Unwinding a toroid? - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 11th November 2007, 01:55 AM   #1
StalfoS is offline StalfoS  Canada
diyAudio Member
 
Join Date: Jan 2005
Location: Vancouver, British Columbia
Default Unwinding a toroid?

I have a 25V dual secondary transformer and I need the secondaries to be closer to 20V. I am thinking of attempting to unwind a few of the coils. Is there anything I should know before I attempt this? Would you advise for or against it?


(I wasn't sure what forum to post this it. It is here because it is for a
Tripath AMP5 from 41Hz.)

Cheers.
  Reply With Quote
Old 11th November 2007, 02:54 AM   #2
Kinnja is offline Kinnja  United States
diyAudio Member
 
Join Date: Jan 2007
Default Re: Unwinding a toroid?

Quote:
Originally posted by StalfoS
...it is for a
Tripath AMP5 from 41Hz.)

Cheers.

I don't know what forum would be the most correct one for a transformer question, but the Class D forum gets a lot of discussion on the Tripath amps.
  Reply With Quote
Old 12th November 2007, 12:12 AM   #3
diyAudio Member
 
Join Date: Oct 2004
Location: Maui, Hawai'i, USA
Stalfo,

I haven't done this myself, but I asked Dave Slagle about it and his reply was that the secondaries are almost always wound on the outside (the primaries are always the same, so they keep primary-wound cores ready to build to demand), and are almost always bifilar wound (for ease of winding) so that it's pretty simple to just unwind a few turns at a time, scrape the wires, connect up, and see what you get.

Only time there's a snag is if the center is potted (like the Amveco ones), it's a booger to get that out of there.

You might want to go a little lower than 20v; I just put an Amp 9 on the end of a 15V 50VA Amveco, and I'm getting about 21Vdc at the chip, so maybe 18-19 volts would be optimum.

Aloha,

Poinz
  Reply With Quote
Old 13th November 2007, 03:44 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
if you reduce the voltage from 25V to 20V you will also reduce the VA rating.
new VA rating = old VA rating *20/25.
__________________
regards Andrew T.
  Reply With Quote
Old 13th November 2007, 05:27 PM   #5
diyAudio Member
 
Join Date: Oct 2004
Location: Maui, Hawai'i, USA
Why? Same primary wire gauge and turns, same core, same secondary wire gauge, lower secondary DCR.

I don't think so, but I'll go look it up.

Aloha,

Poinz
  Reply With Quote
Old 13th November 2007, 05:33 PM   #6
Banned
 
Join Date: Jan 2005
I make toridal transformers for a living. its no problem unwinding some turns to alter the voltage output, just unwind the outer mylar insulation, go one turn at a time till you get the voltage you want, then wind the mylar insulation back on and presto.
  Reply With Quote
Old 13th November 2007, 05:49 PM   #7
diyAudio Member
 
Join Date: Feb 2004
Location: North of Toronto
Quote:
Originally posted by AndrewT
Hi,
if you reduce the voltage from 25V to 20V you will also reduce the VA rating.
new VA rating = old VA rating *20/25.

Quote:
Originally posted by Poindexter
Why? Same primary wire gauge and turns, same core, same secondary wire gauge, lower secondary DCR.


Poinz
I believe Andrew is correct in regards only to the secondary, as the guage of wire in secondary winfding is rated for the amperage, this is so independently from the Voltage delivered.

But the primary now is loaded less because you have to consider the VA input as well as the VA on output.

The VA of a transformer will have to consider both the input as well as the output. This is for a design heat rise parameter. THere will be less heat generated at the primary when the secondary Voltage is reduced for a given current output.

So i believe it is not just asimple VA on the output concern, but is a total transformer concern.
__________________
Sheldon D
  Reply With Quote
Old 14th November 2007, 07:46 AM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
having thrown away 5/25 (20%) of the VA rating, you now have that same 20% spare VA capacity in the primary.
You can add a third or even a fourth winding to use up that spare capacity.
What you cannot do is increase the output current of the original windings. Modern transformers mostly use 3.1A/sqmm. A thicker wire will improve the regulation and decrease the IsquaredR loss.

If you want the full VA rating in just one pair of secondary windings then you must replace the whole secondary with the correct larger diameter wire to maintain the same heat loss and the same regulation. That is a lot of work.
__________________
regards Andrew T.
  Reply With Quote
Old 14th November 2007, 01:39 PM   #9
Shaun is offline Shaun  South Africa
diyAudio Member
 
Shaun's Avatar
 
Join Date: Aug 2001
Location: Cape Town, South Africa
Another way to reduce output voltage is to add turns to the primary winding. If connected correctly, it will reduce output voltage. Reversing the connection polarity of the additional winding will boost the output voltage (which is how I actually used it).

Doing this will generally require adding more turns (to the primary) than what one would have removed (from the secondary). It also requires making an elongated bobbin, narrow enough to thread through the centre of the transformer.

To make the bobbin, one can use a plastic ruler or similar flat, rigid plastic or wooden object. Cut a V shape into both ends, then wind up a number of turns of enamelled copper wire over the length of the bobbin. This can then be threaded though the transformer....
__________________
Shaun Onverwacht
|||||||||| DON'T PANIC ||||||||||
  Reply With Quote
Old 14th November 2007, 09:28 PM   #10
diyAudio Member
 
Join Date: Feb 2004
Location: North of Toronto
Quote:
Originally posted by AndrewT
Hi,
having thrown away 5/25 (20%) of the VA rating, you now have that same 20% spare VA capacity in the primary.
You can add a third or even a fourth winding to use up that spare capacity.
What you cannot do is increase the output current of the original windings. Modern transformers mostly use 3.1A/sqmm. A thicker wire will improve the regulation and decrease the IsquaredR loss.

If you want the full VA rating in just one pair of secondary windings then you must replace the whole secondary with the correct larger diameter wire to maintain the same heat loss and the same regulation. That is a lot of work.

Quote:
Originally posted by Shaun
Another way to reduce output voltage is to add turns to the primary winding. If connected correctly, it will reduce output voltage. Reversing the connection polarity of the additional winding will boost the output voltage (which is how I actually used it).

Doing this will generally require adding more turns (to the primary) than what one would have removed (from the secondary). It also requires making an elongated bobbin, narrow enough to thread through the centre of the transformer.

To make the bobbin, one can use a plastic ruler or similar flat, rigid plastic or wooden object. Cut a V shape into both ends, then wind up a number of turns of enamelled copper wire over the length of the bobbin. This can then be threaded though the transformer....

Good point!, which concept applied to the secondary:

Actually adding bucking turns to the secondary of the same guage wire will in fact maintain the same VA of the transformer
at a lower voltage but should then maintain the same VA rating.
This is a relatively panlless way to lower the voltage but maintain the VA.

There would be a small extra IR drop due to the extra wire length. But this is a second order effect and could probably be ignored. (i.e. 20% 0f 20%= 4%)
__________________
Sheldon D
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Unwinding a steel laminate core inductor? Formula? AudioLapDance Parts 1 18th February 2009 01:27 AM
unwinding inductor chainenoble Multi-Way 2 31st January 2007 09:29 AM
Unwinding Toriodal Transformers DJED Parts 14 3rd January 2005 04:15 AM
Unwinding my inductor to obtain lower inductance michael Multi-Way 11 6th January 2004 06:10 AM
Unwinding secondary peter5 Parts 3 20th October 2003 04:35 AM


New To Site? Need Help?

All times are GMT. The time now is 02:55 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2