1. Field of the Invention
The present invention relates to an automatic thawing i,e., "defrosting," apparatus for a microwave oven, and particularly to an improved automatic thawing apparatus of a microwave oven capable of advantageously thawing a food therein by amplifying thawing state detection signals to a predetermined level using at least one amplifier and by judging food thawing state using the amplified signals.
2. Description of the Conventional Art
Referring to FIG. 1, a conventional automatic thawing apparatus of a microwave oven includes a power supply circuit 1 in order to supply power to the elements thereof requiring, a key input circuit 2 for selecting a desired function using control keys, a display circuit 3 for displaying the operation states in response to key selections from the key input circuit 2, a thawing state detection circuit 4 for detecting a thawing state of the food using a sensor 6, and a microcomputer 5 for controlling the thawing operation in response to the signals inputted from the key input circuit 2 and the thawing state detection circuit 4.
The detailed operation of the conventional automatic thawing apparatus of the microwave oven will now be explained.
To begin with, power is applied to the corresponding elements thereof by the power supply circuit 1, and a user selects a desired function via the key input circuit 2. Thereafter, the microcomputer 5 performs a predetermined operation in response to the key signals inputted from the key input circuit 2. At this time, if the key signals inputted from the key input circuit 2 are for thawing the food, the microcomputer 5 judges the food thawing state by receiving the signals detected by the thawing state detection circuit 4. Here, the thawing state detection circuit 4 includes a variable resistance Rs of the sensor 6, one end of which is connected to the power supply voltage Vcc and a fixed resistance Rf, one end of which is connected to ground. Here, the variable resistance Rs and the fixed resistance Rf are directly connected in series. Since the resistance value of the variable resistance Rs varies in accordance with the thawing state of the food, the power supply voltage VCC is divided at the output terminal connected between the variable resistance Rs and the fixed resistance Rf. Thereafter, the capacitor C which is connected to ground and in parallel with the fixed resistance outputs the divided voltage to the microcomputer 5, where the divided voltage represents the thawing state detection signal for indicating the thawing state of the food.
The microcomputer 5 judges the thawing state of the food in accordance with the signal outputted from the thawing state detection circuit 4. In accordance with such judgement, the output of the microwaves and the thawing time are controlled and then the food is properly thawed thereby. In addition, the display circuit 3 displays the operation mode and the thawing time.
However, referring to FIG. 2, the signal level difference between the signal which the thawing state detection circuit 4 outputs to the microcomputer at the initial state of the food thawing operation and the corresponding signal at a finishing time of the thawing operation is small. Therefore, the correct thawing state might not be obtained due to external noises in the thawing state detection signal, so that the output of the microwave energy or the thawing time are not advantageously controlled.