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Home Built Transformer 120 volt primary 48 volt secondary with CT

Nigel Goodwin

Super Moderator
Most Helpful Member
#21
I know many people, who use Microwave as defrost / preheat... And they run it for tens of minutes (try defrost 2kg of meat, it will run nearly hour or more...). By your posts all of them should have fried transformers. Is there no safety and every microwave producer is sure, that nobody runs it for more than few minutes? Are you really sure about your posts?
I've never understood what they are on about?, microwaves run perfectly happily pretty well permanently - they even have timer settings for an hour or more.

Obviously with extended use they get quite hot, but they are designed to do so - maximum heat is probably reached in 10-15 minutes?.

However, I wouldn't suggest that a microwave oven transformer is a good choice to rewind, as the laminations are welded.
 

MrAl

Well-Known Member
Most Helpful Member
#22
Hi, I know this is old thread, but I want to build PS from MOT and need to ask...

I know many people, who use Microwave as defrost / preheat... And they run it for tens of minutes (try defrost 2kg of meat, it will run nearly hour or more...). By your posts all of them should have fried transformers. Is there no safety and every microwave producer is sure, that nobody runs it for more than few minutes? Are you really sure about your posts? If so, please prove it with something stronger than thoughts & feelings...

No hard feelings. I work as designer in automotive and I know well, how high are security reserves when you sell something. This surprised me a lot considering how dump usual users are. Sorry because of my "english" :)
Hello,

This may or may not matter in your case, but when microwaves are put on the defrost setting they do not run at full power. They run at either reduced power or at full power but then turn on and off.
The real question i think is not if they can run for one or two hours, it's if they can run for one or two weeks non stop at full power.

Lots of people have rewound mic transformers, but there are drawbacks. One is that it is just hard to do, and second i dont think there is any way to get the leakage inductance lower. That can be a benefit with a DC power supply though.
The long term use might come into question, such as if the finished power supply has to run for weeks at a time. It certainly should be able to run for a few hours i would think.

Also, this has to be rethink'ed if you remove the shunts. Of course good testing is always recommended to be sure it works for what you want it to work for. Most people get their mic transformers from old microwaves anyway so it doesnt matter as much if it doesnt work.
 

gary350

Well-Known Member
Thread starter #23
Wow this was 7 years ago. Did I ever mention I finally found the correct math and built the transformer I need and it works great. Did I not post a picture? Pictures are probably still on photo bucket but photo bucket has changed I cannot log on unless I pay a fee. Photo bucket can kiss my ***.
 
Last edited by a moderator:

MrAl

Well-Known Member
Most Helpful Member
#24
Wow this was 7 years ago. Did I ever mention I finally found the correct math and built the transformer I need and it works great. Did I not post a picture? Picture is probably still on photo bucket but photo bucket has changed I cannot log on unless I pay a few. Photo bucket can kiss my ass.
Hi,

Sounds like the ol' bait and switch. They get you hooked, then later change it so you have to pay.
You can post pics right here though so that's the best bet.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#25
This may or may not matter in your case, but when microwaves are put on the defrost setting they do not run at full power. They run at either reduced power or at full power but then turn on and off.
If I recall correctly?, defrost is 30% ON and 70% OFF, on a 10 second cycle. However, you can happily run a microwave on full power for extended periods - and as I mentioned previously, the transformer reaches maximum temperature relatively quickly.
 

MrAl

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Most Helpful Member
#26
If I recall correctly?, defrost is 30% ON and 70% OFF, on a 10 second cycle. However, you can happily run a microwave on full power for extended periods - and as I mentioned previously, the transformer reaches maximum temperature relatively quickly.
Hi Nigel,

Have you ever run one for two weeks straight on full power?
Or maybe for 24 hours straight?
 

MrAl

Well-Known Member
Most Helpful Member
#28
No, but I have for 3 or 4 hours, and they reach maximum temperature in about 15-50 minutes.
Hi,

Ok, well the reason i was asking is because meltdowns might take some time when there is something wrong.
We had one meltdown on life test once after we all went home for the night. The next morning, everything that was not metal had melted and dripped out of the converter output transformer. What a mess. I think that was a 2000 watt unit or maybe 3000 watts.
Output transformer had to be redesigned.

Power supplies often have to run for days, weeks, months. We had some that had to power computers that had to stay on 24/7. That's when reliability issues really start to crop up.
 

schmitt trigger

Well-Known Member
#29
To avoid guessing:

If one has access to an AC real-power meter, one can take a power-in reading and a power-out reading, substract both and you will obtain total transformer losses, core plus copper.

There are electrical tables for target efficiencies depending on the transformer’s power level.
If I remember correctly, a 1 kva transformer should be between 90% and 95% efficient.
 

gophert

Active Member
#33
Yes, but the lower power settings (like defrost) run at low duty cycles rather than a constant reduced power. You can hear the microwave cycle between on and off cycles - usually 5 to 10 second cycles with defrost function being about 30% power. So a long defrost cycle is not really pushing a MW transformer at its limits.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#34
Yes, but the lower power settings (like defrost) run at low duty cycles rather than a constant reduced power. You can hear the microwave cycle between on and off cycles - usually 5 to 10 second cycles with defrost function being about 30% power. So a long defrost cycle is not really pushing a MW transformer at its limits.
As I mentioned earlier, Defrost is 30% duty cycle, on a 10 second cycle - however you can happily stick them on 100% duty cycle (Full Power) for extended periods as well.

You can't very well reduce the power in a linear fashion (and there would be no point in doing so) as you would reduce the heater voltage accordingly, preventing it oscillating. Burst fire control (which is what this is) is perfectly fine for heating applications.
 

MrAl

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Most Helpful Member
#35
As I mentioned earlier, Defrost is 30% duty cycle, on a 10 second cycle - however you can happily stick them on 100% duty cycle (Full Power) for extended periods as well.

You can't very well reduce the power in a linear fashion (and there would be no point in doing so) as you would reduce the heater voltage accordingly, preventing it oscillating. Burst fire control (which is what this is) is perfectly fine for heating applications.
Hi,

I've actually experimented quite a bit with reducing power in a linear fashion. This was using a variac. It's a little tricky, but it works.
I found out by accident when our power line went down in the summer months repeatedly, i started to notice a longer and longer required cooking time. Once i got the variac, i could try it at different settings.
As i said though it is tricky, and it does not cook at all below a certain input voltage like maybe 90v but i cant remember offhand.
I tried this with a regular mic oven not my Panasonic.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#36
Hi,

I've actually experimented quite a bit with reducing power in a linear fashion. This was using a variac. It's a little tricky, but it works.
I found out by accident when our power line went down in the summer months repeatedly, i started to notice a longer and longer required cooking time. Once i got the variac, i could try it at different settings.
There's obviously a small range it will work over, simply because an oven needs to work if the mains is a bit low - however, you can'r reduce it enough to be of any use - and as I said before, what would be the point anyway?.
 

MrAl

Well-Known Member
Most Helpful Member
#37
There's obviously a small range it will work over, simply because an oven needs to work if the mains is a bit low - however, you can'r reduce it enough to be of any use - and as I said before, what would be the point anyway?.
Hi,

I am not sure what you mean here. I can get 100 percent down to 1 percent if i wanted to.
But the purpose is to have continuous low power cooking power v.s. pulsed low power cooking. There seems to be a difference in the resulting cooked food. One advantage seems to be more tender meats. I see this with my Panasonic too because that has 'inverter' technology.
Also, it's a little fun to play around with :)
Someone else i know from online conversation uses a series impedance to lower power to a set level for slow cooking beans.
 

MrAl

Well-Known Member
Most Helpful Member
#39
I'm absolutely sure you can't get 1% power from a microwave with a variac, you 'might' get down to 70% perhaps?, but I would have thought any less would be unlikely?.
Hi,

Well then what do you base that on?

As i am sure you know, the input power to most devices is related to the input voltage. If we start with a device that takes 1000 watts and is resistive and reduce the input voltage by 50 percent the input power drops to 25 percent of 1000 which of course is 250 watts. So why not just reduce the voltage more to reduce the input power even more.
The mic cooking power is related to the input power, so the less input power the less cooking power.

It's actually easy to try though. Just reduce the input voltage to a standard non inverter mic oven and note that the cooking power goes down more and more as you reduce the voltage. I actually did this experiment and measured the input power for several input voltages.
The cooking time of any object placed in the oven of course gets longer and longer as the voltage is reduced because there's just less power available.
Granted it is not the same as having a resistive element because that is more linear, but with less power in less power out and so longer cooking times.

The oven i used for the tests was a 700 watt oven i think. Obtained from Target stores when it was on sale for only about 30 dollars USD. In contrast, my Panasonic was something like 130 dollars USD or something like that.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#40
Hi,

Well then what do you base that on?
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.
 

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