General > General Technical Chat

Microwave oven PWM frequency

<< < (3/17) > >>

John Heath:
A microwave oven is switching on and off every 8 m seconds as well as every second or so. The reason for this is it is against the law for a microwave oven to run off a filter DC power supply. It must be a unfiltered supply raw off the full bridge rectifier thus on and off every 8 m second. The reason for this law is to sweep a gigantically stupid law passed before under the rug. The gigantically stupid law was to have a 1000 watt microwave  and a tiny 100 m watt WIFI in the same frequency range. The solution , equally as dumb , was to out law filtered power supplies in microwave ovens so that the tiny 100 m watt WIFI can sneak a few bytes out in a 2 m second window between power cycles of a monster 1000 watt microwave. Is it just me or did someone drop the ball here? Thank god for 5 GHz WIFI.

tooki:

--- Quote from: John Heath on February 13, 2018, 01:46:06 am ---A microwave oven is switching on and off every 8 m seconds as well as every second or so. The reason for this is it is against the law for a microwave oven to run off a filter DC power supply. It must be a unfiltered supply raw off the full bridge rectifier thus on and off every 8 m second. The reason for this law is to sweep a gigantically stupid law passed before under the rug. The gigantically stupid law was to have a 1000 watt microwave  and a tiny 100 m watt WIFI in the same frequency range. The solution , equally as dumb , was to out law filtered power supplies in microwave ovens so that the tiny 100 m watt WIFI can sneak a few bytes out in a 2 m second window between power cycles of a monster 1000 watt microwave. Is it just me or did someone drop the ball here? Thank god for 5 GHz WIFI.

--- End quote ---
Do you have any sources for this supposed law and its supposed effects?

Microwave ovens predate WiFi by around half a century. I doubt they're bound by laws regarding WiFi.

I'm not even sure what you mean about a microwave operating from unfiltered DC. I mean, the magnetron isn't operating off DC, nor off mains frequency.



As for why microwave ovens and WiFi operate in the same frequency range: it's on purpose, because of the same root cause, though for different intents. It's the frequency range that is best absorbed by water. In a microwave oven, this is desirable to achieve optimum efficiency in heating the food. In WiFi, which operates in the unlicensed ISM band, the ISM band was placed in that range for the same reason: it's absorbed by water. This naturally limits the range of the signals by absorption by atmospheric water vapor, allowing the same frequencies to be reused in relatively close physical proximity to each other.

Halcyon:

--- Quote from: John Heath on February 13, 2018, 01:46:06 am ---A microwave oven is switching on and off every 8 m seconds as well as every second or so. The reason for this is it is against the law for a microwave oven to run off a filter DC power supply. It must be a unfiltered supply raw off the full bridge rectifier thus on and off every 8 m second. The reason for this law is to sweep a gigantically stupid law passed before under the rug. The gigantically stupid law was to have a 1000 watt microwave  and a tiny 100 m watt WIFI in the same frequency range. The solution , equally as dumb , was to out law filtered power supplies in microwave ovens so that the tiny 100 m watt WIFI can sneak a few bytes out in a 2 m second window between power cycles of a monster 1000 watt microwave. Is it just me or did someone drop the ball here? Thank god for 5 GHz WIFI.

--- End quote ---

I think it might be you who has dropped the ball this time. This is the first time I've heard of such a thing as well. This is certainly not the case in Australia. Yes, microwave ovens operate within the same frequency range as 2.4 GHz Wi-Fi however in my experience it's rarely a problem if your microwave is operating / built correctly. Microwave ovens are designed to keep as much of the RF inside the cavity as possible. If it's leaking significant RF, then you have bigger issues than just your Wi-Fi dropping out. Microwave leakage testing can be a bit of a wank (like PAT testing) and there are people making money off it, but significant levels can cause problems. There are a number of standards and studies out there which dictate "safe" limits, but leakage of 5 milliwatts per square centimetre or less is considered acceptable.

I have absolutely no dramas with my 2.4 GHz Wi-Fi when my microwave is operating at full power (1200 watts). When it first starts up and ramps up to full power, it can draw as much as 1900 watts @ 240v. Even old devices manage full signal strength with no perceivable difference in throughput.

My microwave also doesn't switch off the magnetron at those intervals, in fact, it's ALWAYS on, regardless of power setting as it has an inverter.

Red Squirrel:
I can kind of tell when the magnetron turns on/off.  It's almost like I can "feel" it, not like feel myself getting zapped because that would mean my microwave is leaking, but there's an odd feeling of magnetic force in the area. Hard to explain.   Same thing when there is a power outage.  When the power comes back on I can feel and hear the magnetic force of the initial "hit".  If I'm in bed it will actually wake me up at times, and I look at the clock and sure enough it's flashing 12:00.    Not sure if that's normal though, maybe I'm crazy? lol.

But yeah microwaves just turn the magnetron on/off in couple second intervals based on the setting.  For defrost you actually want that, because any water will heat up, and that water will help melt ice to create more water.  If it was on continuously the water would evaporate.  Ice does not really get affected as well by microwaves so it would take a long time to melt.  I'm not sure what things like the pizza setting or popcorn setting do though, is there actually a science behind those? 

Halcyon:

--- Quote from: Red Squirrel on February 13, 2018, 02:21:20 am ---I'm not sure what things like the pizza setting or popcorn setting do though, is there actually a science behind those?

--- End quote ---

They just vary the duty cycle (or in the case of inverter models, the output power) of the oven throughout various stages of cooking based on a program. Some programs might start off slow then ramp up towards the end of cooking, some might stop part way through and prompt you to stir/turn whatever you're cooking. It's really nothing fancy or complicated, they are just based off tried and tested recipes by the manufacturer. The microwave doesn't "know" when the food is actually done, it just bases it off pre-programmed weights, cooking sequences and power levels.

For example, if I tell mine to defrost 500 grams of chicken, it knows based on that weight, it should take 12 minutes and 1 second (at a certain power level) to defrost that mass of particular meat.

My microwave (Sharp Carousel) also comes with its own cookbook and the microwave itself has built-in programs based on those recipes. Most brand name microwaves will have something similar.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod