I've been trying to put together a home made food dehydrator and I was passed on a big old broken Sharp 40L combi microwave to use as the body. It's got a 1.4kW heating element in a compartment at the back and a big fan that draws air from the oven body into that compartment to warm it then blows it out again. I had hoped to use that to dry my fruit as both work fine, but checking it with my multimeter I get an odd reading. The heating element has a resistance of 40 Ohms which translates to the expected 1.4kW, but the fan has a resistance of 180 Ohms which works out as a massive 320 W draw. Now in use it works OK as I said, and sounds no louder than any other oven fan, but is this sort of consumption par for the course for oven fans? And I can't check it while running because I've stripped out all the nasty HT bits (after safely discharging the big capacitor should anyone be fool enough to copy me based just on this posting - the insides of microwaves are nasty places, children, read lots more before picking up your screwdriver).
Combination Microwave fan power
- Thread starter windylewis
- Start date
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1400w = 6 amps ..... 320w = 1.4 amps.
Compared to your element the fan draws peanuts. But it is probably an old inefficient shaded pole motor.
That aside how do you intend to prevent the fruit from catching fire or the dryer overheating & burning your house down?
Compared to your element the fan draws peanuts. But it is probably an old inefficient shaded pole motor.
That aside how do you intend to prevent the fruit from catching fire or the dryer overheating & burning your house down?
The theory of drying fruit is pretty simple: cut it thin (and you can get a cheap gadget on Ebay that will peel core and slice it thin all in one go) and warm it to somewhere between 40 and 60 degrees then add a bit of breeze to blow out the air that has already picked up moisture and swap it for some that is nice and dry. Start the thing up and leave it for anywhere between 8 and 36 hours.
If you want to buy a ready made one then the US made Excalibur is pretty good but pricey, while the cheaper ones are a bit rubbish. But have a look for food dehydrators on the internet and you'll find lots of home made ones of varying quality, going from a foil lined cardboard box with a lightbulb at the bottom upwards. I thought that I could do better than those for a lot less cash.
Now with anything that will be left running that long you want it to use as little electricity as possible. I had originally thought that that a combi microwave would be a great thing to build on as it already has a fan and heatsource at the back. The heater could be wired up to a controller that would monitor the inside temperature and switch it on for a couple of seconds or so a minute. A PID controller (a fancy box of electronics that over time learns how not to overshoot its target temperature) would do the job, or maybe an Arduino type controller connected to a solid state relay. But as I expect the heater to average out to using about 100 watts of electricity an hour then using 300 to blow that heat around seems silly. Time for a small rethink I guess.
But in other ways the microwave is a great container for doing this. Power passes first of all through a small board that has lots of nice filtering stuff on it so that can be reused. Then there are lots of nice little thermal fuses scattered around inside that can be wired up as well, and there's even a little low voltage transformer for powering any microcontrollers. Now I just need to track down another heatproof fan.
If you want to buy a ready made one then the US made Excalibur is pretty good but pricey, while the cheaper ones are a bit rubbish. But have a look for food dehydrators on the internet and you'll find lots of home made ones of varying quality, going from a foil lined cardboard box with a lightbulb at the bottom upwards. I thought that I could do better than those for a lot less cash.
Now with anything that will be left running that long you want it to use as little electricity as possible. I had originally thought that that a combi microwave would be a great thing to build on as it already has a fan and heatsource at the back. The heater could be wired up to a controller that would monitor the inside temperature and switch it on for a couple of seconds or so a minute. A PID controller (a fancy box of electronics that over time learns how not to overshoot its target temperature) would do the job, or maybe an Arduino type controller connected to a solid state relay. But as I expect the heater to average out to using about 100 watts of electricity an hour then using 300 to blow that heat around seems silly. Time for a small rethink I guess.
But in other ways the microwave is a great container for doing this. Power passes first of all through a small board that has lots of nice filtering stuff on it so that can be reused. Then there are lots of nice little thermal fuses scattered around inside that can be wired up as well, and there's even a little low voltage transformer for powering any microcontrollers. Now I just need to track down another heatproof fan.
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