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Home Built Transformer 120 volt primary 48 volt secondary with CT

MrAl

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Most Helpful Member
The fact that a microwave oven uses a magnetron, and a magnetron is a type of valve - reducing the mains to the oven reduces the heater voltage, preventing the magnetron from oscillating. A common fault in microwaves is poor crimping on the push-on terminals for the magnetron heater, and the small loss of voltage caused by the 'high' resistance (a fraction of an ohm) prevents the magnetron oscillating reliably.

The first thing you do when repairing a 'not heating up' microwave (after discharging the HV capacitor!!!) is remove the terminals, give the tag crimps two or three squeezes with a pair of side cutters, and then solder them as well. Then you see if it now works, quite often it does, and if not you start fault finding properly.
Hi again,

That sounds very convincing. The only thing i can add is that maybe there is a critical point that we might reach where it no longer works, but up to that point the output power decreases from 100 percent down, even though not exactly in a linear fashion. As i said i noted that there was a certain voltage, and it was not too low really, where it would no longer cook at all.

It doesnt take much to try though. If we connect a variac to the oven and turn down the power little by little and check each time for cooking power, we see it go down more and more with input voltage and then stop altogether at some voltage like 90v or maybe it as 80v.
I tried cooking hamburger meat and boiling water for testing. Boiling water is a pretty good test because the less the heating power the longer it takes to boil the water and there is a formula we can use to calculate that too approximately.
 

gophert

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It it was Easter time instead of Christmas, we could also determine the frequency of the microwave would shift as you turn down the power. In the first half of the video, they have a history lesson, then they determine the wavelength of a microwave's output, then the speed of light.

 

Nigel Goodwin

Super Moderator
Most Helpful Member
Hi again,

That sounds very convincing. The only thing i can add is that maybe there is a critical point that we might reach where it no longer works, but up to that point the output power decreases from 100 percent down, even though not exactly in a linear fashion. As i said i noted that there was a certain voltage, and it was not too low really, where it would no longer cook at all.
Which was what I said, you can't reduce the power much in that way without it stopping working.

It doesnt take much to try though. If we connect a variac to the oven and turn down the power little by little and check each time for cooking power, we see it go down more and more with input voltage and then stop altogether at some voltage like 90v or maybe it as 80v.
I tried cooking hamburger meat and boiling water for testing. Boiling water is a pretty good test because the less the heating power the longer it takes to boil the water and there is a formula we can use to calculate that too approximately.
Water is (or was) the standard method of measuring the output power of a microwave, although later ovens use a different method in order to give supposed higher power - a modern 800W oven is what was called a 650W previously.
 

MrAl

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Most Helpful Member
Which was what I said, you can't reduce the power much in that way without it stopping working.
Well input power from 1000 watts to 200 watts or even 400 watts is still significant to me, but for what we are talking about, which is cooking power, the cooking power goes down all the way to zero, which means that it must go through the range 0 to Max.
Who cares if the input power range is only 200 to 1000, the cooking power is 0 to max as the input voltage is changed.
 

gary350

Well-Known Member
I have collected several microwave ovens that people did not want. Some have nothing wrong people decided they wanted it gone. Some have a broken door handle. Some have a bad circuit board or a function of the PC board no longer works. Replace the bad circuit board with a toggle switch the oven works find. I save the sheet metal covers on the oven to cut up to build other projects. I save all the transformers to build other projects. I have 5 microwave ovens that I have not tested yet and don't plan to until my next project. I have 6 transformers on the shelf I plan to use the largest one for my next project 1 or 2 turns of 800 amp wire on the secondary. I hope I can heat 1/2" diameter solid steel rods red hot so I can roll the rods into a coil, also make 90 degree bents and hooks.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Some have a bad circuit board or a function of the PC board no longer works.
To be fair, the circuit boards (on electronically controlled ovens) are exceptionally reliable, the only real problem you tend to get is that the membrane keypads are prone to failure. You also get the occasional lightning damaged board, but you can't blame the oven for that :D
 

MrAl

Well-Known Member
Most Helpful Member
They don't reliably go down as low as that, and does anyone use anything other than full power and defrost?.
Hi,

Well i use my Panasonic all the time at around 3 and sometimes level 4.
That's why i wanted to test the other mic at low levels.

Yeah my membrane keypad is already showing signs of wear. The '3' is worn out already. I know i use that almost all the time, but i think it should be made to last longer. Lucky the key still works.
 

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