|
|||
|
|
|||
|
|
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.
During the capacitor charging time there is no voltage to the magnetron
because the current takes the course of least resistance. In other words,
rather than take a path through ground and up to the plate of the magnetron,
the current swings up through the diode. The voltage across the capacitor
will rise to the transformer secondary voltage to the maximum 2800 volts.
As the transformer secondary voltage begins to decrease from its maximum
positive value (at time increment
T2
on
the sine wave graph), the capacitor will attempt to discharge back through
the diode. The diode is like a one-way street in that it will not conduct
in this direction. Thus, the discharge path is blocked, and the capacitor
remains charged to the 2800 volts.
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.
|
All materials appearing on this website may not be reproduced, stored in any retrieval system, or used in any way for commercial purposes without the express prior written permission of the copyright holder.
Send correspondence to: Microtech, P.O. Box 940, Gonzalez, Florida 32560
Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.
 Are you shopping for electronics online? With wholesale prices on everything from dvd players to televisions, overstock.com has an unbeatable selection of electronics at prices you just can't beat... why would you shop anywhere else? |
You are visitor number 9161
