A Failed Bidirectional Diode In A Microwave Oven
My wife advised me that our microwave oven was not working. It is a Brastemp model BMS45 Ative! as shown below:
A test using a glass filled with water proved that the set was not working. After 2 minutes, the water remained cold.
I opened the oven and made another test, while measuring the presence of voltage in the primary of the high voltage transformer. The meter showed 127 Vac, meaning that the problem was somewhere in the high-voltage circuit (transformer, HV diode, capacitor and magnetron tube). This moment I decided to put in practice an old idea: a quick way to monitor externally (i.e., when the oven is still closed) the presence of mains voltage to the transformer. It´s simple: a LED installed in the front panel powered from the input of the transformer (low-voltage side – 127 or 220 Vac, depending on the region. In my case is 127 V). I have a lot of AC adapters in my junk box and some LEDs. I chose one adapter with 6 Vdc – 300 mA output, connecting its input in parallel with the HV transformer primary terminals. The 6 Vdc output was connected to a red LED (could be other color, of course) through a 330 Ω resistor. See the picture below:
The AC adapter was fixed in the open space between the control board and the HV transformer.
I drilled a small hole in the front panel to install the LED, as shown below:
With this arrangement, one can count on a way to quickly monitor the arrival of AC voltage in the HV transformer, even if the oven is still closed. Undoubtedly, an efficient way to streamline any maintenance, either this present one or another in the future. Of course, I did such a modification IN MY OWN OVEN. In the case of a work in a customer´s set, this must be done upon agreement.
Further tests revealed an interesting thing: for long time running (let us say 5 minutes instead of the normal 1 to 2 minutes), the water in the cup became tepid (not hot)! This revealed, in one way or another, that the set was working, although in a limited extent. Evidently, a problem in the voltage doubler circuit (capacitor + HV diode), thus freeing the HV transformer. The circuit composed of diode and capacitor doubles the voltage from the secondary of HV transformer (nearly 2 kV) to 4 kV, generating a pulsating DC voltage that is applied to the magnetron. If one of these two components is bad, there is a possibility that the magnetron receives nearly only half the voltage normally employed (2 kV pulses in this case), which explains the low performance. As a result, some conclusions can be taken: 1) the magnetron is probably good, 2) one of the other two components (diode and capacitor) is faulty and 3) half the voltage, ¼ of power from the magnetron, which explains the low performance. A more detailed operation of the voltage doubler can be found in my another article https://jestineyong.com/a-bad-hv-capacitor-in-a-microwave-oven/
The capacitor was tested with an analog multitester, and showed to be in a good condition, according to the deflections of the instrument needle.
For the diode test, a novelty: I saw two diodes. One of them is the normal rectification diode, that takes part of the voltage doubling for the magnetron (with the cathode of this diode connected to ground in the diagram below). The other diode is a special one called the double diode or bidirectional diode. It does not take part on the voltage doubling operation, i.e., it does not act as a rectifying diode. Its operation is like a varistor. It has two diodes in opposition and is connected in parallel with the capacitor, as seen below. Externally it has two bands in the body.
One data sheet of this bidirectional diode can be found in https://html.alldatasheet.com/html-pdf/139337/FCI/HV-6X2P1/55/1/HV-6X2P1.html
In the normal working of the circuit, it never conducts current – as if were not included in the circuit. The relevant part is what we could say the zener voltage of the diodes (higher than 2 kV when reverse polarized), and they only conduct if the voltage reaches a value over the nominal, thus protecting the capacitor and the magnetron. Therefore, their philosophy is: they are in the circuit to NEVER WORK (in normal use). The same as for vehicle insurance: you bought it to preferably never use!
As seen in the picture, the two components – HV diode and bidirectional diode – are sold as one set, i.e., you change the entire set, even if only one of them is bad.
My tests revealed that the problem was in the double (bidirectional) diode. In normal tests with the analog multitester in the resistance scale x10K (that uses a 9 V battery to overcome the direct polarization voltage), a good component must not conduct in any of the positions of the test probes (inverting them for tests). In my case, I got current with the test probes in a certain position, and no current inverting them. Of course, one of the two internal diodes was shorted. This explains why the warming inside the oven was low (producing nearly – perhaps – half the voltage, or ¼ of the power radiated by the magnetron).
I went downtown and bought the set of two diodes in the electronic sector (very cheap). After replacing it, the oven began to work normally.
I closed the oven, leaving the LED installed in the front panel, aiming at a future service.
1) Reinforcing the alert: Always discharge any eventual voltage in the high-voltage capacitor before touching or service any part of the HV circuit.
2) I would like to write more maintenance articles, but as I don´t work directly with this, the occasion only happens when it´s a set of my own, or from a relative (or friend). Anyhow, I always read all the publications from others.
This article was prepared for you by Henrique Jorge Guimarães Ulbrich from Curitiba, Brazil. Retired telecommunication and electronics technician. Loves electronics, telecommunications, cars, wines and grand children.
Please give a support by clicking on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.
P.S- If you enjoyed reading this, click here to subscribe to my blog (free subscription). That way, you’ll never miss a post. You can also forward this website link to your friends and colleagues-thanks!
Note: You can read his previous article on Battery Problem In Head Lamp Retrofitted