• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Home Built Transformer 120 volt primary 48 volt secondary with CT

gary350

Well-Known Member
Thread starter #1
I used a 900 watt microwave oven transformer to build this with a 48 volts secondary. I decided since I only need 36 turns of wire I would not take the welded laminations apart. I could have removed the welds in the milling machine but I think it will be more work than fishing the wire around and around through the 2 holes.

I tested the house voltage it is 122 VAC.

I tested the microwave oven transformer my Amp Clamp reads 10 Amp idle current with no load on the 2000 VAC secondary coil. This seems high?

Idle current is high 122 x 10 = 1220 watts.

I cut the secondary winding off with the milling machine then drill through the holes with a .500" drill. I pulled what was left of the secondary winding out with plyers.

There are 89 turns of wire on the microwave oven primary. It is an open winding with 9 layers and 10 turns on each layer except for the very last layer it as 9 turns. Core area on this transformer is 3.3984 sq inches.

I did the math several times just to double check to see how many turns should be on the primary but I kept making math errors. One time the math said the primary needs 2 turns. Another time in was over 8000 turns. I tried one more time got a goofy answer so I guit.

Next, with an 89 turn primary on 120 VAC the secondary winding of 48 VAC needs to be 35.6 turns with a center tap. That is 18 turns for 24 volts each side of the center tap.

I put 10 turns on the secondary then tested the transformer With 120 VAC on the primary I get 9 volts on the 10 turn secondary. A added 1 turn at a time and checked it again each time 13 turns gave me 12 volts. I figure I need 13 more turns to get 24 volts but 26 turns gives me 25 volts. Remove 1 turn it gives me 24 VAC.

I have 2 secondary windings each with 25 turns of #14 enamel coated copper wire that tests 24 VAC each. Perfect.

I varnished the windings and let it dry over night. I had to make 3 terminals to glue to the fiberglass tape then wrapped it with more fiberglass tape. I soldered both center taps to the center terminal and the other 2 wires to the other 2 terminals varnished it and let it dry.

I tested each 24 volt winding each one reads 24 VAC. I tested both windings in series and it reads 50 VAC. How can that be??? I test it again with a different volt meter and I still get 24 VAC + 24 VAC = 50 VAC. How is that possible???

Amp clamp still reads 10 amp idle current on the primary like before nothing has changed.

I removed the transformer shuts current is now up to 12 amps on the primary and the secondary voltage is up from 50 to 54 volts.

Secondary winding 10 turns = 9 volts, 13 turns = 12 volts, 25 turns = 24 volts, 50 turns = 50 volts.

I have built a lot of transformers in the past, they all worked. This one is weird. Probably because I did not change the primary winding.

Here are the pictures.

http://i43.photobucket.com/albums/e358/gary350/T1.jpg

http://i43.photobucket.com/albums/e358/gary350/T2.jpg

http://i43.photobucket.com/albums/e358/gary350/T3.jpg

http://i43.photobucket.com/albums/e358/gary350/T4.jpg

http://i43.photobucket.com/albums/e358/gary350/T5.jpg

http://i43.photobucket.com/albums/e358/gary350/T6.jpg

http://i43.photobucket.com/albums/e358/gary350/T7.jpg
 
Last edited:

DerStrom8

Super Moderator
Most Helpful Member
#2
I'm sure you already know this, but I need to say it anyway.

PLEASE be EXTREMELY careful when playing around with microwave oven transformers! They can kill you instantly with a single touch. I know you have worked with Tesla coils before, so I'm sure you're familiar with them, but I wanted to post this in case anyone else wants to try what you're doing too.

To anyone considering using a microwave oven transformer for anything,

DO NOT EVEN ATTEMPT TO USE ONE UNLESS YOU HAVE EXTENSIVE HIGH VOLTAGE EXPERIENCE AND SOMEONE TO WATCH YOUR BACK! These things are very dangerous and are not to be played with!

That's all I have to say.
 
Last edited:

RCinFLA

Well-Known Member
#3
Because a microwave oven transformer is relatively low duty cycle they typically run less turns per volt for a given core area on their windings then a continuous duty transformer. This make the unloaded inductance lower then a normal transformer and therefore more reactive idle current.

The 10 amps unloaded primary current is mostly inductive reactance so it isn't anywhere near 1220 watts. If it was the transformer would burn up is a matter of minutes. It probably has a real power dissipation of about 75 to 100 watts, which is still quite high for that sized transformer. If you run it in continuous duty use the core will get pretty hot and waste a lot of power.

Because the core is seam welded you cannot take the laminates apart. Being that you didn't need that many turns for secondary it made it manageable to feed wire through the hole although you have to be careful not to knick the wire insulation on edge of core while pulling wire through.
 
Last edited:

DerStrom8

Super Moderator
Most Helpful Member
#4
Because a microwave oven transformer is relatively low duty cycle they typically run less turns per volt for a given core area on their windings then a continuous duty transformer.
What are you talking about? A microwave oven runs on alternating current, so duty cycle isn't even in the picture.


EDIT: I apologize if that sounded rude. It is not supposed to ;)
 
Last edited:

RCinFLA

Well-Known Member
#5
What are you talking about? A microwave oven runs on alternating current, so duty cycle isn't even in the picture.


EDIT: I apologize if that sounded rude. It is not supposed to ;)
Low duty cycle means it isn't run that long a time. Continuous duty means it can run 24/7.

Most of the time a microwave oven in not run more then a few minutes at a time. A very long period for a microwave would be 15-20 minutes on time and if things get too hot the power is cycled to allow for some cool down time.
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
#6
What are you talking about? A microwave oven runs on alternating current, so duty cycle isn't even in the picture.
A microwave oven is made as cheap or cheaper than is possible. So its transformer core is too small, its winding wires are too thin and it does not have enough turns of wire. So the transformer gets hot.

It survives because of its very low duty-cycle:
1) How often is a microwave oven used? Once a week? Once a month?
2) How long is a microwave oven used at full power? 30 seconds. 2 minutes? The transformer probably takes 10 minutes to fully heat up.
 

gary350

Well-Known Member
Thread starter #7
Ok, I finally got my math correct. With a core area of 3.398 sq. in. the transformer needs 205 turns on the primary. My guess inductive reactance is too low making it pull a high idle current on the primary. That does not explain all the goofy numbers, 24 volts + 24 volts = 50 volts??? 13 turns = 12 volts but 26 turns = 25 volts???

Every time the number of turns doubles the voltage per turn increases. 13T = 12V, 25T = 24V, 50T = 50V.
 
Last edited:

RCinFLA

Well-Known Member
#8
Ok, I finally got my math correct. With a core area of 3.398 sq. in. the transformer needs 205 turns on the primary. My guess inductive reactance is too low making it pull a high idle current on the primary. That does not explain all the goofy numbers, 24 volts + 24 volts = 50 volts??? 13 turns = 12 volts but 26 turns = 25 volts???

Magnetic field is directly related to the number of turns in a coil. With 1/2 the number of turns needed the magnetic field will be about 1/2 too. The secondary is picking up a weak magnet field. It is obvious with the numbers above the more turns on the secondary the higher the voltage is per turn.
A continuous duty transformer would have about twice the transformer core area for that much power. A 900 watt microwave magnetron output power needs about a 1400 VA transformer. That would be about 7 sq inches for core area and about 0.7 turns per volt winding for continuous duty. (but a microwave oven magnetron is also skimped on design so it is not rated for continuous operation either).

A continuous duty 60 Hz transformer with a 3.4 sq. inch core would be rated at about 400 VA with about 1.4 turns per volt winding.

Welding the laminates together eliminates much of transformer buzz but increases the eddy current losses by the core. I never thought it was a good thing to do to a laminate E-I core.
 
Last edited:

DerStrom8

Super Moderator
Most Helpful Member
#9
Low duty cycle means it isn't run that long a time. Continuous duty means it can run 24/7.

Most of the time a microwave oven in not run more then a few minutes at a time. A very long period for a microwave would be 15-20 minutes on time and if things get too hot the power is cycled to allow for some cool down time.
A microwave oven is made as cheap or cheaper than is possible. So its transformer core is too small, its winding wires are too thin and it does not have enough turns of wire. So the transformer gets hot.

It survives because of its very low duty-cycle:
1) How often is a microwave oven used? Once a week? Once a month?
2) How long is a microwave oven used at full power? 30 seconds. 2 minutes? The transformer probably takes 10 minutes to fully heat up.
Hmm, I guess I've never heard "duty cycle" used that way. Very strange....

Thanks for your patience guys.
 
#10
I have wound AC line transformers before and I need about 10 times the area that professionals use because my wire has thick insulation. The guy who started this thread seems to know what he is doing. The first strange post I saw was a warning about high voltage which of course was obsolete because the HV secondary was gone. I looked at the primary current waveform on a scope once and it is decidedly not a nice sine wave. That means the iron is getting into the saturation region which means hysteresis loss. That agrees with a number of the posts above. I don't like the primary current being so high for fresh unmodified transformers because it pops my 3A breaker. So I plan to add more primary turns but put them on the secondary side of the magnetic shunt. I wish it were otherwise because the magnetic shunt enables the transformer to act like a constant current source, which does not draw too much current when shorted (electrically). I will wind as many wires as will fit and then measure the open circuit voltage and short circuit current. With luck it might be a good 12V transforer.
I measured the secondary voltage of a number of good Microwave transfoormers and they were all about 2KV.
A rule of thumb: If you add 10% more turns to the primary you can cut the hysteresis heating significantly and have a much nicer transformer.
 
Last edited:

gary350

Well-Known Member
Thread starter #11
I talked to a friend that is a retired electrical engineer he worked for a company that maked transformers for about 30 years. He says the microwave oven transformer is designed to come on at full power and stay at full power all the time. It does not have an idle current like most transformers. The low number of primary turns pulls a very high current all the time therefore the transformer does not need a load to make it come up to full power.

I learn something new every day. It looks to me like I need to start over. The primary winding needs to have 205 turns of #20 wire. The secondary needs to be 82 turns of #16 wire with a center tap. All I need is 320 watts max 24-0-24 vac. The 3.398 sq. in. core area can handle 1000 watts.
 
Last edited:

RODALCO

Well-Known Member
#12
You are better off with a 240 / 120 Volts 1 or 2 kVA step down transformer, look on the web or secondhand shop for a deal

Run the 240 Volts winding at 120 or 110 Volts, the out put will be near 60 or 55 Volts

By adding a second TX rated at the current you need, say 5 Amps, reverse the secondary winding to get your 48 Volts required.
 

RODALCO

Well-Known Member
#13
I missed that last post # 11

For a 24 - 0 - 24 Volts winding my suggestion wouldn't work unless you can find a 240 / 110 TX with a split 55 - 0 - 55 Volts winding
 
Last edited:

tcmtech

Banned
Most Helpful Member
#14
A second better choice for making custom wound transformers is to use a common HID type ballast as the base unit.

They are continuous rated and many have multiple taps on the primary windings plus they are just as easy to take apart and put back together as most cheap microwave oven transformers.

Over the years I have made a few into step up/down issolation transformers just by removing the seconday and putting a matching primary on from a second ballast.

Its just a thought.
 

gary350

Well-Known Member
Thread starter #15
I have a 3KW step down transformer 480/240 VAC step down to 120 VAC.

I spent several hours searching all the electronic supply places, ebay and CL no one has a 24-0-24 VAC transformer that is 10 amps. I found one that is 2 amps, 5 of those in parallel would work but it will cost over $400 plus postage.

Today I put the MOT in the milling machine vise and milled off the 4 welds. Then I removed all the laminations from the transformer. Tomorrow I will make a turning fixture to wind the transformer in the lathe if I have time. If it is not raining I want to do a 20 mile bicycle ride, about 2 hour ride. I will use enough laminations to have a core area of 3.5156 sq. in. so the transformer will have 200 turns on the primary and 80 turns on the secondary with CT.

I am retired and having fun building this transformer so I really does not matter that I have to build it to get what I need.

http://i43.photobucket.com/albums/e358/gary350/100_1873.jpg

 
Last edited:

RCinFLA

Well-Known Member
#16
I talked to a friend that is a retired electrical engineer he worked for a company that maked transformers for about 30 years. He says the microwave oven transformer is designed to come on at full power and stay at full power all the time. It does not have an idle current like most transformers. The low number of primary turns pulls a very high current all the time therefore the transformer does not need a load to make it come up to full power.

I learn something new every day. It looks to me like I need to start over. The primary winding needs to have 205 turns of #20 wire. The secondary needs to be 82 turns of #16 wire with a center tap. All I need is 320 watts max 24-0-24 vac. The 3.398 sq. in. core area can handle 1000 watts.
Bad info! Don't mix current and power.

Read up on reactive loads and power factor. The transformer would be toast in about 25 minutes if it was dissipating 1220 watts with no load. Ask yourself where 1220 watt of power would go.
 
Last edited:

DerStrom8

Super Moderator
Most Helpful Member
#17
Post removed.
 
Last edited:

tcmtech

Banned
Most Helpful Member
#18
Finding 24 volt transformers is easy. I found hundreds of them on line in just a few minutes.

Search for 24 volt control or 24 volt buck/boost and you should not have any trouble finding ones in the sizes you need. :)
 

MrAl

Well-Known Member
Most Helpful Member
#19
Hi gary,

What we normally see in the transformer primary is excitation current. This excitation current is out of phase with the line voltage and so does not cause too much power heating, except in the wire itself which has to carry the current.

On the other hand, what you are probably seeing is high excitation current due to a primary winding that doesnt really have enough turns for the core area, so it is probably causing core saturation which would cause more current at the peaks. You can verify this with a scope and current probe.

To correct the problem, you would add more turns to the primary. With the core area at about 3.4 square inches you might get away with a 1 volt per turn ratio, meaning you would need at least 120 turns on the primary. This would probably get you going. In any case, adding turns to the primary will help quite a bit and make the transformer much more usable.

You can take it from there, adding a secondary and see what you can get out of it. With 120 turns on the primary you'll get 24v output with 24 turns (or a little more to make up for loading effects).

You probably want to consider loading effects too, calculating the primary and secondary resistance and calculating the voltage drop with a given load current. The wire gauge should allow for at least 600 circular mils per amp or better.
 
Last edited:
#20
Hi, I know this is old thread, but I want to build PS from MOT and need to ask...

I know many people, who use Microwave as defrost / preheat... And they run it for tens of minutes (try defrost 2kg of meat, it will run nearly hour or more...). By your posts all of them should have fried transformers. Is there no safety and every microwave producer is sure, that nobody runs it for more than few minutes? Are you really sure about your posts? If so, please prove it with something stronger than thoughts & feelings...

No hard feelings. I work as designer in automotive and I know well, how high are security reserves when you sell something. This surprised me a lot considering how dump usual users are. Sorry because of my "english" :)
 

Latest threads

EE World Online Articles

Loading

 
Top