Introduction: Microwave Isolation Transformer

Picture of Microwave Isolation Transformer

Everything started when I set up my new workshop. I always wanted to have an isolation transformer so I decided to make one. I also had some microwave transformer and the case laying around.

Step 1: Why and Parts Need

Picture of Why and Parts Need

What is an isolation transformer?

An isolation transformer is used to protect people from electrical shocks. (

!!!DANGER!!! Using an isolation transformer
can be very dangerous, because no safety switch is working! Using it is anyone’s own risk!!!

Parts need:

· 2 identical high-quality microwave transformer (high-quality because no name microwaves normally use an aluminum coil.)

· Case

· 2 wall plugs

· 7A fuse and fuse holder

· 4A fuse and fuse holder

· 4A thermal fuse

· LCD Volt and ampere meter (ebay:

· 3 switches

· C13 Plug (from broken pc power supply)

· Board

· Bridge rectifier

· 10µF 25V capacitor

· 1K resistor (1K works every time J)

· 3mm diffuse LED (color of your choice)

· 3mm LED holder

· Wire

· Some screws


· Safety plug (to connect an antistatic necklace)

· Zip ties

· Zip tie sockets

· Wire ends

· Shrink tube

Tools need:

· Soldering iron

· Hacksaw

· Wig welder

· Dremel

· Hot glue gun

· Pliers

· Screwdriver

· Hammer

· Vice

Step 2: Take Apart Transformer

Picture of Take Apart Transformer

Take the first transformer and cut the weld with the hacksaw on both side away. Then knob with the hammer to the side of the weld. The bottom should break away and it should look like image 2. Open the vice and bust the coil to the bust. Now take a piece of wood and put it in the middle of the iron core. Now take the hammer (the one on the picture is to small :-) ) and knob on the piece of wood. The coils should go out and you will get the pieces on picture 4 (and a small red coil who is missing on the picture). You will only need the primary coil (the one with the thick wire). Do the same with the second transformer.

Step 3: Reassembly

Picture of Reassembly

Now take one of the iron core and mound the red coil in (I forgot it on the pictures). After that take one of the primary coil and put it over the red one. Put the second primary coil over it and fill it with the metal sheet. It should close nice with the iron core. Take the bottom and lay it to the top of the iron core. Press it with two clams together.

Step 4: Weld It Together

Picture of Weld It Together

Stick some insulating tape to the coil to protect it when welding. Weld the bottom to the iron core. Remove the tape.

Step 5: Install Transformer and Power

Picture of Install Transformer and Power

I made the case at school so I have no pictures of the manufacturing :-) . Put the transformer in the case and mark the holes of the transformer to the case. Take the transformer out and drill the holes with the dremel. Now put the transformer back in and screw it to the case. Next you have to make a hole for the C13 Plug with the dremel. Install it. Solder wires to the plug.

Step 6: Fuse, Switch and PE

Picture of Fuse, Switch and PE

Next you have to install the main switch. Mark the holes to the case, drill the holes and install the switch. Solder the wires coming from the plug and two new ones to the switch. Connect one of the new wires to the fuse F1 like in the schematic. After connecting the fuse F1, put the fuse F2 and the thermal fuse 3 in there holes. Also the switches S2 and S3 have to be mounded in there holes. I also connected a safety plug to add a antistatic necklace. Wire them as it is in the schematic.

Step 7: Wire Transformer and Fuse

Picture of Wire Transformer and Fuse

Now take the wire from the switch and the one coming from F1 and connect them to the “Primary” coil of the transformer. Add wires to the “secondary” coil and wire fuse F2 with one of these wires. Wire the second side of F2 with one side of F3.

Step 8: Install LED

Picture of Install LED

To power the led I decided to make a small circuit. The circuit takes the 6v AC from the secondary coil and rectify it. I also had this capacitor so I added it to stabilize the power a little bit, so the led isn’t flashing. The resistor protects the led from getting over current. Drill a hole through the board and the case and secure the circuit with a screw to the case. Don’t forgot a space holder. Connect the transformer and the led to the circuit. Drill a hole over the main switch and mound the led with the led holder.

Step 9: Install LCD

Picture of Install LCD

To install the LCD you have to make a cutout with he size of the panel and put it in. Next you have to connect the wire from the transformer and the fuse to the voltage input of the LCD. Connect the current transformer to the current input of the LCD.

Step 10: Wall Plugs

Picture of Wall Plugs

Make a hole for the wall plug and mound them in. Now take wires and connect them to the LCDs voltage input, put the current transformer over one of the wires and connect the other side to the wall plug.

Step 11: Tie

Picture of Tie

Tie the wires together and secure them with Zip tie sockets to the case.

Step 12: Close and Measure

Picture of Close and Measure

Close the case and power it on. Now take a multimeter and measure the voltage between the two connections of the wall plug and the case. There should not be more than a few volts. Now measure the voltage between the two connections of the wall plug. There should be your mains voltage.

Step 13: Use It!!

Picture of Use It!!

I use mine when I’m testing and repairing TV’s, receivers, radios, power supply’s…. But be sure: Microwave transforms aren’t made for continuous operation, so don’t make everything over it.

Step 14: Future Development

Picture of Future Development

I wrote that Microwave transforms aren’t made for continuous operation, so I maybe will install a pt100, a fan and a relay to control the temperature that on a specific temperature the fan turns on and on a higher temperature the relay turns the transformer of…

Thanks for reading my instructable please leave a comment if you have a question.



SagirF (author)2017-04-09

That bridge rectifier and capacitor are overkill just to power an LED. Just use a 400V or higher non-polarized capacitor in series with the LED, and since it's a Diode itself, it will rectify to 30Hz (or 25Hz) pulsed DC. Capacitive reactance is useful here. Reactance = 1/(2*pi*frequency*Capacitance). (340V peak - 1.8V LED) / 0.005A = ~68k resistor. 47nF nonpolar will do (for 240V AC, 340V peak).

Since most people can't really tell the difference between 30Hz and a continuous light source, there shouldn't be a problem.

elmesito (author)2014-03-19

Nice job, but there is one thing that I find concerning.
You have used a mix of blue and red sockets, both connected to the same source, which is 240V. According to the universal standard, Yellow sockets are 110V, Blue ones are 240V, and red ones are 415V. This means that you probably now have 240V equipment wired up with a red socket, that could accidentally plugged into a 415V, with a mighty BANG!!! And this could be potentially deadly!!!

Please be careful, and replace the red socket with a blue one, so that no accidents can happen. Electricity is not to be played with, and the colours are not to make things pretty, but simply safe.

alterator (author)elmesito2014-03-29

yeah. usually we take our isolation transformers on a trip.

Gabse (author)elmesito2014-03-19

thanks emesito for your comment, it are two identical plugs. And yes normaly its important to use the correct collor, but in this kase it doesent mater, cause i knew that the transformer makes 230V and when mi using it im using dont look which color i use.

MattsAwesomeStuff (author)2014-03-23

Finally, someone who doesn't insist on using a chisel to remove old windings. I recommend not re-welding the whole seam back together at the end as this makes eddy currents higher. Instead, just tack weld the 4 corners. It only has to be mechanically sound, not electrically connected.

Another tip is, if you can find a 3rd transformer the same, remove the magnetic shunts and try to sneak in a 3rd primary. Split it about half way and add each half onto the primary and secondary. The extra 50% turns will reduce waste heat by 90+%. I've been meaning to add followup videos to my MOT disassembly tutorials, but, I have not got around to it yet.

Gabse (author)MattsAwesomeStuff2014-03-25

That with the eddy currents i already knew, but all microwave transformers are weld together. It would be better to put the sheets together like they are in normal transformers, but is a lot of extrawork. That with extra turns i didnt knew, i will make in V2.

mrabensteiner (author)2014-03-20

Well done gabs ;)
I think is one of your best works
Can you phublish your cnc machine when you finished? :)

Gabse (author)mrabensteiner2014-03-21

sory mrabensteiner, my cnc is only a mod of a finish cnc and not a Instructable

mrabensteiner (author)Gabse2014-03-21

Ok, thanks gabs for your comentar

throbscottle (author)2014-03-19

Clever idea to use the coil from another transformer. I would never have thought of that :)

Gabse (author)throbscottle2014-03-19


tfletcher8 (author)2014-03-19

Excellent project. Thanks for sharing your work.

Gabse (author)tfletcher82014-03-19


The Lightning Stalker (author)2014-03-18

I find an angle grinder or die grinder with a cutoff disc works the best for cutting out the welds.

I used a hacksaw because the cut gets cleaner than with a angle grinder so it's easier to weld it together

Whenever I used a hacksaw, it would catch on the laminations and pull them apart and flex them. Plus, it takes such a LONG time. I guess I'm just not that patient.

Todd Gehris (author)2014-03-15

Nice Instructable. I kept noticing your wrote "whit". I think you meant "with".

Gabse (author)Todd Gehris2014-03-15

Thanks for the note, I will correct it immediately.

geraldpaxton (author)Gabse2014-03-19

Might want to also check words (mound}{mount), (knob}{knock), and was that second small coil used for anything? I'm guessing English is your 'second' language....good job! While you're at it...check out the use of (there, their, they're, you're and your), we all have trouble with those!


Gabse (author)geraldpaxton2014-03-19

thank you for your comment. Yes english is my third language. The second small red coil is for separation of the two big ones. thank you for your comment. yes english is my third language. The second small red coil is for separation of the two big ones. On my next instructable i will ask my teacher for correcting the grammar. :-)

kevtoms49 (author)2014-03-19

a nice tidy well set out piece of kit,every tinkerer should have one.Well done

Gabse (author)kevtoms492014-03-19


LiamOSM (author)2014-03-19

very nice. You have a very neat, tidy assembly in that box.

Just a note- welding the core of the microwave transformer can sometimes cause the layers to come apart. I simply use an epoxy to stick them together and it works wonderfully.

Gabse (author)LiamOSM2014-03-19

Thanks for your comment,

i weld it because is a lot easier and cheaper than buying a special epoxy

lrothman (author)2014-03-19

Looks great!

However, for additional electrical isolation, you should add a sheet of mylar or other high voltage insulation between the two windings and also ground the core once it's reassembled. One of the issues I see is flash-over from the primary to secondary if a voltage spike/surge appears on the primary side. With an insulating sheet between the windings, any arcing will go to the core, which is grounded.

Gabse (author)lrothman2014-03-19

There is already a second isolation around the coil as you can se at the image of reassembly. I also put put the red coil of the second transformer between the primary and the secondary coil. The transformer is groundet over the case.

Bowtie41 (author)2014-03-18

Long pointy screws!Ouch or sparks waiting to happen.Please be careful and grind off with dremel,slip rubber caps on,or dig up some nuts/bolts.Not bashing,just seen lots of accidents that way,sometimes long after it was built.

Or, even better, use some bolts & nuts.

That's in my comment.

Gabse (author)Bowtie412014-03-19

Yes it would be better, i only had this long wodscrews ad home when i made it.

BurgersBytes (author)2014-03-19

What is the mysterious "red coil"? You glossed over it a few times. Where did you get it?

Gabse (author)BurgersBytes2014-03-19

The red coil is the second secondary coil of the microwafe transformer. It is normaly used to heat up the magnetron. I use it to power the led.

ElegantAndrogyne (author)2014-03-19

Nice case, kinda like 50/60s electronics... or anything you can see in Fallout :)

carlos66ba (author)2014-03-17

A very useful device and very well explained instructable.

Gabse (author)carlos66ba2014-03-18


samchamb (author)2014-03-18

nice ible , I have read some where that the quick and dirty method is to place 2 identical trasformers back to back (sec) . I prosume this would work with microwave tx but would of cause not be a good idear due to the hi voltage secondrys .

Gabse (author)samchamb2014-03-18

that would also work, but it wouldn’t be very efficient.

About This Instructable



Bio: I am an 20 years old mechatronics and I’m living in the north of Italy. My interests are working on electronics, playing computer and ... More »
More by Gabse:858D SMD Hot Air Reflow Station HackAdjustable High Current Lab SMPS Out of a Standard ATX PSLaptoprecycling-Touchpad
Add instructable to: