Adjusting a SMPS output - 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 19th September 2010, 08:03 PM   #1
Patman is offline Patman  United States
diyAudio Member
 
Join Date: Jul 2010
Exclamation Adjusting a SMPS output

Hello everybody on diyAudio,
I've got a challenge for the electrical engineers on the forum.

I've pulled a switch mode power supply from an old lexmark printer (got it a few years ago) and I'm trying to incorporate it into a small amp-on-a-chip project. I'm set on this ps because it outputs both 24v and 3.3v, perfect for driving the amp and some decorating LEDs.
Here's the problem: this amp supposedly runs best at 24v, but can operate between 8v-28v. I already know that if I lower the input voltage (say, to 12v) it'll give less gain, which I need for this particular application. This will also allow me to run the case cooling fan (an old CPU cooler) without risking burning it out.

Is there a way (and if so, how?) to lower the 24v output section of this supply, and maintain the 3.3v rail intact? I've been following the traces on the board, and I'm taking a rough guess that if I would be able to modify the driver circuit for the driver ICs, the output could be adjusted.

Also, should I be shielding the enclosure to prevent EMI?

As for filtering, I've noticed one particular fluorescent fixture in my house (on the same breaker as my workbench) generates a lot of buzzing in the speakers when switched on. Is there anything I could/should do about this? The PS has pretty good filtering as it is, I think.

I'd love any help I could get. I need to finish this one quick!
  Reply With Quote
Old 19th September 2010, 10:27 PM   #2
diyAudio Member
 
Join Date: Mar 2010
You can't adjust the outputs of the power supply separately. They are in a fixed ratio, determined by the turns ratio of the power transformer. Having said that, however, there are some supplies that have separate feedback paths and if you are lucky, you have one of those. Rare, though.

Shielding is a good idea; those units are practically radio transmitters.

The fluorescent lamp can be a problem; I doubt there is much you can do with it. If the EMI is line propagated you can try ferrites on the power leads. But if it's radiated from the lamp itself you are out of luck, short of screening it with some mesh.
  Reply With Quote
Old 20th September 2010, 12:42 AM   #3
luka is offline luka  Slovenia
diyAudio Member
 
luka's Avatar
 
Join Date: Sep 2005
Location: in Slovenia :)
Send a message via Skype™ to luka
you could do it, but you would need to redo the transformer..
__________________
home page @ http://www.classdaudio.tk/ @ 24/7 all year long
I FEEL SLOVENIA
  Reply With Quote
Old 20th September 2010, 05:24 AM   #4
Patman is offline Patman  United States
diyAudio Member
 
Join Date: Jul 2010
Quote:
Originally Posted by bob91343 View Post
You can't adjust the outputs of the power supply separately.
Remember Bob, this is a switching power supply, not a linear one. I should upload pictures of it, but it's controlled by two small 8-pin ICs. One that (I assume) controls the main power MOSFET (i.e. the PWM cycle) for the 24v system and the other (that I assume) controls a secondary MOSFET [positioned after the first] for the 3.3v output.
  Reply With Quote
Old 20th September 2010, 07:49 AM   #5
diyAudio Member
 
Join Date: Mar 2010
I would imagine the main FET controls all the output. Each output has its own winding on the transformer and the ratio between the outputs is fixed. The secondary FET might be for the standby supply, which these beasts all have, so it can know when to switch on. The green wire needs to be grounded to make the supply start up.
  Reply With Quote
Old 20th September 2010, 08:50 AM   #6
Patman is offline Patman  United States
diyAudio Member
 
Join Date: Jul 2010
I'll post pictures, they should help decipher this.
From what I see, the main FET goes to the only active coil on the transformer, which supplies the other half of the board. from there it goes to the other IC and eventually the second FET, which runs to the 3.3v.
  Reply With Quote
Old 20th September 2010, 10:28 AM   #7
wwenze is offline wwenze  Singapore
diyAudio Member
 
Join Date: Mar 2008
SMPS providing multiple output voltages, for some the rails are group regulated, for others individually regulated. Or the power source for the lower voltage rail may be taken from the higher voltage rail.

If you can manage to google the function of the chips that should answer the questions.
  Reply With Quote
Old 20th September 2010, 01:23 PM   #8
Patman is offline Patman  United States
diyAudio Member
 
Join Date: Jul 2010
Yes, this ps takes the low voltage from high, but has an SMD IC regulating it via a secondary MOSFET (I think...)
  Reply With Quote
Old 20th September 2010, 04:22 PM   #9
wwenze is offline wwenze  Singapore
diyAudio Member
 
Join Date: Mar 2008
Try cross-loading.
  Reply With Quote
Old 20th September 2010, 05:27 PM   #10
Patman is offline Patman  United States
diyAudio Member
 
Join Date: Jul 2010
Cross-loading? explain.
  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
What happens with an output choke on a SMPS rickmcinnis Power Supplies 1 10th September 2010 06:30 PM
Variable output SMPS Razor-Edge Power Supplies 7 4th August 2007 12:37 AM
output rectifier choice for SMPS zilog Power Supplies 3 18th February 2007 05:56 PM
SMPS regulated output MaXiZ Power Supplies 1 24th December 2005 09:10 AM
Non-symetrical SMPS output cm961 Parts 4 21st August 2003 11:27 PM


New To Site? Need Help?

All times are GMT. The time now is 04:09 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