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Old 11th Apr 2019, 11:07 am   #1
Steve G4WCS
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Default Toroidal Transformer voltage reduction by removing turns

Dug out the Transformer I was intending to use for my 20 amp 13.8V Power Supply last night.

I thought it had 2 X 21V secondaries at 12.5A ( which is a bit on the high side but it was a brand new £10 bargain )

It hasn't, it has 2 X 25V secondaries.

Are there any pitfalls to reducing the secondaries output voltage by un shrink wrapping and removing turns on the secondary?

Is this the best way to go about it?
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Old 11th Apr 2019, 11:20 am   #2
Herald1360
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Default Re: Toroidal Transformer voltage reduction by removing turns

You will need to be careful to remove exactly the same number of turns from each secondary- especially if you then parallel them up into a bridge. A bit less dramatic if you use a centre tap full wave circuit as then you'll just add some 50Hz to make the ripple a bit worse.

Connect one end of each secondary together before and after mods and measure the voltage between the other ends. With the secondaries in phase it should be 0V.

Don't load the reduced secondaries to any higher current than the original rating.

Although that 12.5A secondary rating sounds overkill, it may not be when the rectifier circuit's transformer utilisation factor is considered. If continuous full load operation is envisaged, you'll need to allow for this. FWIW, a bridge is better than a full wave in this respect.

You could perhaps reduce the secondary voltage by using a suitable bucking transformer to feed the primary- no mods needed then and the extra TX will need a secondary current rating to match the main transformer primary current so it will be somewhat smaller than the main one......
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Last edited by Herald1360; 11th Apr 2019 at 11:31 am.
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Old 11th Apr 2019, 11:35 am   #3
kalee20
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Default Re: Toroidal Transformer voltage reduction by removing turns

Nothing to add to what Herald1360 has written, everything is there!

You should be able to tweak the voltage to well within half a volt of what you actually want. You'll have fun, and end up with a good project.
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Old 11th Apr 2019, 12:02 pm   #4
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Default Re: Toroidal Transformer voltage reduction by removing turns

I would guess that the on load voltage of that transformer would be much closer to 20 volts.
I would try it on load before any modification.
It is strongly recommended to de-rate transformers if running on continuous rectifier loads.
The charging pulses going into the capacitors often make transformers get quite hot.
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Old 11th Apr 2019, 2:56 pm   #5
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Default Re: Toroidal Transformer voltage reduction by removing turns

Might be easier to wind some turns over the existing windings and connect in reverse phase, can then drop the voltage to whatever you want.
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Old 11th Apr 2019, 6:17 pm   #6
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Default Re: Toroidal Transformer voltage reduction by removing turns

Quote:
Might be easier to wind some turns over the existing windings and connect in reverse phase
If done on the primary side the output will be as balanced as it ever was and you only need one winding.
 
Old 11th Apr 2019, 8:35 pm   #7
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Default Re: Toroidal Transformer voltage reduction by removing turns

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
Might be easier to wind some turns over the existing windings and connect in reverse phase
If done on the primary side the output will be as balanced as it ever was and you only need one winding.
But many more turns. To get lower output voltage, you will need to add primary turns or subtract fewer secondary turns.

If the two output widings are not identical, then it may be better to run each into its own bridge rectifier, then recirculating currents will be eliminated.

Last edited by TrevorG3VLF; 11th Apr 2019 at 8:38 pm. Reason: Add a bit.
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Old 11th Apr 2019, 9:00 pm   #8
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Default Re: Toroidal Transformer voltage reduction by removing turns

Just out of interest I measured the BNOS 12/25 toriod.
Label 240 V /18V 500 V.A , Low voltage winding over wound by the 240 volt winding on the top.

Off load open circuit secondary 18.76V at 240Volts ( true rms meter)

The winding resistance was 2.85 Ohm primary , 0.0183 Ohm secondary. (Kelvin Bridge)

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Last edited by G4_Pete; 11th Apr 2019 at 9:13 pm. Reason: added V
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Old 12th Apr 2019, 7:42 am   #9
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Default Re: Toroidal Transformer voltage reduction by removing turns

Yes, high current secondaries are often wound on first to prevent any crushing of the more delicate primary.
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Old 12th Apr 2019, 7:58 am   #10
Steve G4WCS
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Default Re: Toroidal Transformer voltage reduction by removing turns

Food for thought, thank you, I'll have a play.
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Old 16th Apr 2019, 2:43 am   #11
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Default Re: Toroidal Transformer voltage reduction by removing turns

If the secondary is on the outside (wound over the primary) easiest to undo turns. With careful examination, you may find that the secondaries are bifilar wound ( many toroids are).

Mylar tape is recommended after you have the correct voltage, or the voltage you require. At LEAST 3 layers, but 5 layers is better.

For current ratings, multiply the name plate rating by .62 to get the correct output current. Because toroids run such high flux densities, the charging current peaks can be literally HUGE.


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Old 11th May 2019, 10:08 pm   #12
Steve G4WCS
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Default Re: Toroidal Transformer voltage reduction by removing turns

An enjoyable couple of hours and 15 turns per winding later.

Off load ac before 28.5
After 18.3

Rectified DC

6A load 21V
13A load 19.5V
26A load (briefly due to the smell of warm croc leads) 17.5V

From careful shopping around.
Transformer £11
Load resistors £9
Cap bank and rectifier £8
2x heatsinks with 8 tested 2N 3055’s £14
got most of the other bits and pieces except a crowbar thyristor and mc3423.
Happy days
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Old 12th May 2019, 7:00 pm   #13
kalee20
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Default Re: Toroidal Transformer voltage reduction by removing turns

Well done! You've got the voltage you wanted, no volts thrown away is silly resistors, just exactly right straight from the transformer!
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