In the high-voltage section of a microwave oven, the diode (rectifier) and the capacitor function together to effectively double the already-high voltage. This is called a voltage-doubler circuit.
In order to effectively understand the voltage-doubler circuit used in microwave ovens, it is first necessary to understand the difference between effective voltage and peak voltage. Measured with a common voltmeter, the voltage in the standard household receptacle is 115 VAC (± 10%). The actual voltage alternates through one complete cycle every 60th of a second, as shown in the sine wave of Figure 1 . Because the voltage is continuously varying, the value reflected on the voltmeter is only the effective value of this voltage. The sign wave actually reaches a peak value of 1.414 times the effective value. So the peak voltage at a standard wall outlet would be:
Peak voltage = 1.414 X 115 VAC = 163 VACKnowing peak values and their relationship to effective values is important to understanding the operation of a voltage-doubler circuit.
Voltage-doubler circuits are fed with the stepped-up AC voltage from the high-voltage transformer's secondary (or output) winding. Typically, a transformer would step up 115 volts to about 2000 volts, which would have an approximate peak value of 2800 volts. We will use this value in analyzing the operating sequence of a voltage doubler. Please note that the values of voltages shown are peak, no-load, theoretical values. Under actual circuit operation, the load of the magnetron tube may decrease the output of the voltage doubler by as much as 40 percent.
Refer to Figure 2B . At time T3 , the transformer secondary (output) voltage swings into the negative half-cycle and increases in a negative direction to a negative 2800 volts, with polarities as shown. The transformer secondary and the charged capacitor are now essentially two energy sources in series. The 2800 volts across the transformer winding adds to the 2800 volts stored in the capacitor and the sum voltage of 5600 volts is applied to the magnetron cathode .
There are two fundamental characteristics of this 5600-volt output that should be noted. First, because a voltage doubler is also a rectifier, the output is a DC voltage. Second, the resulting output voltage that is applied to the magnetron tube is actually a pulsed DC voltage. This is because the doubler generates an output only during the negative half-cycle of the transformer's output (secondary) voltage. So, the magnetron tube is, in fact, pulsed on and off at a rate of 50 or 60 times per second, depending on the frequency of the line voltage.
However, microwave oven problems can be
diagnosed just as conclusively, and certainly more safely without checking
the high voltage. Therefore,
MEASURING THE HIGH
VOLTAGE IS STRONGLY DISCOURAGED.
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