Newbe v. simple Transformer question - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

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 22nd September 2002, 02:46 PM   #1
diyAudio Member
 
Join Date: Sep 2002
Location: London, UK
Question Newbe v. simple Transformer question

Hi there,
I have very little knowledge of electronics (though Im learning fast Im in the process of building my first psu. However I have a very simple question. If my transformer delivers 25-0-25v (Am I correct in thinking that this is 2x +25v as opposed to -25v and +25v?)this will then give something in the region of 35v after rectification how can I transform this into +/-35v? Im sure the answer is very simple I just can't get my poor head around it.


Any help would be gratefully appreciated


Jamie.
  Reply With Quote
Old 22nd September 2002, 03:03 PM   #2
Rarkov is offline Rarkov  England
diyAudio Member
 
Rarkov's Avatar
 
Join Date: Aug 2002
Location: North East, UK
Send a message via ICQ to Rarkov Send a message via Skype™ to Rarkov
Hi,
I'm afraid you've got the wrong end of the stick
The diagram for a transformer is more helpful than the 25-0-25 description! What this means is that there are two secondary windings. One will output 25V, with respect to ground and the other will give -25V. The reason for this is that they are centrally tapped and that centre tap goes to ground. What you can do is connect ground to the bottom pin and ignore the centre one. Then you will get 50V from the top pin. (Vice versa will give you -50V) but using this method - you cannot get +-50V

I hope that is clear enough! I have enclosed a crap diagram to confuse you more!!!
Gaz

P.S. This is my first message as a Prophet!
Attached Images
File Type: jpg trans.jpg (3.3 KB, 294 views)
  Reply With Quote
Old 22nd September 2002, 03:11 PM   #3
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
Careful. The notion of + and - is not quite correct when referring to AC, since each wire goes alternately + and - depending on where you're at in the cycle. You can instead refer to phase.
Sometimes transformers have 2 secondary windings (e.g. 2x25V) and sometimes a single winding with a 'tap' in the centre (e.g. 50VCT). You can use the 2-winding version as a centre-tap version if you connect the proper wires together (getting the phase right).
Phases are normally shown as dots or absence of dots as the equivalent of +. To connect two windings in series, connect a dotted lead with a non-dotted lead (equivalent of putting batteries in series + to -).
If you take a 50VCT transformer, use a bridge rectifier and 2 filter capacitors in a conventional arrangement (see for example, the power supplies at www.sound.au.com for any of his bigger amps), you will get +/-35VDC (or 70VDC total).
Read the articles at www.sound.au.com for much more information.
  Reply With Quote
Old 22nd September 2002, 04:16 PM   #4
diyAudio Member
 
Join Date: Sep 2002
Location: London, UK
Great so just to see if Ive got this right, if I connect the black and the yellow together I can use this as 0v. Then the orange would be v- and the red v+ is that about right?

Also if a transformer is said to be 18-0-18 is there anyway of knowing weather it is centrally taped or if it has two secondary windings (ie 2x25v)?? Sorry about these elementary questions but you've got to start somewhere I suppose.
Attached Images
File Type: gif transfo.gif (7.2 KB, 284 views)
  Reply With Quote
Old 22nd September 2002, 04:19 PM   #5
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
Yes.. but you must use a rectifier. Then you can obtain +,0,-
Pavel
  Reply With Quote
Old 22nd September 2002, 04:30 PM   #6
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
Like this...
Attached Images
File Type: gif attachment.gif (7.6 KB, 287 views)
  Reply With Quote
Old 22nd September 2002, 04:47 PM   #7
diyAudio Member
 
Join Date: Sep 2002
Location: London, UK
Thanks!! This is a great website!
  Reply With Quote
Old 22nd September 2002, 04:51 PM   #8
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
You are welcome. Just happy to help :-))
  Reply With Quote
Old 22nd September 2002, 05:00 PM   #9
diyAudio Member
 
Steve Eddy's Avatar
 
Join Date: Sep 2002
Location: Sacramento, CA
Quote:
Originally posted by jnxw2
Great so just to see if Ive got this right, if I connect the black and the yellow together I can use this as 0v. Then the orange would be v- and the red v+ is that about right?
Black and yellow together forming the center tap is right. But the rest doesn't matter. It's the rectifier that establishes the DC polarity. Doesn't matter how you hook up the orange and red leads.

As someone else mentioned, dots are sometimes used on the transformer's schematic to indicate which leads have the same relative polarity. And this is important to know when you have a transformer with dual secondaries which can be wired in series or parallel.

For example, if you wanted to create a center tapped transformer, if you connected the two leads with the dots or the two leads without the dots to form the center tap, you wouldn't get any output as the two secondaries would be cancelling each other.

Similarly, if you wanted to wire the two seconaries in parallel, if you wired the lead with the dot on one secondary to the lead without the dot on the other secondary, and vice versa, you'd also get no output.

But once you've got the secondaries wired correctly, the concept of + or - is irrelevant as far as the rectifier is concerned.

Quote:
Also if a transformer is said to be 18-0-18 is there anyway of knowing weather it is centrally taped or if it has two secondary windings (ie 2x25v)?? Sorry about these elementary questions but you've got to start somewhere I suppose.
Not necessarily. Some manufacturers will use the xx-0-xx format even if the transformer has dual secondaries to indicate the center tapped voltages. Though if xx-0-xx is the ONLY designation given, there's perhaps a greater chance that the transformer is internally wired for a center tap.

se
  Reply With Quote
Old 22nd September 2002, 05:02 PM   #10
diyAudio Member
 
Steve Eddy's Avatar
 
Join Date: Sep 2002
Location: Sacramento, CA
Quote:
Originally posted by PMA
Like this...
Right. And if you swap the red and orange leads, you get the same result.

se
  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
Newbe question about power chips Musiclear Solid State 2 17th November 2006 06:51 PM
ANOTHER newbe question!!! Dannyball Multi-Way 11 27th May 2004 03:13 PM
simple transformer question owel Parts 3 12th March 2004 06:41 PM
Newbe and first question josefr Digital Source 1 28th March 2003 09:57 PM
Simple Aleph 2 transformer question BrianGT Pass Labs 14 7th April 2002 10:27 PM


New To Site? Need Help?

All times are GMT. The time now is 12:39 PM.


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