UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Reply
 
Thread Tools
Old 11th Apr 2019, 11:07 am   #1
Steve G4WCS
Pentode
 
Steve G4WCS's Avatar
 
Join Date: Sep 2004
Location: Preston, Lancashire, UK.
Posts: 219
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?
Steve G4WCS is offline   Reply With Quote
Old 11th Apr 2019, 11:20 am   #2
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 13,209
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......
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O

Last edited by Herald1360; 11th Apr 2019 at 11:31 am.
Herald1360 is offline   Reply With Quote
Old 11th Apr 2019, 11:35 am   #3
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 4,973
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.
kalee20 is offline   Reply With Quote
Old 11th Apr 2019, 12:02 pm   #4
Refugee
Dekatron
 
Refugee's Avatar
 
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 4,109
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.
Refugee is offline   Reply With Quote
Old 11th Apr 2019, 2:56 pm   #5
woodchips
Heptode
 
Join Date: May 2010
Location: Grantham, Lincolnshire, UK.
Posts: 627
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.
woodchips is offline   Reply With Quote
Old 11th Apr 2019, 6:17 pm   #6
merlinmaxwell
Dekatron
 
merlinmaxwell's Avatar
 
Join Date: Jul 2008
Location: Maidenhead, Berkshire, UK.
Posts: 8,377
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.
__________________
Cats have staff, it's dogs that have owners.
merlinmaxwell is offline   Reply With Quote
Old 11th Apr 2019, 8:35 pm   #7
TrevorG3VLF
Octode
 
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,221
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.
TrevorG3VLF is offline   Reply With Quote
Old 11th Apr 2019, 9:00 pm   #8
G4_Pete
Hexode
 
G4_Pete's Avatar
 
Join Date: Feb 2014
Location: Southampton, Hampshire, UK.
Posts: 338
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)

Pete

Last edited by G4_Pete; 11th Apr 2019 at 9:13 pm. Reason: added V
G4_Pete is offline   Reply With Quote
Old 12th Apr 2019, 7:42 am   #9
Dickie
Heptode
 
Join Date: Sep 2004
Location: St. Albans, Hertfordshire, UK.
Posts: 971
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.
__________________
Regards,

Richard, BVWS member
Dickie is offline   Reply With Quote
Old 12th Apr 2019, 7:58 am   #10
Steve G4WCS
Pentode
 
Steve G4WCS's Avatar
 
Join Date: Sep 2004
Location: Preston, Lancashire, UK.
Posts: 219
Default Re: Toroidal Transformer voltage reduction by removing turns

Food for thought, thank you, I'll have a play.
Steve G4WCS is offline   Reply With Quote
Old 16th Apr 2019, 2:43 am   #11
joebog1
Heptode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 795
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.


Joe
joebog1 is offline   Reply With Quote
Old 11th May 2019, 10:08 pm   #12
Steve G4WCS
Pentode
 
Steve G4WCS's Avatar
 
Join Date: Sep 2004
Location: Preston, Lancashire, UK.
Posts: 219
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
Attached Thumbnails
Click image for larger version

Name:	E1CF7AA6-FF8A-453E-AD64-EF67108EC0B8.jpg
Views:	39
Size:	54.2 KB
ID:	182904   Click image for larger version

Name:	7A22D182-B513-4DC1-901A-D2E9986310C0.jpg
Views:	39
Size:	58.4 KB
ID:	182905   Click image for larger version

Name:	4A82F1B8-71D3-4131-8204-20E819D5D53B.jpg
Views:	41
Size:	71.2 KB
ID:	182906   Click image for larger version

Name:	B1E26EA2-2304-461B-95CC-75C9751C4C45.jpg
Views:	42
Size:	77.8 KB
ID:	182907  
Steve G4WCS is offline   Reply With Quote
Old 12th May 2019, 7:00 pm   #13
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 4,973
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!
kalee20 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 6:44 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.