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Using an old microwave transformer

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Douglas Johnston24/01/2021 15:17:29
729 forum posts
34 photos

I have started to make a small spot welder using an old microwave transformer ( I never throw anything away! ) and have a question I can't seem to get an answer to despite trawling the net.

When the redundant secondary winding was removed I noticed that there were two stacks of about half a dozen iron laminations separating the primary and secondary windings. I believe these are called shunts, but I have no idea what their function is and whether they can be removed to create more space for the thick welding cable secondary winding.

Can some electrical guru explain the function of these shunts and should I leave them in or discard them ?


john fletcher 124/01/2021 15:37:18
669 forum posts

Well, when I made my spot welder several years ago, the shunt went out, as I though it provided an alterative path for some of the magnetic flux. I used two turns (I think) of plastic covered PYRO and used the OUTER as the conductor ignoring the inner. For the jaws I fortunately had a length of damaged earth rod. It worked OK on thin stuff, but I think a bigger transformer from an industrial. cooker would be much better. Anyway, space became a premium in the workshop, so out it went. As an alternative, if you need to join sheet steel panels together is to drill a hole in ONE piece and with your stick weld or Mig tack both together, through the hole. I've seen car restoration folk do this, and works well. john

Douglas Johnston24/01/2021 17:29:03
729 forum posts
34 photos

Thanks for the reply John, so it would seem that I can dispense with the shunts if I need the extra space for the secondary winding. I would love to know why they are included in the first place for use in the microwave.


John Rudd24/01/2021 18:33:33
1428 forum posts
6 photos

As I understand it, the purpose of the shunt(s) is (are) to aid regulation of the transformer in either current or voltage......

SillyOldDuffer25/01/2021 09:57:11
6877 forum posts
1539 photos

Last night I found a good explanation on the web, this morning I can't find it!

Gist of it was Microwave Oven Transformers are designed to power a pulsed cavity magnetron with the minimum sized core and copper. The transformer is intended to deliver high-voltage / high amperage pulses and magnetic shunts are used to stop the transformer passing excess current and burning itself out. An inductive choke would do much the same job, but a few extra laminations inside the transformer are much cheaper.

Rewinding the secondary for other purposes the advice was to remove the shunts. In theory more turns should be added to the primary to compensate for removing the shunts, but in practice they work without. I suppose the worst that could happen is the primary burning out and junking the transformer,

Ages ago I built Tesla Coils and looked into MOTs because they're an attractive source of cheap beefy high-voltage power. Unfortunately, MOTs are exceptionally dangerous, perhaps the closest you can get in the home to a full-throttle industrial accident. Apart from the kilovolt shock hazard they deliver more than enough current to char flesh. Several amps! Those lucky enough to survive electrocution spend months in hospital recovering from severe burns.

Don't play with an unmodified MOT!


Douglas Johnston25/01/2021 11:16:29
729 forum posts
34 photos

Thanks for that information Dave, the theory behind the operation of the shunts would probably tax my aging brain to the limit, but at least I can ditch the shunts without any problem. I wonder if modern microwave ovens still use these large transformers or if there is new electronic wizardry to replace them.


Frances IoM25/01/2021 11:21:30
1002 forum posts
27 photos
I took apart a modern microwave oven which used a HF + much smaller transformer to provide the EHT for the magnetron - the HF was derived by an electronic circuit fed from the rectified mains
John Haine25/01/2021 11:23:08
3661 forum posts
206 photos

A quick Google suggests that modern microwave ovens use switch mode supplies, which would be lighter, smaller, and more controllable. Certain the one we bought a few years back to replace the first one (which died after 20 odd years) is too small and light to have a huge chunk of iron and copper inside it.

And looking further also suggests that the magnetron is being replaced with solid state RF power devices.

Edited By John Haine on 25/01/2021 11:38:18

Joseph Noci 126/01/2021 06:30:10
853 forum posts
1051 photos

The shunt is as has been said to provide a degree of protection - both to the magnetron and the transformer by providing current limiting. When current flows in the secondary, an opposing magnetic field is set up in the transformer core. This reduces the induction in the secondary, and therefore output voltage. This is overcome when there is only one magnetic path, and so all the primary flux cuts the secondary winding, and current is then only limited by the winding resistance ( and AC impedance which we will ignore for this discussion)

However, if a second flux path is provided - the 'shunt'- The secondary generates the opposing magnetic field, the strength of which depended on load current, but the Primary flux wishes to be maintained, primary flux is forced to pass more and more through the shunt, and less through the secondary leg, hence output voltage drops as does current, and a balance is achieved.

This is easily seen in the inexpensive buzz-box stick welders - these often use normal E-I core laminations, with the primary and high-amperage secondary windings on the center leg of the E laminations. All flux cuts the secondary all the time, current is max and limited only by wire and cable resistances. Typically the hi-current secondary open circuit voltage is 18 to 26 volts.

The lower current winding are in series with the high current primary and wound on one of the outer legs of the E laminations. The resulting voltage is higher - from 30 volts up to even 50 volts. This ensures a good arc is struck at the start. But then, after the arc starts, the current flow in the outer leg secondary generates an opposing flux which forces a portion of the primary flux to flow in the opposite leg of the E core, and not in the secondary leg, so dropping the secondary voltage an current.

( Contrary to intuition, the HIGHER the secondary voltage in the outer leg windings, the LOWER the secondary current - this is because the high voltage strives to force a high weld current, which in turn generates a higher opposing flux, forcing the voltage to drop and current with it.)

Here I converted a buzz-box core to a 5000 ampere spot welder - the core is simply a square core , similar in operation to a toroidal core, and as a buzz-box welder it had a shunt across the center which I removed. The secondary winding are over the primary. The white upper section with the black stripes are pieces of wood held in place with tie-wraps to stop the core laminations buzzing ( its a buzz box..) at high currents!

The yellow cables are 20mm diameter, two in parallel and in the central part of the core they occupy the space where the shunt sat.

The core is not ideal - it has lots of leakage - desirable in cheap stick welders to have some sort of welding 'control' in lieu of proper current regulation ( variable saturable inductors, etc) and so does not provide best performance in a spot welder application. You want the best magnetic coupling ( secondary under the primary, tight wound) , good silicon steel laminations, properly insulated ( lacquer/varnish) , very low resistance in the weld current path ( only copper, no brass, steel, aluminium, etc).

I have 3.8v at no load on the secondary - at 5000 amps flowing the voltage drops to 2 volts typically, at the weld, and is 3.2volts at the ens of the yellow secondary wires - 1 v is lost in the rest of the cables, the weld arms, connections, etc - thats only 0.2 milliohms!!! Resistance rules in a spot welder!

heavy wiring and flex.jpg

All very long winded,

The bottom line is if you leave that shunt in place, you will a Spot Welder Not Make...

And Removing it will raise the primary magnetisation current a fraction but can be ignored.





Edited By Joseph Noci 1 on 26/01/2021 06:38:29

Douglas Johnston26/01/2021 11:53:14
729 forum posts
34 photos

Wonderful explanation Joseph and that spot welder you have made really looks the business. The spot welder I have in mind is a much more modest affair for use with very thin gauge metal. I also want to experiment with band saw blade welding with another microwave transformer along the lines of an article in MEW 41.

Perhaps I should start collecting some more old microwave ovens in case I find other ideas to investigate, since it would seem more modern ones are less likely to have a good old beefy transformer.


Joseph Noci 126/01/2021 13:56:22
853 forum posts
1051 photos

Yep, collect those uWave ovens if you can - those transformers are very useful for all sorts of things - excellent for high current power supplies ( for the likes of radio hams) , beefy audio amplifiers, small welders , etc - If you try to purchase such a core with primary winding it costs a fortune!


Regarding bandsaw blade welding - The current required for blade welding varies hugely, and generally underestimated - the thin blade, sort of 0.25 mm thick wood blades, the shiny bright silver types, up to 10mm or so width, weld easily with around 300 / 400 amps - within the capability of an MOT style spot welder. The metal shop type blades - 0.6 to 0.8mm thick x 12 to 16mm wide are a different story - A 0.8 x 16mm blade yields a surface contact area of 12 sq mm - that is a large area and needs lots of current to heat orange/white! Easily 2000 amps or more. Too little amps creates a tendency to extend the weld time, which just heats up a larger portion of the blade, annealing the nearby teeth to uselessness - the weld is also invariably suspect. The weld region must be very limited in width and that you achieve with a very high, short blast of current.

I have an 'IDEAL BS-1 blade welder - it works well for the woodshop blades, but is very marginal for the metal saw blades...So, I hook the big spot welder up to the IDEAL's terminal for the heavy blades!


welder bs1 front.jpg


Even more reason to collect more transformers - if you can get a few identical ones, you can parallel the secondaries and double/triple, etc, the current easily.


Edited By Joseph Noci 1 on 26/01/2021 13:58:27

Douglas Johnston27/01/2021 11:28:52
729 forum posts
34 photos

Hi Joe, you are a mine of good information. With regard to the bandsaw blade welding, I want to try welding 1/2" by 0.025" blades for my ubiquitous 6 by 4 inch horizontal bandsaw and as you mentioned the MOT should be up to the task. I think it will take quite a bit of experimenting to get the correct conditions of power and preload etc but it will keep me amused for a while during lockdown!


john fletcher 127/01/2021 12:28:39
669 forum posts

I silver solder my 6 x 4 band saw blades. I chamfer the two ends using my belt sander, I made a simple jig to hold each ends, one at a time, getting both the chamfer to the same angle. Then using another little jig ( a piece of angle iron this time) to hold both ends together, I apply the flux, heat things up and solder. A easy quick job and, I mix and match bits to make another blade, now have more than eight. Rarely do I get a broken one, worn out yes. Wouldn't be without the bandsaw, there was an article some where on making a larger table for ripping lengths of wood John

Ian P27/01/2021 13:14:05
2465 forum posts
101 photos


When dismantling an old oven to extract the transformer be aware that there may still be lethal charge in the capacitor even though its disconnected from the mains. In theory the oven manufacturer will have fitted a bleed resistor but its best to be safe.

I have no bad experiences or anecdotal knowledge but suggest that anyone contemplating repurposing the transformer does some research beforehand.

Ian P

Douglas Johnston27/01/2021 13:36:58
729 forum posts
34 photos
Posted by Ian P on 27/01/2021 13:14:05:


When dismantling an old oven to extract the transformer be aware that there may still be lethal charge in the capacitor even though its disconnected from the mains. In theory the oven manufacturer will have fitted a bleed resistor but its best to be safe.

I have no bad experiences or anecdotal knowledge but suggest that anyone contemplating repurposing the transformer does some research beforehand.

Ian P

Very well said Ian, you can't be too careful when dealing with mains and charged capacitors, I have had a few minor shocks in my time and am now ultra careful when dealing with electricity.

With regard to bandsaw blades, I too have used silver soldering with good results and don't really need to try welding but just thought I would give it a try after reading the article in MEW. That article by the way was very interesting but resulted in a device that was rather over engineered. My idea is to pare the idea down to the bare essentials. It may well not work but as they say- nothing ventured...


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