It is certainly possible, just plenty of hassle to open up the core to get access to the windings and later put it all back together without any excessive humming from the laminations that are now no longer super tight...
The transformers also normally run at brink of saturation and are very wasteful so there isn't much headroom, I'm not sure if rewinding the secondaries will help with that aspect.
The transformers also normally run at brink of saturation and are very wasteful so there isn't much headroom, I'm not sure if rewinding the secondaries will help with that aspect.
What i saw on the web was 2 ways of rewiring the coil : cut throu the iron and have completely access to the secondary coil, and the other, he doesn't cut throu the iron, all the iron stays intact and he cuts the secondary coil wires. But i'm no having much sucsess throu this second way because the wires are really glued to the inside of the iron and its very dificult to remove the secondary coil with the tools i have here.
"The transformers also normally run at brink of saturation and are very wasteful so there isn't much headroom, I'm not sure if rewinding the secondaries will help with that aspect."
Can you please explain to me this better ? I'm not understanding what yu're saying when you talk about saturation ...
Thanks all.
"The transformers also normally run at brink of saturation and are very wasteful so there isn't much headroom, I'm not sure if rewinding the secondaries will help with that aspect."
Can you please explain to me this better ? I'm not understanding what yu're saying when you talk about saturation ...
Thanks all.
Here is some explanations:
http://www.ti.com/lit/ml/slup124/slup124.pdf
https://coefs.uncc.edu/mnoras/files/2013/03/Transformer-and-Inductor-Design-Handbook_Chapter_2.pdf
But i like to say that it is not very promising to rewound this microwave oven transformer, like you say - 100V and 9A. This means apparent power 900VA but this microwave oven transformer iron core just dont fit this kind of power.
If you need 900VA power then you need a core which central leg cross section is approximately 36 square cm. ie - if you have, say E-I iron core like here: https://upload.wikimedia.org/wikipedia/commons/thumb/7/71/Scrapless_Lam.jpg/220px-Scrapless_Lam.jpg
and this core has a 40mm center leg (nr2 measuring in this drawing) then you need 80mm thick stack to form good core.
Rule of thumb is: square root from power you need multiplied by 1.2 (this will give you a some headroom too). Example if you need 900VA power then square root900=30 multiplied 1.2 and you get 36square cm central leg cross section.
If you dealing with toroidal or strip (banded) core, then you need smaller cross section core. I think you dont really want to wrap it yourself without a winding machine but if you really want to do this and you have adequate core and copper wire then we may help you if you need information etc. number of needed turns and choice of wire diameter... But you need to understand electrical safety rules too!
Tried this, the laminations are not interleaved, but simply welded together. I removed the secondary windings & calculated(guessed) to the number of windings for a tube heater, did the windings, reassembled as best possible & powered it up to measure how much voltage. I was rather close, but the profound buzzing we all hear when firing up a Microwave oven was still present...a truly unacceptable sound level.
----------------------------------------------------------------------Rick............
----------------------------------------------------------------------Rick............
Here is good one if you really wish to wrap yourself 
I was 14 if I wrap my first transformer, over 1kVA power
Practical Transformer Winding
I was 14 if I wrap my first transformer, over 1kVA power
Practical Transformer Winding
MOT's are not good quality. They are run very high up the B/H curve, and most have magnetic shunts that increase the leakage reactance to very high values. About the only thing you could use a re-wound one for is a battery charger - where you actually want a transformer with those characteristics. Almost any other transformer would be more suitable for a custom rewind job to build an amplifier.
The other thing they're good for is making Tesla coils. For that you just use the original HV winding. And put a couple in series -, and add a voltage multiplier.....
The other thing they're good for is making Tesla coils. For that you just use the original HV winding. And put a couple in series -
, and add a voltage multiplier.....These are the transformers that i have here, i tried to remove the secondary coil in one of them but no sucsess. I end up on damaging the primary coil a litlle bit. The secondary coil is really glued and when i'm in a place with appropriate tools, i gonna have to remove that part of the iron to have full acsess to both coils.
These are the two transformers:
![IMG_20171027_090320[2].jpg](https://www.diyaudio.com/community/data/attachments/584/584119-292d8038cb31795833fbfcf7a84c9970.jpg "IMG_20171027_090320[2].jpg")
![IMG_20171027_090441[1].jpg](https://www.diyaudio.com/community/data/attachments/584/584154-984f9bdd35a8c6d22cdf6fd832799819.jpg "IMG_20171027_090441[1].jpg")
![IMG_20171027_090531[1].jpg](https://www.diyaudio.com/community/data/attachments/584/584168-b1415ecefa13e4043d053ac819cb949e.jpg "IMG_20171027_090531[1].jpg")
![IMG_20171027_090542[1].jpg](https://www.diyaudio.com/community/data/attachments/584/584183-ffd9c94d9729fb8eafc0edec73641254.jpg "IMG_20171027_090542[1].jpg")
![IMG_20171027_090627[1].jpg](https://www.diyaudio.com/community/data/attachments/584/584193-7a863317d49bea70646b143f00cf6971.jpg "IMG_20171027_090627[1].jpg")
![IMG_20171027_090635[1].jpg](https://www.diyaudio.com/community/data/attachments/584/584204-d0ac0434b2636a05852d07c323e3e91f.jpg "IMG_20171027_090635[1].jpg")
These are the two transformers:
When you get this old coils out and wound new secondaries this place, they work good.
BTW - core seems maybe 200-250VA if compared to coin
A microwave transformer core is a poor choice for an audio amplifier.
Microwave ovens are built CHEAP as possible. They work the iron VERY hard. This is acceptable for short-time cooking. I believe if you just connect that primary, no load, after an hour the winding will be HOT and breaking-down. I would be very wary about leaving it run all-day like a hi-fi amp often does.
The magnetron tube has wide parameter variation. The transformer is designed to "sag" so that a high-current magnetron gets lower voltage so it will not over-cook itself or your food. As wg_ski says, there is a shunt to reduce flux to the secondary. This built-in sag is much more than we want in an audio amp supply.
Use MOTs for SHORT-time loads: popcorn, welding, melting gold.
The Chinese will, in much of the world, sell you an appropriate transformer for audio amplifiers at a price scarcely higher than I would pay just for new secondary wire.
Microwave ovens are built CHEAP as possible. They work the iron VERY hard. This is acceptable for short-time cooking. I believe if you just connect that primary, no load, after an hour the winding will be HOT and breaking-down. I would be very wary about leaving it run all-day like a hi-fi amp often does.
The magnetron tube has wide parameter variation. The transformer is designed to "sag" so that a high-current magnetron gets lower voltage so it will not over-cook itself or your food. As wg_ski says, there is a shunt to reduce flux to the secondary. This built-in sag is much more than we want in an audio amp supply.
Use MOTs for SHORT-time loads: popcorn, welding, melting gold.
The Chinese will, in much of the world, sell you an appropriate transformer for audio amplifiers at a price scarcely higher than I would pay just for new secondary wire.
Micro-wave transformers as stated in earlier replies cannot be used for normal duty PSUs.
They have one very high voltage winding. This makes a good air cored inductor if you can remove it without damage.
The magnetic shunt must be removed. The easiest way to get the HV secondary and the iron shunt out is to cut the welds along the EI core.
This opening up also allows one to access the primary winding.
You will find that the primary winding uses a lot less than 50% of the winding window.
This is the main reason the transformer cannot be used for normal duty. There is too little copper in the primary. You need to add many more turns to reduce the near saturation of the core.
Winding a new primary is an onerous task.
Winding an extra partial primary to add to the existing primary is also an onerous task.
Think carefully before you attempt either.
An alternative use:
remove the iron shunt.
add the existing primary and the existing secondary in series and place back into the EI core.
Adjust the EI gap when you place the Is back on top of the Es.
This makes an acceptable gapped iron cored inductor.
They have one very high voltage winding. This makes a good air cored inductor if you can remove it without damage.
The magnetic shunt must be removed. The easiest way to get the HV secondary and the iron shunt out is to cut the welds along the EI core.
This opening up also allows one to access the primary winding.
You will find that the primary winding uses a lot less than 50% of the winding window.
This is the main reason the transformer cannot be used for normal duty. There is too little copper in the primary. You need to add many more turns to reduce the near saturation of the core.
Winding a new primary is an onerous task.
Winding an extra partial primary to add to the existing primary is also an onerous task.
Think carefully before you attempt either.
An alternative use:
remove the iron shunt.
add the existing primary and the existing secondary in series and place back into the EI core.
Adjust the EI gap when you place the Is back on top of the Es.
This makes an acceptable gapped iron cored inductor.
Just one another thing, when we put a resistor in series with a capacitor, in the very beginning of the charging process of the capacitor, the current that flows in the circuit is given I=(V/R) ( it's like the capacitor doesn't exist )... But in a power supply ( transformer + diodes + capacitors ), in the very beginning of the charge process of the filter capacitors, the current won't exceed the max supported by diodes ? How can i calculate the initial current in this case ?
Thanks all.
Thanks all.
posted before.
Add up all the resistance in the secondary charging circuit.
if the transformer is already started and up to full open circuit voltage and the capacitors are fully discharged then you have Ipk=Vemf/{total of all resistances}
For the primary circuit where the transformer has zero remnant flux again you add up all the primary circuit resistances and divide them into the mains voltage that exists at the moment the switch is closed. Ipk~Vmains/{total of all resistances}
Add up all the resistance in the secondary charging circuit.
if the transformer is already started and up to full open circuit voltage and the capacitors are fully discharged then you have Ipk=Vemf/{total of all resistances}
For the primary circuit where the transformer has zero remnant flux again you add up all the primary circuit resistances and divide them into the mains voltage that exists at the moment the switch is closed. Ipk~Vmains/{total of all resistances}
microwave transformers are built for cheap, so low duty, they are not built for continuous service. especially when idling, the core is close to saturation, such that a high no.load current is running, heating up the primary.
in my opnion, elcheapo microwave transformers are hardly suitable. if you have a chance to find transformers from pro-restaurant units , take those. if a choice of 220 and 240V is available , always take the 240 tap.
in my opnion, elcheapo microwave transformers are hardly suitable. if you have a chance to find transformers from pro-restaurant units , take those. if a choice of 220 and 240V is available , always take the 240 tap.
Hi, just another question... Amplifiers's PSU's can be used to build adjustable ( or not ) and simmetrical ( or not ) dc power supply's ? Is this kind of transformer ready to deliver a dc signal with the objective to feed something ? I ask this because an audio wave is always oscilating between negative tension and positive tension giving to the psu time to rest a little bit ( at least , the amplifier ( itself ) stop's requiring psu energy for a little bit ) , i mean...
Thanks All.
Thanks All.
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