Microwave oven (no kidding :D)

[ben999]

Hi guys,

My microwave oven broke down last month.

Not only it's a pain in the backside but it is also more waste for the planet.

A quick search on the internet educated me about safety and how to test parts.

Turns out the main board is dead (display works fine, most buttons too but "start" and "stop" buttons are useless).

New motherboard is difficult to source and is no more available from manufacturer (i bought it 2 years ago but must have been some fairly old design... )

So back to drawing board and soldering iron:

• arduino nano
• 2.8" TFT colour display
• rotary encoder
• salvage "power board" from microwave (gives 12V and 5V)

I am pretty well on my way design-wise and need a final advice on transistors ; following are 2 arrangements regarding what i would call the "oven door transistor" (the bottom PNP one).
The 3 NPN transistors at the top drive a relay each one (magnetron, plate and heating resistance). Each transistor is driven by the arduino via a resistor.

Please accept my apologies if any of my sketches make your eyes bleed.

Which one would be the least bad and how could it be improved.

Thanks a lot for reading

EDIT: Sketches drawn with https://www.falstad.com

[skywatch]

@ben999 You need flyback diodes across relay coils, this is not optional.

[ejlane]

Yes on the flyback diodes.

However, the upper transistors look like they're NPN, not PNP. The lower transistor would be PNP.

Otherwise, it looks like it would work, as long as all of the transistors are sized correctly for the current on the relay coils.

But I would be really hesitant to try to control that kind of power with an arduino. What happens if your code has an error and locks up? What if you have the magnetron running but not one of the heaters? I don't have answers, as I don't understand the working of them that well, but code errors running a high voltage radiator device like that make me a bit nervous.

I like being able to turn on the microwave and then ignore it and do other things while it's running, but it would take me a lot of watching it before I would feel comfortable with something like this.

On the other side, you're only controlling 3 relays, and it's not a nuclear reactor or anything, so I'm likely being overly cautious. But I am an engineer, and worrying about "what-if" is kind of my job, so I lean that way anyway...

[ben999]

@skywatch great, thanks a lot for your suggestion. A bit of research on the internet educated me about these diodes looks like they already exist on the board that carries the relay. However, i deeply thank you for the info. This forum and internet in general is such a powerful tool

@ejlane your concern is fully justified i also come from an engineering background and i like to think "worse case scenario".
The more one look the more one find that the very vast majority or microwave ovens work the same way : there are 3 industrial switches built in the door lock. 2 of them are cabled on the "power side", one is for signaling ("software side"). So we are pretty safe on that matter.
But you are so right about safety. Magnetron do emitt dangerous waves and capacitors and stuff packed in the oven can be deadly because of high voltage.

Thanks both for suggestions. I will come back with updated sketches and beg for advice once more