I need to convert the normal household microwave into a research one with controllable output power. I thought if i varied the voltage supplied to the magnetron, i can control its power. But i need a definitive answer before i start working based on this theory.
Hi Mahmoud Omar Amer, would it be acceptable in your setup to control the mean radiated power by generating periodic bursts and varying the duration of the bursts (like pulse width modulation)? Or do you need a continuous wave?
In the first case: That works, but you have to switch the supply voltage of the magnetron of about 3000 V DC. You cannot use SCRs because with DC, they cannot be switched off (except with high effort). Some decades ago, I used 5 high voltage npn transistors in series, controlled via optocouplers, each driver supplied by a DC/DC converter. Today, there are MOSFETs with 4,5 kV VDS available, e.g. here:
https://www.mouser.de/ProductDetail/IXYS/IXTT1N450HV?qs=sGAEpiMZZMshyDBzk1%2FWi0SMbum6dx%252BZxQUT2PmVt7g%3D
so you could avoid the effort of driving several transistors in series.
Hello prof Joerg Fricke
thank you for the reply, i am just new to this area so i am going to tell you simply what i have in mind so that you have the full picture.
I want to heat some samples up but i don't want the microwave at full power and i don't even want it to work in the on-off duty cycles that are already in the microwave when you select lower power setting.
so i thought if i can control the AC voltage going to the magnetron itself with SCR it would be great. and SCRs can be turned off easily with AC.
Hello Mahmoud Omar Amer , if I understand correctly you want to control the AC voltage (220 V) which powers the whole microwave oven.
With this approach, I see at least two problems:
- Common dimmers are not meant to drive transformers or generally inductive loads, so the manufacturer does not guarantee that both halfwaves of the AC voltage at the output enclose exactly the same area in the voltage over time diagram. But different areas mean a DC voltage component, and a DC voltage might cause high current in inductive loads, damaging the load. You could avoid this problem by using a dimmer especially designed for inductive loads. I guess the dimmer of your thermostat is not of this kind because it is designed to control heating equipment, i. e. resistance with negligible inductance.
- In the 50 .. 100% range of the dimmer (full amplitude), the supply voltage of the magnetron will remain at maximum (peak rectification) but the heating of the cathode will be reduced. I never tried to operate a magnetron this way but I guess if the result were satisfactory then some microwave ovens would reduce the power by reducing the temperature of the cathode. But I don't think they do.
I understand that controlling by the duty cycle of a PWM signal can have disadvantages at low frequencies but the MOSFET I suggested in my first reply can be switched with frequencies in the kHz domain.
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