Microwave oven transformer as a 12V power source

Any words of wisdom on this before I consider trying it for myself.

According to this: http://www.users.on.net/~endsodds/psrewind.htm, you can remove the secondary windings, add 10-20 turns of suitably amp rated cable and re-task the transformer as a 12V high amp power source.

Since you are unclear on how transformers are built and work (if you weren't you wouldn't have asked the question) the only responsible suggestion I can make is "don't even try"
Working with mains voltage systems is potentially lethal and not something beginners should be doing.

What specific reason do you have to attempt this potentially lethal project?

Taking apart the mains transformer from a microwave with hammer and chisel and rebuilding it for 12V using instructions off the internet when you don't have a clue what you're doing => http://www.darwinawards.com/

Any words of wisdom on this before I consider trying it for myself.

Make sure you haven't missed any premiums on your life insurance policy.

Pete

Well I have actually built a bench DC power supply with a transformer I extracted from a discarded guitar amplifier.
Although the rectification electronics I did was on the low voltage side.

Not quite the same as what I am contemplating here I know.

I am conscious of the dangers involved with mains voltage and I am not necessarily going to proceed unless I am confident of all the facts. Perhaps never.

The reason for using the laminated core is to limit current induced in the core itself by the primary windings.

So why don't the welds, across the laminations that bind them together, create circuit paths through which current can flow within the transformer core?

Welding the core laminations together seems to defeat the purpose of electrically insulating the individual core laminations with varnish.

No, because you only need to prevent current from circulating - with one weld-seam there is
no return path. Two weld seams and yes you'd create a single shorted turn that would
probably glow red hot...

For the same reason you can bolt a toroidal transformer to a panel using a metal plate and
bolt, but if you add another conducting path around the outside of the toroid to the plate
you'd cause mayhem.

el_supremo:
Taking apart the mains transformer from a microwave with hammer and chisel and rebuilding it for 12V using instructions off the internet when you don't have a clue what you're doing => http://www.darwinawards.com/

Any words of wisdom on this before I consider trying it for myself.

Make sure you haven't missed any premiums on your life insurance policy.

Pete

Yes Pete I know all about the Darwin awards and I don't plan to get a mention on them.

I am very conscious of the dangers of mains voltage.

You would probably be horrified at the fact that I do my own minor electrical work around the house - replacing busted light switches and power points etc.

IF I was to attempt this I would be enclosing the transformer in a plastic box or something, earthing the core and fusing the mains power to it - I have some old fashioned fuse wire laying around in my garage. I have also had a suggestion from another forum to use a dimmer switch to control and limit the current through the primary winding.

But I am not yet clear whether or not drawing 30A from such an arrangement for an extended period could potentially cause the primary windings to fail. Or simply cause the secondary voltage to sag.

The fact that folks make spot welders out of MOTs, that draw hundreds of amps at low voltage for short periods of time, would suggest that they could handle 12V / 30A. But until I am confident I am not going to bother with this as it would be a waste of effort.

MarkT:
No, because you only need to prevent current from circulating - with one weld-seam there is
no return path. Two weld seams and yes you'd create a single shorted turn that would
probably glow red hot...

For the same reason you can bolt a toroidal transformer to a panel using a metal plate and
bolt, but if you add another conducting path around the outside of the toroid to the plate
you'd cause mayhem.

But hang on.......if you melt the edges of the laminations together then I don't understand why they haven't created a return path.

Why can't a current flow from the edge of one lamination, across the weld to the opposite lamination, through that lamination to the opposite weld, back along the opposite weld to the original lamination,...............

As I understand at present, any induced current would flow across the laminations if they weren't individually varnished, i.e. in the same direction as the current induced in the secondary windings.

There are 3 welds on the MOT core I have. Two either side of the join between the E and the I part of the core and one at the top of the E.

So if any induced current flows as I am assuming it would, then there appears to be a complete electrical circuit through the welds and through the body of the laminations.

Clearly there is something wrong with my assumptions here because these transformers don't glow red hot when still in the microwave oven. So where are my assumptions faulty?

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Actually there is another issue with this that I have not considered....

If it is going to make a horrendous sound like this then it makes for a fairly impractical power source - it would be extremely irritating.

Still interesting as an exercise however.

Transformers are -not- a simple component. Especially when starting with a microwave oven transformer.

polymorph:
Transformers are -not- a simple component. Especially when starting with a microwave oven transformer.

Are you prepared to elaborate on that?

http://www.pupman.com/listarchives/2000/February/msg00373.html

On the transformer saturation topic, here are a few thoughts of mine:

A given transformer is most likely to saturate when its primary is run
overvoltage and there is NO LOAD current drawn from the secondary.
This results in the highest flux density in the iron core. If there
is not enough iron, saturation will start to occur. A good example
of this is the Microwave Oven Transformer. These are built to a low
budget and contain the bare minimum of iron to minimise weight. If
you run one of these with its secondary open circuit, you will notice
it growl when you feed it with much more than its rated input voltage.
This is accompanied by a steep rise in magnietizing current, (The tell
tale signs of core saturation.)

http://sound.westhost.com/xfmr.htm
http://sound.westhost.com/xfmr2.htm

Often, people using microwave oven transformers will wind a few extra turns to put in series with the primary circuit to bring it further from saturation under no load conditions.

And those magnetic shunts that your link has you removing are there to help prevent saturation under no load.

http://www.randomfunprojects.co.uk/shed_charger.html

One of the problems associated with microwave oven transformers is that, due to core saturation, they take very high magnetising currents and get hot even at low loads. To improve the efficiency of this power supply additional turns were added to the transformer primary and bring the core out of saturation. The number of turns needed was determined simply by adding turns and measuring the current drawn until it reached a sensible level, from memory this was about an extra 50 turns. The wire used was from the primary of another similar transformer and was actually made of aluminium with coloured enamel to make it look like copper. I joined this to the existing primary and to the copper power wires with crimp connectors. The transformer had two secondary windings of 1.5 mm2 pvc insulated wire with an output of around 18 Volts, and an extra winding of small pvc insulated equipment wire which supplys a few milliamps to some of the control circuits. This final winding wasn't really needed but since I was winding my own transfomer I included it anyway. I could have instead connected a voltage doubler to one of the other windings.

polymorph:
Often, people using microwave oven transformers will wind a few extra turns to put in series with the primary circuit to bring it further from saturation under no load conditions.

What about the idea that was suggested to me of puting a standard dimmer switch in series with the 250V primary to limit and control the current?

That would be far easier, and perhaps safer, than trying to extend the primary winding.

polymorph:
And those magnetic shunts that your link has you removing are there to help prevent saturation under no load.

14 Volt 25 Amp battery charger/power supply

Well my understanding is that those magnetic shunts are there to limit current in the secondary windings rather than the primary windings.

And that a side effect of them is that the increase current unnecessarily in the primary due to leakage inductance making the transformer some what inefficient.

But then the microwave manufacturers don't give a shit because they are not the ones paying for the electricity.

Well look all this is good because I think I can assume that I have rendered this particular MOT unusable.

Because when I was getting the secondary windings out I used a drill bit and ground into the central part of the core a little as a result. So I wont use this core in case I have compromised the insulation between the laminations in the central part of the core where the eddy currents are concentrated.

I think a safer way to remove the windings would be to use an angle grinder carefully on the welds joining the E to the I, dismantle the core and get the windings out with a block of wood. Then put the primary windings back, along with new secondary windings, and re-assemble the the core. Another hint in the video I was watching was to heat the whole thing up in order to soften the glue before removing the windings.

I'm also thinking of using a MoT (Class H) that I have to build a step down transformer. I've checked both of the windings and none is connected to the core. It doesn't have any shunts installed either. I connected it to mains (230V) in reverse, i.e. mains to secondary (HV) winding and measured the output from the primary around 22/23V. The no-load current in the HV winding was pretty much zero.

So the idea is to use it in reverse, add a soft-start circuit, some filtering caps and a high power rectifier and have a 23VDC high amperage PSU.