis of the electrode 7. Alternating phase AC voltages are induced at the midpoint terminal C and one end A point of the secondary winding of the transformer 2, and at the midpoint terminal C and other end B points. The alternating current induced between the end A of the secondary winding of the transformer 2 and the middle terminal C is the path of the wire 3, the electrode 7, the cleaning liquid 6, the cleaning liquid tank 5, the diodes 9 and 10, and the resistor 11. And a voltage proportional to the flowing current is generated across the diodes 9 and 10 and the resistor 11. When the cleaning liquid 6 does not exist in the cleaning liquid tank 5, the resistance value between the electrode 7 and the cleaning liquid tank 5 is infinite, so that the electric current does not flow through the cleaning liquid 6, but the electric wire 3 and the cleaning liquid are actually cleaned. Due to the stray capacitance between the enclosures, a small leakage current flows between the points A and C of the transformer 2, the diodes 9 and 10 and the resistor 11,
As a result, a slight potential difference is generated across the diodes 9 and 10 and the resistor 11. Further, the alternating current induced between the other end B point of the secondary winding of the transformer 2 and the midpoint terminal C point reaches the open end of the electric wire 4 via the electric wire 4. A slight leakage current flows between the points B and C of the transformer 2, the diodes 9 and 10, and the resistor 11 due to the stray capacitance between the electric wire 4 and the casing of the cleaning device. Since it has been wired, the electric wire 3,
The stray capacitance between the electric wire 4 and the casing of the cleaner is equal, and the AC voltage applied to the electric wire 3 and the electric wire 4 are substantially equal, and the phases thereof are opposite. Therefore, the leakage current due to the stray capacitance is the secondary current of the transformer 2. The currents flowing through the diodes 9 and 10 and the resistor 11 become zero due to the stray capacitances canceled by the windings. Further, even if electric noise generated when turning on and off a large-capacity induction motor that injects the washing liquid 6 of the dishwasher onto the dish to be washed is superimposed on the electric wires 3 and 4, the same as above. Offset by reason.

When the cleaning liquid 6 exists in the cleaning liquid tank 5, the alternating current induced between the points A and C of the transformer 2 is the electric wire as described above.
3, electrode 7, cleaning liquid 6, cleaning liquid tank 5, diodes 9 and 10,
A voltage proportional to the flowing current is generated across the diodes 9 and 10 and the resistor 11 by flowing through the route of the resistor 11. This voltage is doubled and rectified by the diodes 13 and 14 via the capacitor 12,
It is smoothed by a π-type filter composed of a resistor 15 and capacitors 16 and 17, and is guided to one input terminal of a voltage comparator 18. Voltage comparator 18
A reference voltage preset by the variable resistor 19 is applied to the other reference input terminal of the. Then, when the input voltage of the voltage comparator 18 exceeds the reference voltage, the output terminal of the voltage comparator 18 is turned on, and it can be seen that the cleaning liquid 6 in the cleaning liquid tank 5 exists.

FIG. 2 shows the actual measurement values obtained by this device. The liquid level detector of the dishwasher according to the present invention configured as described above uses the frequency of the AC voltage source 1 of 40KHz, 5V.
pp, the transformer 2 has a winding ratio of 1 to 2 (with a midpoint tap), and the electric wires 3 and 4 use a 0.3 mm diameter twin core vinyl sheath with a length of about 3 meters and a resistance of 11 = 1 KΩ. As a result of operating with the capacitor 12 = 0.1 μF, the resistor 20 = 20 KΩ, the resistor 15 = 100 KΩ, the capacitor 16 = 0.47 μF, and the capacitor 17 = 10 μF, the results shown in FIG. 2 were obtained. That is,
When the cleaning liquid tank 5 does not contain any liquid, the voltage at the input terminal of the voltage comparator 18 is 10 to 20 millivolts. This is a noise signal, and if the reference voltage of the voltage comparator 18 is set to 400 millivolts, whether the cleaning liquid 6 exists in the cleaning liquid tank 5 or not can be sufficiently identified. (In the case of the conventional device, the same specifications as above are used for about 400 to 6
A noise voltage of 00 millivolts appears at the input of the voltage comparator 18).

If the cleaning liquid tank 5 is filled with normal tap water,
The voltage at the input terminal of the voltage comparator 18 is about 700 millivolts. In addition, when the cleaning liquid tank 5 was filled with a normal dishwashing basic detergent solution having a concentration of 1%, a voltage of 2 V or more was obtained. In addition, it is possible to detect the concentration of the basic detergent liquid for dishwashing stored in the washing liquid tank 5 based on the above-described actual measurement result.

(Effects of the Invention) The liquid level detecting device of the dishwasher according to the present invention has the same length of the same length, in which the voltages of approximately the same voltage and opposite phase induced at both ends of the secondary winding of the transformer are closely wired. Power is supplied to the first wire and the second wire, and the stray capacitance between each wire and the cabinet of the dishwasher is made the same, so the current leaking from each wire to the cabinet of the dishwasher through the stray capacitance is offset. It
Therefore, the noise signal due to the leakage current is not generated.

In addition, the noise voltage in the Monmon mode, which is generated due to an external factor and is superimposed on the electric wire, is canceled by the secondary winding of the transformer, so that the presence or absence of the cleaning liquid in the cleaning liquid tank is higher than that of the conventional device. It was possible to detect with the / N ratio, and a highly reliable cleaning liquid detection device in the cleaning liquid tank was realized.

Further, it is not necessary to use an expensive electric wire such as a coaxial cable as the electric wire, and it is possible to use an inexpensive parallel electric wire.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a liquid level detecting device for a dishwasher according to the present invention, FIG. 2 is an operation diagram showing measured values of this device, and FIG. 3 is a circuit diagram of a conventional device. 1 ... AC voltage source, 2 ... Transformer 3 ... First electric wire, 4 ... Second electric wire 5 ... Cleaning liquid tank, 6 ... Cleaning liquid 7 ... Electrode, 8 ... Insulator 18 ... Voltage comparator, A ... One end of secondary winding B ... The other end of secondary winding, C ... Middle point terminal

Claims (1)

[Scope of utility model registration request] 1. A primary winding of a transformer having a midpoint in the secondary winding and having opposite phases at one end and the other end thereof and generating substantially the same voltage is excited by an AC voltage, One end of the secondary winding is connected to an electrode provided in a cleaning liquid tank made of metal with a first electric wire, and the other end of the secondary winding is a second electric wire that is in close contact with the first electric wire. An open end extended to the vicinity of the electrode, and an output generated between the middle point of the secondary winding of the transformer and the cleaning liquid is added to one input terminal of a voltage comparator having a reference voltage applied to the other end. A liquid level detection device for a dishwasher, which detects the presence or absence of a cleaning liquid in the cleaning liquid tank.