Rewinding a microwave transformer

Hi guys.

Are there any power transformer guru's that can advise me on a transformer rewind?

I'm planing on a full rewind, primary and secondary of an old MOT 240v in 38v to 40v out. The transformer is to be used as a high current variable bench power supply, about 10 amps. I'll be using a stacked bobbin to separate the windings.

My dilemma is this,I'm using Silvio Kliac's transformer calculator to give me a primary and secondary turn count and wire size, but the software doesn't consider the hight of the core and I can not fit the suggested number of turns on this core.

Suggested number of primary turns is 496 of 1mm wire. Can I simply reduce the primary turns to say 480 and recalculate the secondary to compensate?

regards to all,

Wal

Hi,

I would NOT recommend using a microwave transformer for a power supply.

Microwave transformers are a form of leakage transformer. That is they have a magnetic shunt in the core, between the primary and secondary windings.
The shunt provides a path for the magnetic flux when the secondary is overloaded.
This protects the transformer if the secondary is overloaded or shorted. Good if you are making a spot welder, but useless for a regulated power supply, because the output voltage drops significantly even under a light load.

You need a plain old 50 Hz power transformer. 20 years ago a power transformer from an old TV's was a good source. But modern TV's all use switch mode power supplies using a small high frequency ferrite transformer that are useless for what you are doing.

When I rewound transformers many years ago, I only removed the secondary winding, because you know that the primary turns had been set to give the correct flux density for the transformer core.

If you know what the rated output voltage was (or can measure the voltage under full load), counting the turns that were on the secondary, allows you to determine the secondaries turns per volt.

ie, if I wanted 40V at 10A I would find a transformer that had a AV rating of over 400VA.
ie, I would try to find a 24V x 20A transformer or a 12V 40A transformer.

If the output was 24V and there was 100 turns on the secondary, the turns per volt is 100/24 or 4.17
So to get 40v output you need 40 x 4.17 turns = 167 turns.

You then select the largest wire diameter that will still allow you to fit the 167 turns on the secondary bobbin.
You want the largest diameter wire possible, so that the secondary does not get too hot under full load.

Regards Steve.

[QUOTE=steve-37;1889272]Hi,

I would NOT recommend using a microwave transformer for a power supply.

Microwave transformers are a form of leakage transformer. That is they have a magnetic shunt in the core, between the primary and secondary windings. :tears:

Thanks for your reply Steve, I very much appreciate it.

I thought the shunts were put in place to compensate for the crude and low cost manufacturing process.

When completely stripped down it's basically just a 50 hz EI laminated core. I understand that in it's original state, the core is over driven as it only has 240 primary windings, hens the need for shunts and a cooling fan. I'd hoped that a complete rewind with no shunts would eliminate most issues. Old transformers are now hard to come by and I don't mind admitting that I'm a bit of a tight ass so am not prepared to outlay $190 od to have one made.

Wal

[QUOTE=walcen;1889281]

Quote:

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Originally Posted by steve-37

Hi,

When completely stripped down it's basically just a 50 hz EI laminated core. I understand that in it's original state, the core is over driven as it only has 240 primary windings, hens the need for shunts and a cooling fan. I'd hoped that a complete rewind with no shunts would eliminate most issues. Old transformers are now hard to come by and I don't mind admitting that I'm a bit of a tight ass so am not prepared to outlay $190 od to have one made.

Wal

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Hi,
I agree that if you rewind the transformer and leave out the shunts, you should end up with a suitable transformer.
I think you should be OK to set the number of primary turns between the original 240 turns, and the suggested 496 from Silvio Kliac's.
I would be happy trying 450 turns.

Once you have wound the transformer (fit a 5A slow blow fuse in series with the primary, and earth the core) and test it under no load for a few hours, and check the output voltage and temperature rise. Should only rise a couple of degrees. And is mainly due to the hysteresis losses in the magnetic core.

Then I would connect the full 10A load for a few hours, and again check the output voltage and temperature rise. Should get warmer, but not so hot that you cannot leave you hand on the core for a minute. a rise of less than 30 - 40 deg C should be OK.
The added temperature rise is due to the resistive losses in the windings.

Steve.

[QUOTE=steve-37;1889516]

Quote:

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Originally Posted by walcen

Hi,
I agree that if you rewind the transformer and leave out the shunts, you should end up with a suitable transformer.
I think you should be OK to set the number of primary turns between the original 240 turns, and the suggested 496 from Silvio Kliac's.
I would be happy trying 450 turns.

Once you have wound the transformer (fit a 5A slow blow fuse in series with the primary, and earth the core) and test it under no load for a few hours, and check the output voltage and temperature rise. Should only rise a couple of degrees. And is mainly due to the hysteresis losses in the magnetic core.

Then I would connect the full 10A load for a few hours, and again check the output voltage and temperature rise. Should get warmer, but not so hot that you cannot leave you hand on the core for a minute. a rise of less than 30 - 40 deg C should be OK.
The added temperature rise is due to the resistive losses in the windings.

Steve.

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Thanks very much Steve, that,s great advise. I have a cousin who is an old amateur radio operator, he's been helping me out a bit but he has never done a full rewind. I'll go back to my cad drawing and see how 450 turns will fit, I'm clad I drew it out because I nearly got tricked, the secondary needs to be twice the thickness and it takes up 4 time the space.

regards

Wal

First off I would agree with other forum members regarding the rewinding of a microwave oven transformer and unless you have cheap supply of copper wire it simply isn't worth the hassle if you take into account cost of wire.

If you are determined however I would go about it this way, in the first instance.

Leave the primary in place, and cut away the secondary completely ,

Wind on around 10 turns of secondary wire size doesn't matter and measure the output voltage, from there it is simple calculation to get turns required for the voltage you want, for a transformer of this size I'm guessing it is about 1.75 - 2 turns per volt, the bigger the core the less turns per volt .

Measure and calculate the space you have for the secondary in squ mm, remember for an EI core you only need measure one side.

Pick a safe starting figure for the current density of say 3 amps per square mm, you will need wire of approx 3 squ mm cross section (3 x 3 = 9 amps current carrying = very close to 10)

Multiply needed turns (add an extra few turns to allow for wire resistance) by 3 to get total space taken by wire, add 20% to allow for imperfect winding and insulation. If it fits you are good to go. From memory I think about 1.8 - 2mm thick wire is approx 3squ mm cross-section.

If the wire won't fit try the same again but assume a current density of 5amps per square mm, this you will need wire of 2 squ mm cross section (thinner) and again calculate if it will fit.

Any thing from 3 - 7 amps per squ mm should be ok, obviously the lower the current density the cooler it should run, the wire however will thicker and a bit harder to wind.

Modern transformers especially plug pack types can go as high a 10 amps per sq mm, they do get quite hot though and I would not recommend current densities that high.

I know that there is a certian amount of satisfaction in doing your self but unless you have the wire on hand I don't believe it is worth it, keep a lookout on gumtree and the 'bay for a 2nd hand transformer of the size you want or go to your local Jarcar and check out the shelves for a transformer that looks like it have been there for some time (faded and tatty looking package) make the manager an offer, if it has been there for a few years he may just want to get rid of it.

Lastly judging from the wording of your question I'm assuming you are a newbie with electronics (apologies if you are not) so DO NOT be tempted to power up a microwave oven transformer just to test the output voltage it can be as high as 2kV, if you make a mistake it WILL kill you.

[QUOTE=walcen;1889281]

Quote:

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Originally Posted by steve-37

When completely stripped down it's basically just a 50 hz EI laminated core. I understand that in it's original state, the core is over driven as it only has 240 primary windings, hens the need for shunts and a cooling fan. I'd hoped that a complete rewind with no shunts would eliminate most issues. Old transformers are now hard to come by and I don't mind admitting that I'm a bit of a tight ass so am not prepared to outlay $190 od to have one made.

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I understand the satisfaction of rolling your own but what about something like this
http://www.altronics.com.au/p/mc5745...l-transformer/
$120 if you sign up for an Altronics trade account.
Then you know it's safe plus all the other advantages of a toroidal unit.

[QUOTE=BobL;1889803]

Quote:

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Originally Posted by walcen

I understand the satisfaction of rolling your own but what about something like this
http://www.altronics.com.au/p/mc5745...l-transformer/
$120 if you sign up for an Altronics trade account.
Then you know it's safe plus all the other advantages of a toroidal unit.

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Wow 16000 + posts!! you have been a busy boy, my hat is off to you.:2tsup:

Yes I've looked at Altronics and seriously considered buying one of those, But money's tight and I do like to roll my own as much as possible, It's my Philomathical nature.

If I fail I may still consider a toroidal.

Thank you

Wal

Steve, I've worked out my turn counts using this formula from this website
http://www.google.com/translate?u=ht...&en=es&en=UTF8
with a constant of 42. it suggests 432 pri and 72 sec

7.3 cm x 3.2 cm = 23.36 cm



42/23.36 = 1.8 turns per volt


240v x 1.8 = 432 primary turns.


40v x 1.8 = 72 secondary turns.

I'm struggling with a decision to use a single bobbin that I know I can fit all the turns on, or a stacked bobbin that might be a little touch and go but will offer better isolation.:dunno:

regards

Wal

[QUOTE=walcen;1890353]

Quote:

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Originally Posted by BobL

Wow 16000 + posts!! you have been a busy boy, my hat is off to you.:2tsup:

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No hat tipping required .

Then there are the 6000+ posts on a Wood milling forum, and a few thousand on a bunch of other forums . . . . .

Some people might say - look at all that time wasted when he could have been doing something useful , or that bloke has too much to say for himself, or . . .. . :)

Good luck with it.

Quote:

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Originally Posted by walcen

I'm struggling with a decision to use a single bobbin that I know I can fit all the turns on, or a stacked bobbin that might be a little touch and go but will offer better isolation.:dunno:
Wal

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Hi Wal

If you have a stacked bobbin for the transformer, I would use it.
If you are unable to get all of the turns on, I would just reduce the number of turns by say 10-20 %, remembering that the original transformer only used 240 primary turns, so it would be quite OK to reduce the turns, but don't go below 240 turns.
Just keep to the correct primary/secondary ratio.

When you wind the secondary, I usually increase the number of secondary turns by 3-5% to account for any resistive losses.
When you test the transformer under full load, you may find the O/P is a little high.
If you want, you can remove a few turns without dismantling the transformer. Its much easier than adding turns if the output is to low.

Regards Steve

Serioulsy ..... rewinding a transformer is more trouble than it is worth ..... point 1 .... where are you going to get the wire from.

Another particularly important issue is the safety and compliance of the completed transformer ..

I got a box of trannies ...and a few biguns.

One good source of suitable trannies is dead power amplifiers ...... its become so expensive to replace an output stage in a power amplifier these days in comparison to buying a new one .... and som of the designs are so margiannly stable that they simply cant be repaired.

what voltage and VA rating are you looking for .... I know i've got some 120 is VA 24 volt O core billitons ....I got a 500va torroid .. not sure of the voltage I think I got a complete 450 watt per channel amp with a blown output stage .... torroid, case filter caps the lot.

cheers