Re: Why voltage drops with very small load? - 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 12th February 2006, 09:27 AM   #1
diyAudio Member
 
Join Date: Dec 2005
Location: New York
Default Re: Why voltage drops with very small load?

I built a very simple AC/DC transformer with the following parts

2 x 9V 20VA transformer
4 x MUR860 diodes
4 x 560uF 50V caps (just have these on hand)

I used various resistors to load the power supply and the output DC voltage changes as follows

No loading 12.84V
175 ohm loading/current shows 65mA/voltage reads 11.85V
125 ohm/95mA/11.78V
100 ohm/115mA/11.66V
50 ohm/225mA/11.27V
36 ohm/280mA/11.14V

Considering the transformer is 20VA with other parts well exceed 1A, why does the voltage drops when the current increases to such a small loading?

Thanks
  Reply With Quote
Old 12th February 2006, 09:41 AM   #2
Warp Engineer
On Holiday
 
AudioFreak's Avatar
 
Join Date: Oct 2001
Location: Queensland, Australia
I'd expect it to settle at around 10.5V at rated load. A transformer with low VA like that will have poor regulation which explains the higher than expected no load voltage. At only 9VAC secondaries, the diode drop is a significant portion of the rectified voltage which accounts for the deficit between VAC x 1.414 vs. the actual loaded output.
__________________
- Dan
  Reply With Quote
Old 12th February 2006, 11:42 AM   #3
moamps is offline moamps  Croatia
diyAudio Member
 
Join Date: Oct 2002
Location: Croatia
Default Re: Re: Why voltage drops with very small load?

Quote:
Originally posted by Aaron323
Considering the transformer is 20VA with other parts well exceed 1A, why does the voltage drops when the current increases to such a small loading?
Hi,
That's normal for unregulated power supply.
For more info see
http://hyperphysics.phy-astr.gsu.edu...rectct.html#c4

Regards,
Milan
  Reply With Quote
Old 12th February 2006, 03:52 PM   #4
dnsey is offline dnsey  United Kingdom
diyAudio Member
 
Join Date: Mar 2004
Location: Shropshire, England
If you take into account the internal resistance of the supply, the variation is easily explained.
  Reply With Quote
Old 13th February 2006, 12:40 PM   #5
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Voltage drop is not proportional to the load current, the apparent resistance is reduced as the load is increased. Voltage will also drop if you (or your neigbours) turn on appliances connected to the same mains line, this is another funny feature of unregulated power supplies.
  Reply With Quote
Old 14th February 2006, 01:10 PM   #6
diyAudio Member
 
Join Date: Dec 2005
Location: New York
Thank you for all the information. If I changed the transformer from 20VA to 500VA, will the voltage also drops with a load of 250mA?
  Reply With Quote
Old 14th February 2006, 01:46 PM   #7
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
It will also drop. Consider it this way: the first few volts of voltage drop appear very easily in that kind of unregulated power supplies. A much smaller change will happen when the load is increased from, say, 250mA to 500mA. It happens due to the shape of the mains sine wave and the non-linear characteristic of the diodes.
  Reply With Quote
Old 14th February 2006, 01:47 PM   #8
Did it Himself
diyAudio Member
 
richie00boy's Avatar
 
Join Date: Nov 2003
Location: Gloucestershire, England, UK
Voltage will always drop with load, just that the higher the VA rating the less it will drop. Or looking at it another way, the rated voltage is usually at the rated load current, so if you don't load it up fully it will be higher. And the lower the VA rating the worse the regulation so the greater the difference between loaded and loaded output voltage.
__________________
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more.
  Reply With Quote
Old 14th February 2006, 02:22 PM   #9
Warp Engineer
On Holiday
 
AudioFreak's Avatar
 
Join Date: Oct 2001
Location: Queensland, Australia
Quote:
Originally posted by Aaron323
Thank you for all the information. If I changed the transformer from 20VA to 500VA, will the voltage also drops with a load of 250mA?

Yes, as richie and Eva have already said.... It will still drop but regulation for a 500VA transformer will be far superior to that of a 20VA transformer so the difference between no load and loaded voltage output will be less with the 500VA transformer (ie. It will produce a "stiffer" supply).
__________________
- Dan
  Reply With Quote
Old 17th February 2006, 12:02 AM   #10
diyAudio Member
 
Join Date: Dec 2005
Location: New York
Thank you for all the information

  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
Voltage spikes under load tstitans Power Supplies 3 13th August 2009 02:01 AM
Voltage Drop Based On The Load OMNIFEX Solid State 29 9th May 2009 01:14 PM
PS voltage without load duke_tj Power Supplies 5 16th September 2007 06:27 AM
No Load Voltage on Zen V4 Power Supply vsr123 Pass Labs 13 5th May 2003 04:10 PM
BOZ supply voltage drops to 1/2?? Kermit Pass Labs 11 11th December 2002 07:08 PM


New To Site? Need Help?

All times are GMT. The time now is 03:30 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