Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

Power transformer voltage question
Power transformer voltage question
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 23rd October 2005, 07:16 PM   #1
hbarki is offline hbarki  Indonesia
diyAudio Member
 
Join Date: Dec 2003
Location: NoCal
Default Power transformer voltage question

I think I chose the wrong power transformer for my power supply. It is 35VA, 2x22V parallel secondaries, which should give me about 1.4 x 44 = 61 volts of output going to the 7812 and 7912 regulators. My filter caps are rated at 35V, and I want to have a +/- 12V output from the regulator. I think 61V will blow up the regulators and caps.

Other than buying a new transformer, is there any way I can reduce the output AC voltage coming from the secondaries?

Thanks for your help!
  Reply With Quote
Old 23rd October 2005, 07:39 PM   #2
tool49 is offline tool49  Canada
diyAudio Member
 
tool49's Avatar
 
Join Date: Apr 2003
Location: Montreal
Don't forget that each regulator should only see half this voltage so each should only see 30.5V. It is still a bit high though. Technically the LM7812 is good for up to 35V so you should be alright, just make sure it has enough heatsinking.

What you could do is, if you are too worried, use a LM317/LM337 combo instead as these only care about the differential between your input and output. In your case, 30.5-12=18.5V which is way inside the range allowed by the LM3x7 regulators.

Hope this helps!
Sébastien
  Reply With Quote
Old 24th October 2005, 07:41 AM   #3
AndrewT is offline AndrewT  Scotland
R.I.P.
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
you could try two alternatives

1. Add a preregulator with it's output referenced to the final voltage not to ground. See Jung and Borbely (I think it is the super reg).

2. Build a CRCR PSU before the regulator. It will drop a significant voltage before the regulator and when current increases it drops it even further so reducing power dissipation on the regulator.

B.T.W. Tool49 is right, each 22Vac winding after rectification and smoothing will give about 30Vdc. For regulated 12Vdc you should use a 12Vac to 15Vac transformer. By using a higher voltage you are throwing away a large part of the VA rating that you are paying for (space and price). But if you use either or both of the two alternatives you do gain a bit back in return for the excess voltage.
__________________
regards Andrew T.
  Reply With Quote
Old 24th October 2005, 07:53 AM   #4
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: The great city of Turnhout, BE
If you have two 22VAC windings put them in series and ground the mid point. A full bridge rectifier would give you just what you need.

Jan Didden
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.
  Reply With Quote

Reply


Power transformer voltage questionHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Power transformer question keithgreenhalgh Tubes / Valves 4 6th August 2008 07:37 AM
Power transistor rail voltage question yeshu26 Solid State 23 21st July 2004 06:46 PM
Stupid question about voltage and output power Cro maniac Chip Amps 7 6th May 2004 11:33 AM
Voltage from the power transformer in a Krell KMA-100 mk1 Pinhead Solid State 5 29th April 2004 09:20 PM
Power Transformer Voltage trustalbini Tubes / Valves 7 11th May 2003 12:37 AM


New To Site? Need Help?

All times are GMT. The time now is 02:53 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 16.67%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki