A Bad HV Capacitor In A Microwave Oven
My daughter-in-law brought me his microwave oven Model Piccolo from Panasonic with the complaint of no heating. I had already repaired it twice – one for magnetron replacement and the second for HV diode replacement.
The set was plugged into the wall socket. The front display became lit, indicating that the fuse was OK.
Opened the oven. A visual inspection revealed no problem. For test, a cup with water was put inside the oven and a 1 minute running was started. Just after the beginning of the test I hardly could saw a quick small spark flying outside the set. As my eyes were not focused at that area, it was not possible to determine the exact point the spark was generated. I initially supposed it could be the HV transformer, but I wasn´t sure. After 1 minute, the oven stopped and, as expected, the water was still cold.
I prepared the multimeter and connected it to the primary terminal of the HV transformer. Restarted the test and the multitester read the mains nominal voltage, as expected, meaning that the previous circuits (mainly the safety interlock contacts associated with the microwave oven door) were OK. At this point, it´s opportune to inform that the mains voltage in my city is 127 Vac (In Brazil, two voltages are used, depending on the region: 127 or 220 Vac). Throughout this maintenance report, all tests and operation will be based on 127 Vac.
Therefore, the problem is located from the transformer on. The set was disconnected from the wall socket, the capacitor was discharged for safety and a static test was performed on the HV components. The HV diode was OK, as tested with an analog multitester in the scale x10K, as appropriate to this case. The capacitor was also checked with the analog tester – also in the x10k scale – across the two terminals, showing normal initial excursion of the needle as it charged and subsequent return of the needle to its rest point after the charging being completed. The magnetron tube was replaced by another from my stock, well known as good working. Another 1 minute test revealed that the problem was not resolved yet – the water in the cup remained cold.
So, my attention focused on the HV transformer, the only component not yet checked. An ohmic resistance test was done in each winding, resulting OK. The table below shows the results of the ohmic test taken from this transformer and also from another transformer saved from an old Electrolux microwave oven that had been scrapped due to general corrosion. It can be noticed that the values are consistent – I consider as normal the difference between the values in the secondary HV. The isolation values between winding were also measured, resulting OK too.
* The low side of the HV secondary is connected to the core of the transformer.
So the correct connection to ground is obtained by a secure fastening of its body to the microwave oven chassis, using a number of screws that must be efficiently fastened.
However, this type of test only show that the windings are conductive, without revealing another issues as short-circuit between turns. How then can be done a more effective test? The big problem is that the voltage on the secondary can´t be measured in the conventional way, as high values, about 2 kV or more, exist in this point. Such a measure involves two aspects: by the difficulty in getting a meter for this voltage magnitude and by safety issues. Let´s revise the functioning of the high voltage generation circuit for the supply of the magnetron tube as driven from this transformer.
The mains voltage is applied into the primary side. The secondary winding delivers a voltage around 2 kVac. This voltage is applied to a half-wave voltage doubler rectifier, composed of the HV diode and the capacitor (values normally ranging between 0.77 µF and 1 µF). This arrangement delivers a high voltage (positive grounded) which is used to supply the magnetron tube. Note that in this case the arrangement of the magnetron circuit is unusual as compared to conventional electronic tube circuits: the anode (positive side) is connected directly to mass (ground) and the cathode (negative side) is under potential in relation to ground. The tube is a direct heating type, meaning that the 3.3 V filament (supplied from a separate low-voltage winding of the transformer) has two functions: it is the heating resource that allows the internal emission and is also serves as cathode. The connection of the anode to the mass is done due to a simple reason: the anode is directly connected to the housing of the magnetron. During operation the tube become too warm and, besides the heat sink existing in the magnetron itself, the housing – and hence the anode – is directly (mechanically and electrically) connected to the mass of the equipment which optimizes heat dissipation, simplifies installation and avoid the use of additional heat sinks. Other aspects to note are: only one diode and only one capacitor are used; and the use of a voltage doubler means that the secondary of the transformer needs only half of turns. This all renders the circuit to be cost-effective, simple and easiest to maintain, without losing efficiency.
Concerning the DC supply applied to the magnetron tube, it is advisable to say that the voltage is not exactly 4 kVdc neither it presents a pure-DC waveform. As already mentioned, it is very difficult (or almost impossible) to measure the specific waveform in this point with an oscilloscope. Theoretically, however, it is possible to estimate it based on the intrinsic operation of the voltage doubler. For the following explanation let´s assume that the secondary voltage is 2 kVac. The total voltage applied to the magnetron is a sum of two partial voltages obtained in each half-cycle of the AC wave delivered by the transformer secondary. In one half-cycle, when the diode is forward polarized, the capacitor charges at a voltage 2 kV * 1,41 (square root of 2) ≈ 2,8 kV, tending to maintain this charged value constant. During this time, as the diode is forward polarized, there is no voltage supply to the magnetron – only the very small forward voltage of the diode, that´s insignificant for magnetron operation. In the next half-cycle the diode is reverse-polarized and the AC voltage delivered by the transformer secondary (2.8 kVac peak) is summed up to the existing voltage already charged in the capacitor, keeping the same polarity orientation and developing a 5,6 kV pulse that is applied to the magnetron. Hence the magnetron tube is, as a matter of fact, supplied with a DC pulsating voltage at 50 or 60 burst per second (depending on the line frequency – 50 or 60 Hz). Not everybody knows this, but the truth is: when activated, the magnetron does not work continuously – it works only half the time programmed in the front panel. See below how would be the estimated waveform – it can be seen that it consists of 50 (or 60) burst per second, each bursts with a 5.6 kV peak.
The line marked as Diode On (not in scale ) in the above drawing corresponds to the voltage developed in the HV diode in the forward polarization, a very very small voltage, as compared to the 5.6 kV magnitude, is applied to the magnetron. Obviously, this voltage does´t brings about any effect in the magnetron operation.
Coming back to the transformer. I imagined two different forms for testing it, both easy to perform at the bench:
- a) Stepping down the voltage applied to the primary side. As the transformer turns ratio is constant, the voltage developed at the secondary side is reduced correspondingly, allowing the measurement to be done safely and within the normal range of regular multimeters;
- b) Applying the rated mains voltage (127 Vac in my case) at the primary side and measuring on the secondary side through a resistive voltage divider, which also reduces the voltage to the range of usual measuring instruments. Nevertheless, this second method leads to some safety issues (See the important warnings at the end of this article).
Any of these two methods bring an additional advantage: either of them allows the discovering of the transformer turns ratio with considerable accuracy.
I´ve performed the implementation of these two methods as illustrated in the sequence.
Measuring with a stepped-down voltage
A transformer with 127 Vac at primary side and 2 kVac at the secondary has a 1:15.7 turns ratio (in the case of 220 Vac at primary side, this turns ratio is 1:9). I unplugged the female Faston connectors at the primary side, leaving the male ones in the transformer free. This free input received a low voltage obtained from the secondary of a step-down transformer I had in my junk box. One of the output terminals plus the center tape were used to supply the primary of the microwave oven HV transformer. See below the circuit implemented:
The secondary voltage of the chosen transformer is marked as 2 x 7.5 Vac, with the actual voltage measured at each terminal being 7.6 V.
Let´s go to the calculations: with 7.6 Vac applied to the primary, the voltage measured at the secondary was 139.2 Vac, which leads to a 1:18.3 transformer turns ratio.
There are two things to consider about this method: the value is measured without load and the low-power step-down transformer employed makes it practically impossible to put any load at the secondary. Anyhow, it´s a reasonable evidence of the transformer condition, that showed to be good in this case. Based upon this essay, and considering that the turns ratio does not vary, it can be supposed with reasonable accuracy that with 127 Vac applied, the voltage at the secondary will be 2.32 kVac.
Measuring with a voltage divider
In this measuring method the step-down transformer above is not used anymore – the normal transformer connection is maintained, with the mains voltage being regularly applied to the transformer primary through the existing circuit of the set under repair. For the measuring at the secondary side a resistive voltage divider was build using a number of resistors from my stock. The implementation of this voltage divider and the photo of it can be viewed below:
Obviously this building is not the “wonder of the century”. As a matter of fact, it is ugly, but showed itself to be well accurate and was quickly built. This “complex designed” voltage divider takes into account three premises:
1) The measurement at the voltage divider tap corresponds to 10% of the total voltage applied to the high side of the divider. This allows measurements with conventional measuring instruments – multimeters or oscilloscopes;
2) The use of several resistors in series was adopted in order to divide the potential gradients and the dissipation along them – that means don´t concentrate high voltage magnitudes and don´t produce excessive heat in any resistor. This approach leads to some features: increased safety, avoided possibility of arcing, distributed dissipation and the possibility to improve the selection of resistors to be combined aiming at getting the correct values using existing resistor in the workshop, and
3) The burden load to the high-voltage circuit is very small. At 2 kV – in this case – the AC current is about 10 mA RMS.
This third item means that the measurement is performed practically with no load to the high-voltage circuit (as the radiated power from the magnetron, responsible for the heating of foods, is about 800 W for the present microwave oven, this tube consumes almost 20 times greater). Because of this, the measurement is done in a condition near the open state of the output, which tends to increase the value of the voltage being measured, mainly on the DC measurements, as will be seen further on. In this later case, of course, due to the load imposed by the magnetron during the normal operation, the voltage is certainly somewhat less.
This arrangement was connected to the secondary of the transformer (obviously disconnected from the circuit, as can be seen at the right of the photo below – the arrow points to the faston terminal extracted from the HV capacitor) with the primary of the transformer supplied with 127 Vac. The AC voltage at the tap measured 322.6 V, a value something out of expected – by theory it would be just over 200. I didn´t get to understand the reason why this occurs. Perhaps due to the fact the measurement is being made on “no-load” conditions. Could someone explain it? Anyhow, the remaining measurements that follows are all coherent.
The two measurements were repeated on the bench in the other transformer aforementioned. I didn´t take shots of this, as it is not directly relevant to this present maintenance work. Anyhow, it was a good opportunity not only to check the state of this another transformer, but also to confirm the functioning of the resistive arrangement. The values obtained in this additional measurement showed values very close to the first one.
Back to the former transformer: as the measurements suggested it was in good condition. It was reconnected to the circuit and another test was performed. Nevertheless, the problem was still on, i.e., even with all the components hardly checked the water in the cup stubbornly remaining cold!
Up to this moment the resistive divider was used only to measure AC directly from the transformer secondary. That’s when I decided to use it to measure the DC voltage at the voltage doubler output (the point the diode, the capacitor and the magnetron filament are joined). I first unplugged the set from the wall outlet, discharged the capacitor to mass – although knowing it unnecessary in this case, but anyway this is a recommendable practice – and connected the high side of the voltage divider to that point. The set was turned on again and the multimeter was connected to the tap of the voltage divider and … no voltage was measured.
I became very confused. All seemed to be normal, with voltage at the transformer secondary and with all the components being carefully checked. Even so the problem continued.
At this point I had the inspiration to measure the resistance between the output of the voltage doubler (the joining of components mentioned above) and mass, obviously performing again the discharge action in the capacitor. Bingo! Practically zero Ohms. A clear short-circuit but the question arose: where this short-circuit came from?
The magnetron tube was disconnected from the circuit by drawing the filament Faston connector out. The diode was disconnected too (Faston connectors are quick and practical this time). The test on the capacitor was repeated and showed that at least as one capacitor it was still good, with the multimeter needle showing the charging and returning to the rest point. The things changed when I decided to measure the resistance between the chassis of the microwave oven and the two capacitor terminals. The capacitor housing is composed of an aluminum structure that is fastened with a screw to the microwave oven chassis through a special clamp. When I applied the probes of the ohms tester between the chassis and one of the capacitor terminals (those connected directly to the magnetron), the short-circuit became evident. I just found out the point of the problem!
After taken the capacitor out the chassis, another definitive test revealed a resistance of 23.7 Ω between the casing and one of the terminals.
A visual inspection revealed a burnt spot that can be seen in the photo below:
This time I finally discovered the point the spark had been vomited outside the microwave set in the beginning of the troubleshooting. Of course, it was the capacitor. A new capacitor was installed and another ohms test was performed, confirming that the short-circuit does not exist anymore.
In order to perform a final test, again the voltage divider was connected to the cathode point. Turned the set on and finally got the DC voltage, which measured 335 V in the divider tap. As the tap presents 10% of the total voltage, one could suppose that the magnetron supply is about 3.35 kV. But that’s not really true. In this case the value measured serves as a reference only – it just says there is a voltage, without determining it accurately. The reason was already explained: the supply for the magnetron consists of burst voltages for one half-cycle, whereas in the following cycle there is no voltage. The measuring instrument would have to do some kind of special processing, and this not happens. An appropriate measuring would be with an oscilloscope on the tap. If I had one, a photo from the screen would be included in this article for better assessment. For anyone who has one, I would be grateful if I could see a shot from the screen.
To finish the repair work I removed my “complex designed” voltage divider and reestablished and checked the complete circuit, confirming that all was correct. Replaced the cup with water inside and set again the operation to 1 minute. After this time, the oven stopped and the happy end: hot water in the cup. The microwave oven is successfully repaired.
WARNING 1: to everyone that decide to build the voltage divider describe I recommend to mount it in a circuit board and protect the circuit with an appropriate insulating housing. It is useful to build the probes with good quality suitable alligator clips (never forget to discharge the capacitor first). Another point is the heat generated by the resistors: I noticed that they got heated up a bit, so the dissipation of these components shall be properly calculated in a circuit envisaged for constant use.
WARNING 2: measuring the high voltage part of microwave ovens is generally strongly discouraged. It must taken into consideration that oven problems can be diagnosed in a conclusive way, especially considering the low number of components. You only can do measurements in the high-voltage sector if you consider yourself a well prepared engineer or technician, and fully aware of the risks involved.
This article was prepared for you by Henrique Jorge Guimarães Ulbrich from Curitiba, Brazil. Retired electronics technician. Loves electronics, telecommunications, cars and grandchildren.
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