Taming mains voltage - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 16th September 2007, 05:42 AM   #1
diyAudio Member
Join Date: Oct 2006
Default Taming mains voltage

I am running around 123VAC into a 125-0-125 trafo and a 6.3VAC trafo for an Aikido amplifier. Both these trafos are rated for 115V on the primaries. As a result of the higher input voltages, the 125-0-125 puts out 299VAC primary to primary, and the 6.3VAC trafo 7.03VAC.

Based upon current draw, I have come up with the following calculations to estimate what size resistor I would put on the primary side of these trafos to reduce input voltage by 5V (This would ensure that when our mains voltage drops to 120V, the trafos would see 115V and not be underpowered).

125-0-125 VAC:
1. B+ is 312V into 18mA = 5.61W, rounded up to 7W (as an over estimate).
2. Line current would then be 7W / 123VAC = 56mA.
3. Required voltage drop is 5V = 5V / 0.056A = 89.3R
4. Resistor power req. = 5 x 0.056 = 0.28W, which may as well be rounded-up to 2W or 5W resistor.

So around a 90R resistor that is rated to 2W or even 5W (this would be overkill).

For the filament trafo:
1. Power is 7.03V x 2.4A (four x 6mA) = 16.8W so 20W for a little extra margin.
2. line current 20W / 123VAC = 0.16A
3. Req voltage drop of 5V = 5V / 0.16A = 31.25R
4. Resistor power req = 5 x 0.16 = 0.8W, rounded up to 2W or even 5W.

So, a resistor for the filament trafo of 30R rated at 2W or 5W.

Does my math seem OK?

Also, do I need a resistor on each primary (both transformers have two primaries)? If so, would I use half the total required resistance on each primary?

My hope is that by taming my mains voltage, I can reduce the slight buzz and heating that my transformers undergo. It is also a cheap fix, as resistors are cheap.

  Reply With Quote
Old 16th September 2007, 10:25 AM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
the voltage you have measured is the open circuit voltage of the transformer. It show regulation is about 12% this is normal for an EI type.
The mains voltage is about 7% high when you measured it. That is probably a bit high if your nominal supply is 115Vac, but you are probably adjusted at the transformer for a nominal 120Vac.

Multiply 7% and 12% together (1.07*1.12) and apply that to your rated secondary voltage and you get 299Vac.

When you load your transformer the voltage stored on the smoothing cap will be much lower than square root(2)*299.

Do not put a resistor in the primary. Measure your DC voltage and maybe add a series resistor to the secondary before the main cap to give an RC supply.
If you have cascaded CRC or CLC supply then you have the option to add a small resistor before the first cap and another before the second cap. This method of adjusting the DC voltage gains the advantage of reducing the mains hum/buzz on the DC supply.
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 16th September 2007, 03:50 PM   #3
diyAudio Member
Join Date: Oct 2006

You have left me a little confused here. All I am trying to do is to reduce mains input voltage that the transformers see, as they are rated for 115 and get more like 123. After the trafos, my PSU works, with the exception of a slight hum that is due either to some ground loop, or the buzzing of the transformers. I have put in a lot of effort to remove ground loops (maybe not fully successful), but a couple of resistors on the primaries may lower the high voltage seen by the trafos and make them quieter, at least, this is my rationale.

  Reply With Quote


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
LED on mains voltage? AndrewT Parts 43 19th March 2014 11:33 AM
Mains voltage deviation... Bryan Everything Else 7 11th January 2006 07:46 AM
Carver AV-405/AV-505 mains voltage rt_jensen Solid State 0 25th April 2005 03:03 AM
mains voltage fluctuating borloloy Parts 2 28th February 2004 01:38 PM
mains voltage stabiliser GAK Everything Else 1 10th April 2001 07:14 PM

New To Site? Need Help?

All times are GMT. The time now is 05:45 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2