Microwave transformer built power supply

[OCNC]

First, I'm not sure if this thing is powerful enough to give me 15 amps at 24 volts. I don't know the wattage of the microwave. It was small so maybe this transformer won't be big enough.

You can get an idea of the 'size' by measuring the cross-sectional area of the core. For 350 watts continuous (P=E*I) you need about 3.375 square inches.

Second, I'm not sure which winding is the high voltage one.

The high voltage winding is the one with the fine wires. The primary has the heavier gage wire. There is also a two or three wrap winding of heavy gauge wire for the low voltage supply. On the two transformers that I've rewound the secondary is on the top, the primary is on the bottom and the low voltage winding is between them. I beleive that this is standard practice for the industry.

By the way, there was an envelope inside of the chasis with schematics, test procedures, wiring diagram etc. Should I post a scan of the paperwork along with the picture?

The schematic might give you the voltage of the low voltage winding which will allow you to calculate the volts/turn rating which you need for determining how many wraps to wind in the new secondary. You can also measure this directly though.

[Bubba]

Xterrian,
All of the transformers that I have used, have Three windings on them.
The primary winding
The filiment winding which is wound in the same core as the primary winding and generates about 3V
The HV secondary winding which is all by it self and "sometimes has only 1 connector with the other end grounded to the frame of the transformer! Also, the HV winding will have the smallest wire.
Picture(s) will be a help in identifying the various windings. Also, each transformer that I have used is market on it with some identifying numbers. A Google will lead you to suppliers and you can deduce the wattage available from that.
Be glad to help you out if I can.

[OCNC]

Here are a couple of photos of my microwave transformers. The red wire is THHN #14 solid while the gray wire is #12 stripped out of a piece of NM house cable. I plan to rewind the one with the gray wire as it was one or two turns short of the voltage I was looking for. The one with the red wire had a volts/turn of 1.135 volts. The inner secondary has 23 turns for 26v RMS output and the outer secondary has 8 turns for a 9v RMS output. When you're hacksawing off the high voltage secondary, be careful not to cut into the primary. After cutting off the overhanging portions of the secondary I used a 1/4" pin punch to drive out the remaining wires still in the core area. Otherwise I followed Bubba's instructions for rewinding wrapping each layer with some glass filiment tape. I left the original card stock in place.

[Xterrian]

OCNC, it looks like you're using plastic insulated wire. Is that OK, or should I just use magnet wire? Good thing I checked with you guys, I was getting ready to cut into the primary. I'm still charging the camera, pictures soon.

[OCNC]

OCNC, it looks like you're using plastic insulated wire. Is that OK, or should I just use magnet wire?

Guage for guage magnet wire allows for more windings to be placed in a given space than the plastic covered wire. The THHN is rated at 90deg C and magnet wire is 155deg C. I have to imagine that because the windings are a little bit closer than they would be with the THHN the magnetic coupling must be better with magnet wire. Also with magnet wire the windings can be vanrished into a single mass to eliminate vibrations between individual loops of the winding. If you have the magnet wire use it. I didn't so I went with the plastic coated. The max. current I expect to be drawing from this transformer is 6 amps so I don't expect to have any excessive heat issues. When I rewind the gray xformer I'll use magnet wire.

[Xterrian]

Here are the pictures. I stuck the data sheet off of the microwave in there in case someone needs that info.

[smarbaga]

i also got 2 transformers out of fairly new ovens and they ran at about 250 deg f.
one was a ge, the other sanyo, they drew 12 to 12.5 amps 120v on the primary with no load and one with no secondary.
these were to hot to use. i will give it 1 more shot ... the next mike i come across.

[Xterrian]

Here's the rest of the pictures.

[Xterrian]

You can see that I nicked the primary in two places in picture "Transformer4.jpg". I'll just slap a bit of varnish on there. Sorry it took so long to post these, I had to complete several other tasks at the same time. Sunday "Honey Do" list and started cutting MDF for my router. If someone else could put the pictures where they belong in the description, that would be great, if not, I will do it tomorrow night.
Randall.

[Xterrian]

The following is a quote of Bubba's instructional post with my pictures inserted. Let me know if this needs to be done differently.

Building the transformer is quite easy.
Material needed:
Transformer (you have)

Magnet wire of the proper size for the amperage you desire
Insulating "paper", I have used tablet back and gasket material
Tape (I use self adhesive glass tape from MSC)
I also use insulating varnish again can be found at MSC

Tools
Hacksaw
Hammer
Punch
Big screwdriver
Drill(press) with a pretty good sized drill (say 1/2")


The process I use to make my transformers is as follow, as usual, YMMV or you may find a better way to do it.

!ABOVE ALL, WORK SAFELY AND BE CAREFULL!

Process:
Carefully saw off one side of the High Voltage winding and be sure NOT to knick the 110V or ~3V winding. Both of the lower voltage windings have always been wound together on all the transformers that I have messed with.

Now, I have found it easier to get the rest of the HV winding out by drilling into the center of the winding mass to make room to start pulling wires out. Some will wrap on the drill.

When you have a hole in the middle.

start pulling the wires out of the core (one side at a time). Remove ALL the wires and discard. When you get all the wires out, you can either remove the insulating paper or leave it for use on your new transformer winding.


When you have it all cleaned out, I usually use an air hose to be sure all the little chips etc are completely blown out of the clean core area as well as any that may have dropped into the 110/3V windings (you will keep these)

When every thing is clean, I usually put the core in a vice on the bench to hold it firmly. You now want to use your insulating paper to cover ALL exposed edges of the core. You don't want to knick the insulation on your magent wire or you will not have a good transformer (ask me how I know

After wraping the insulation on the core, I use glass tape to hold it in place. There are other tapes that can be used but this is my preference.

Measure the perimiter or the core to see how long a piece of wire it will take to make one turn and then multiply by the number of turns you think you will need. My experience is .9 to 1.0V per turn. Add a few feet of wire to this "estimate". For example, for the 40V AC that I just made, I used a piece of wire about 40ft long and only cut off a little bit at the end!

Start winding and be sure to leave a "tail" sticking out to connect to your circuit when done. Thread the wire through the core and carfully lay each winding next to the last one as best you can. When you fill that layer, I then wrap that winding with a layer of tape. Continue until you have what you think is the correct number of turns for the voltage wanted.

Now we are almost ready to test the actual voltage of the transformer.

BE CARFULL AS YOU WILL BE WORKING WITH POTENTIALLY LETHAL VOLTAGE/CURRENT!

REPEATING:
BE CARFULL AS YOU WILL BE WORKING WITH POTENTIALLY LETHAL VOLTAGE/CURRENT!

Scrape the ends of the wires that you have just wound and using an ohm meter, check to make sure you have continuity of the winding.
Also, check to make sure that you DO NOT HAVE continuity between the winding and the core. (In the former case, the transformer will not work, and in the second, you will see magic smoke in the following test

Set you volt meter to the appropriate scale and connect to the ends of the winding that you just made.
Using a FUSED power cord, hook it to the transformer and get ready to plug it in. Before plugging in, be sure there is nothing that will short out the windings of the transformer or anyone will get shocked.

BE CAREFULL AS YOU WILL BE WORKING WITH POTENTIALLY LETHAL VOLTAGE/CURRENT!

Now plug the power cord in the wall there should be a slight "chunk", but don't worry at this point. If things seem wrong, UNPLUG the cord immediately and find out why.

If all seems well, read the voltage on the meter. Is it the desired voltage that you want???
Hopefully yes, but if not; UNPLUG THE CORD and either add a winding or two or (?) to increase the voltage or remove a winding or (?) and test again.
Repeat as necessary until your voltage is correct.

If you have surplus wire, cut off the excess again leaving enough to allow you to connect to your circuit. I then spray the winding with my insulating varnish and let dry at least overnight before using the transformer.

Ok, now that you have the main winding for your servo, you may want to add another winding for say your 5V/12V controls and breakout board.

Repeat the above process as needed to wind additional winding(s) as necessary using the appropriate sized wire.

WIRE SIZING:
Based on what I have found, wire sizing is based on approximately 750 to 1000 CM per amp of current and having said that, I find that #14 does quite well on my setup of 3 servos each being fed by a gecko with a 5 amp fuse on it and haven't blown anything yet:})

If you work cleanly and carefully, you can make your own transformer this way and have the voltage(s) and current ratings that you want

[Xterrian]

I'll add pictures as I finish this project and hopefully when I'm done someone can put the whole thing in a tutorial area. Let me know if I need to do this a different way.
Next question, I'm not sure how to determine wire size. Stepcalc says I need 14.4 amps to run my steppers. I figure 15 amps to be sure. What size wire should I use?
Thanks,
Randall

[OCNC]

Next question, I'm not sure how to determine wire size. Stepcalc says I need 14.4 amps to run my steppers. I figure 15 amps to be sure. What size wire should I use?

Using Bubba's circular mil rating #14 wire allows for a little over 5 amps continuous and #12 about 9 amps continuous. The success of this transformer for your particular application will come down to the actual duty cycle for your steppers and drives.

I also want to mention that for those who don't have a drill press it works quite well to cut off both sides of the secondary flush to the core and to then drive out the remaining wires with a 1/4" or larger pin punch. It also makes less of a mess. I used a 32 tpi hacksaw blade and let it ride right against the core side as a guide while sawing.

[ynneb]

I just want to say, this is a fantastic thread. You guys have inspired me to start scanning the neighbourhood nature strips for old microwave ovens.

[Bubba]

Xterrian,
The charts indicate a need of 750 to 1000 CM (circular mills) per amp for the wire size.
If you take 15 amps X 750 = 11,250 or 15 X 1000 = 15,000 CM. This give you a range of sizes.
Using a table of wire sizes:
#10 = 10,383 CM
# 8 = 16,510 CM

Now, it boils down the the wire insulation that you will be using because if you are using a smaller wire and pulling the amps, it will run hotter and smaller wire will cause the voltage to droop more under high load. One place I frequent has magnet wire in two temprature ratings 130°C and 200°C.
Also, how accurate do you think your calculations on total load are???? Most probably, this is the max load current rating and do you think you will be running that load ALL the time? I have seen writeups that indicate good engineering principals would be to size the power supply for 2/3 of MAX. In your case, that would be 15 X 2/3 = 10 amps.

Now if we run the calculations based on this load:
10 * 750 = 7500 CM
10 X 1000 = 10000 CM

This give you a range of wire size

#12 = 6530 CM maybe a little light
#10 = 10,383 CM
# 8 = 16,510 CM

Hope this give some background in how to do the wire sizing. A google of transformer wire sizing "may" turn up other ideas. But temprature is going to be one of the main factors that you also have to consider and how well your cooling for the transformer is will also have and effect. Remember that supercooled wire has zero resistance and therefore will not heat and you can get "infinite" current out of it! (I know, we will not be running supercooled stuff in our hobby boxes) [I will now put my insulated flame and cold proof suit on ]

[ynneb]

Just outa interest, does any one know how a machine can wind a toriodal transformer?
I have tried to find a picture of the machine that does it, or even a movie but have had no success. Any links?

[Xterrian]

I'm guessing that the spool of wire is smaller than the torroidal center hole. It would be placed upright in the center and then the torroidal core is spun around it. Just a guess.

Is liquid nitrogen conductive? I do have the cooling fan from the microwave, maybe I will do an extra 4 wrap coil and power it from the transformer. Then I could build a little case for the whole thing to force air over it. Thanks for the help. I was unsure what CM meant. Looks like #10 is my answer.

Thanks again,
Randall

[Al_The_Man]

I have tried to find a picture of the machine that does it, or even a movie but have had no success. Any links?

There was a guy on Europe selling some machines on ebay just recently, looked almost new, I think they were German in origin, went for around $1000.00, probabally a small fraction of what they were worth. I should have posted the link here, for interest.
Al.

[smarbaga]

toroid winding machines are made up a hoop(s) on hinges, this hoop is motor driven.
you place the toroid core in the driving jig, 3 or 4 rubber rollers, this will rotate the core like a car tire.
the hoop is opened and placed through the center of the core, then closed back up.
a pre determined amout of wire is wrapped around the hoop as it is u shaped, and motorized.
then the end of the wire is attached to the core and the hoop and core spin in synchronization until all the wire is wrapped around the core,
then the hoop is filled with mylar tape and that is wrapped around the core.
the primary is now insulated and the process repeats for the secondary winding(s)
almost have to see it to believe it.
perhaps there are different methods, but this is roughly how the machines i have seen work.

[Dylwad]

Another thing to consider, if you cant find large enough magnet wire, you can double up on it, the number of wires doesnt matter much, just the cross sectional area. I used 2 strands of 16 gauge magnet wire i got for free at a local motor repair shop, the transformer has never seen over 120f and gets worked out every day. Its running 3 640 oz in steppers through geckos, all set at 5 amps max, 20x rated voltage~the steppers stay around 140f

I wouldnt use anything but enamel coated magnet wire myself, the plastic stuff looks like fuel to a fire when something goes wrong.

Dylan

[OCNC]

Another thing to consider, if you cant find large enough magnet wire, you can double up on it, the number of wires doesnt matter much, just the cross sectional area.

It will probably easier to wind the smaller wire. I think #10 is going to be pretty stiff.

I wouldnt use anything but enamel coated magnet wire myself, the plastic stuff looks like fuel to a fire when something goes wrong.

Safety First! I agree with you about this. If my use was going to be anything other than a light load and of an experimental/prototypical nature I would have gone with the magnet wire from the start...just using what's at hand to get familiar with all of the parameters involved in the interest of spending wisely on the finished product.