LEARN
HOW
GO BACK MICROTECH HOME PAGE CONTACT US IMPORTANT SAFETY WARNINGS ABOUT THE AUTHOR FREE CASE HISTORY DATABASE OF MICROWAVE OVEN PROBLEMS & SOLUTIONS DOWNLOAD FREE SAMPLES FROM THE CD: The Complete Microwave Oven Service Handbook: Operation, Maintenance, Troubleshooting and Repair

The Microwave Oven Voltage-Doubler Circuit Used in the High Voltage Circuits of Commercial & Residential Microwave Ovens

Theory of Operation

Excerpts from the book The Complete Microwave Oven Service Handbook
-- available on CD-ROM
(CLICK HERE)

and from the video You Can Fix Microwave Ovens
Copyright ©, 1997-2012 by J. Carlton Gallawa

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.

Effective voltage and peak voltage of 60 cycle sine wave 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 VAC
Knowing 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.

The Half-Wave Voltage Doubler

Refer to Figure 2A . During the first positive half-cycle, which is designated on the sine wave graph as T1 , the voltage from the transformer increases accordingly with the polarity shown. The current flows in the direction of the arrows, charging the capacitor through the diode.  Circuit operation of a half-wave voltage-doubler circuit used in a microwave oven high voltage system 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.  Circuit operation of a typical voltage-doubler circuit used in a microwave oven high-voltage system 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.

A Word of Warning

The circuits described here cannot be measured with a normal voltmeter . The powerful voltages combined with the high-current potential make these circuits deadly in nature. If you wish to measure the high voltage, you should first make sure that all your affairs are in order and that your life insurance policy covers death by electrocution. If you still want to measure the high voltage, a special high-voltage meter with special leads must be used. HIGH VOLTAGE SAFETY PROCEDURES MUST BE CAREFULLY OBSERVED .

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.
 

LEARN
HOW

Copyright Information
Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted ©, 1989-2012 by J. Carlton Gallawa. The material available through this site may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

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.

GO BACK MICROTECH HOME PAGE CONTACT US IMPORTANT SAFETY WARNINGS ABOUT THE AUTHOR FREE CASE HISTORY DATABASE OF MICROWAVE OVEN PROBLEMS & SOLUTIONS DOWNLOAD FREE SAMPLES FROM THE CD: The Complete Microwave Oven Service Handbook: Operation, Maintenance, Troubleshooting and Repair

©1998-2012 J. Carlton Gallawa . All Rights Reserved Worldwide

You are visitor number