JPH0244687A - Microwave oven earthless power circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an earthless power supply circuit for a microwave oven that ensures safe operation without the need for grounding the main body of the microwave oven.

Conventional microwave ovens require a high-voltage magnetron power source of about 4.0 OOV, so the main unit must be grounded before use. In addition, when grounding work is required, an earth leakage breaker may be installed and used as shown in the conventional example in Figure 4.The earth leakage breaker 39 uses a zero-phase transformer, and in the event of an earth leakage, the secondary winding The relay coil is energized by the electric power generated and the relay contacts are opened to maintain safety.The configuration of the same parts in FIG. 4 and FIG. 1 will be explained with reference to FIG.

Problems to be Solved by the Invention In such conventional circuits, the earth leakage breaker basically operates only when an earth leakage current flows through the human body or something else, which poses a problem.

The present invention has been made in view of the above problems and provides an earthless power supply circuit with a simple configuration.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention controls the opening and closing of a power source in a metal case of a microwave oven by a signal via a short-circuit detection element such as a resistor or a capacitor.

Operation The present invention can ensure fail-safe properties and provide a safety-guaranteed configuration that does not require electrical leakage through the human body or the like.

Example FIG. 1 shows an embodiment of the earthless power supply circuit of the present invention.

In FIG. 1, a commercial power source 1 is rectified and smoothed by a diode 2 and a capacitor 3. A capacitor 4 and a choke coil 5 are used for noise prevention. A primary coil 8 of a transformer 7 is connected in parallel with the resonant capacitor 6,
Opening/closing is controlled by a switching element including a transistor 9 and a diode 10 connected in series and parallel. The secondary coil 11 of the transformer includes a high voltage capacitor 12 and diodes 13 and 14.
A high voltage is supplied to the magnetron 15 through a voltage doubler rectifier circuit consisting of the following. A heater winding 16 provides a heater voltage to the magnetron 15. The opening and closing of the transistor 9 is performed by a timing control circuit 17. This power supply is supplied via voltage dividing resistors 18 and 19 and a smoothing capacitor 20, and 21 is a smoothing capacitor.

Further, the output of the anode current sensing current transformer 22 of the magnetron 15 is connected to the capacitor 24°25 via the diode 23.
, is supplied to a resistor 26, and both ends thereof are connected to a timing control circuit 17 to constitute a constant anode current control circuit. Resistors 27 and 28 connected to two points on the metal case of the microwave oven to which the anode A of the magnetron 15 is connected are connected to a series resistor 29.30 connected in parallel with a diode 31, and the voltage across them is supplied to 5CR31. . FIGS. 2 and 3 show examples of mounting configurations. A caulking terminal 33 is fixed to a metal case 32 with a fastener 34. 35 is a printed circuit board, and 36 is a lead wire. FIG. 4 shows various potentials and connection diagrams centering on the step-up transformer of the microwave oven. In Figure 4, the magnetron equivalent load Ma has a current difference of 2 between both ends.
, 0OOV (4.0OOV after voltage double rectification is applied to the actual magnetron 15), and one end is connected to the metal case of the microwave oven. In this state, resistors 27 and 28
No current flows through the combined resistance 38 of the combined resistance 37 and resistance 29.30. However, assuming a component failure, especially a short circuit between the primary and secondary circuits of the transformer for some reason, the following will occur. In Japan, commercial power supplies are grounded on one side at the pole-mounted transformer.

If a primary or secondary short-circuit accident occurs here, the metal case to which a human can connect based on the ground potential becomes 2,000Ω in the worst case, and in the worst case, it passes through the human body equivalent to 300 to 500Ω, which can be fatal. Conventionally, the solution to this problem was to ground the metal case or use an earth leakage breaker to detect the leakage current to maintain safety. In the present invention, for example, assuming that the value of the resistor 37 is 999Ω and the resistance value of the resistor 38 is IKΩ, the content will be explained as follows:
Resistance 37.3 depending on which part of the secondary winding is shorted
The voltage between 8 is applied from 0 to 2,0 OOV, and the voltage between resistor 3
A voltage of 0 to 2■ is generated across the terminal 8. 5CR31
For example, if it is 061■, although it depends on the gate sensitivity voltage of
If the metal case is 100 cm or more, 5CR31 is closed, the timing control circuit 17 stops generating pulses, and the inverter also stops generating high voltage.

It goes without saying that the safety level can be further improved by using a more sensitive detection device instead of the SCR or by increasing the value of the resistor 38. In the configurations of Figures 2 and 3, even if either resistor 27 or 28 is open, the resistor 29.3
The structure is designed to ensure safety even if either one of the 0's is short-circuited. FIG. 5 shows a conventional example. Although the present invention shows an example of an inverter power source, a normal power source may be configured to include a relay contact welding or a coil disconnection detection device to control opening and closing of the relay contact.

Further, although an example using resistor voltage division has been shown, a method using capacitor voltage division is also possible. Further, in the embodiment, the reference potential of the detection circuit is placed on one side of the commercial power supply, but it is also effective to place two resistors between the commercial power supplies and use the midpoint potential between the two as a reference. In particular, if the present invention is applied to an inverter power supply circuit as in the embodiment of the present invention, the response speed of the transistor 9 will be fast, and even if the transistor 9 has an open failure or a short failure, the inverter will not operate and the high voltage Since this does not occur, the safety level can be further improved.

Effects of the Invention As described above, according to the present invention, safety can be ensured even when the microwave oven is made earthless with an extremely simple configuration, and is extremely useful in practice.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an earthless power supply circuit for a microwave oven according to an embodiment of the present invention, Figs. FIG. 5 is a circuit diagram of a conventional example, showing the potential relationship centered around the parts. 1...power supply, 2...rectifier, 7...
...Transistor, 9...Transistor (opening/closing means), 15...Magnetron, 17...
・Timing control circuit, 27.28... Resistor (
short circuit detection element), 31...5CR (control semiconductor element), 32...metal case. Name of agent: Patent attorney Shigetaka Awano

Claims (1)

[Claims] An earthless power supply circuit for a microwave oven in which a control semiconductor element is driven by a signal from a short-circuit detection element such as a resistor or a capacitor connected to a metal case part, and the control semiconductor element controls a switching means having a transformer connected in series.