Fuse positioning in PSUs - 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 10th November 2012, 02:48 PM   #1
polsol is offline polsol  South Africa
diyAudio Member
 
Join Date: Aug 2004
Location: Cape Town and Kuwait
Default Fuse positioning in PSUs

Having a fuse on the line input and on the DC output of a PSU is standard practice (well at least the mains input).
However, I have seen designs that include an additional fuse(s) between the transformer secondary and the bridge rectifiers or ones with fuses between the bridge and the caps.
Is one of the latter topologies actually necessary/of benefit?
If they are desirable, how would one calculate their values?

As a side issue, when mounting an unshielded toroid with 'flying leads' is it possible to mount the toroid upside down so the leads are at the bottom - i.e. in contact with the case? I'd guess that this would dangerous but I haven't found a website that mentions this possibility or that warns against it.

Cheers.
__________________
Tony M
  Reply With Quote
Old 10th November 2012, 02:55 PM   #2
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
A fuse between the secondary and the rectifiers can protect the transformer from a shorted rectifier. A fuse before the reservoir cap can protect the rectifier from a shorted cap. Generally, a fuse protects the things near it in the circuit and may provide some weaker protection for things a bit further away.

The fuse value is calculated from the expected RMS current, as for any circuit. This can be difficult when rectifiers are concerned. You could use a guess of 2-3 times the DC current.
  Reply With Quote
Old 10th November 2012, 03:17 PM   #3
polsol is offline polsol  South Africa
diyAudio Member
 
Join Date: Aug 2004
Location: Cape Town and Kuwait
Thanks for the reply DF96, pretty much as I thought.
I guess the question is which is more likely to fail, the bridge rectifiers of the electrolytics.
__________________
Tony M
  Reply With Quote
Old 10th November 2012, 03:17 PM   #4
diyAudio Member
 
Robert Kesh's Avatar
 
Join Date: Oct 2012
So what fuses do you guys think sound best? I like Gossen Metrowatts700V, but would probably use Schurters 1000V if I could afford it.
  Reply With Quote
Old 10th November 2012, 03:24 PM   #5
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
I've been listening to mine, but they seem to be quite silent.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 10th November 2012, 03:26 PM   #6
diyAudio Member
 
Robert Kesh's Avatar
 
Join Date: Oct 2012
My last post was intended to be humorous, by the way.
  Reply With Quote
Old 10th November 2012, 03:28 PM   #7
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Different fuses CAN lead to audible differences, though. I put a 100mA fuse in the power rails of my power amp and the power amp was much quieter than with a 3 amp fuse.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 10th November 2012, 03:29 PM   #8
diyAudio Member
 
Robert Kesh's Avatar
 
Join Date: Oct 2012
Quote:
Originally Posted by SY View Post
Different fuses CAN lead to audible differences, though. I put a 100mA fuse in the power rails of my power amp and the power amp was much quieter than with a 3 amp fuse.
  Reply With Quote
Old 10th November 2012, 03:41 PM   #9
polsol is offline polsol  South Africa
diyAudio Member
 
Join Date: Aug 2004
Location: Cape Town and Kuwait
Doing away with the PSU altogether results in a remarkably low noise floor.
__________________
Tony M
  Reply With Quote
Old 10th November 2012, 03:46 PM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Fuse the mains close rated. (300VA on 220Vac can use T1.25A or T1.6A, you will need a soft start)
Then a fault at the output of the transformer blows the mains fuse.

There is a big advantage in fusing the output of the PSU at the PSU, not at the amplifier.
Once these three fuses are fitted I cannot see any advantage in fusing before the smoothing capacitance. They would need to be so big to survive start up, that they never blow quickly enough to protect anything that survives the first few microseconds of fault condition.
__________________
regards Andrew T.
  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
Toroidal positioning Mankysteve Chip Amps 5 15th September 2010 10:42 AM
Fuse housing melting but fuse not blowing, ppi amp rawadia Car Audio 21 2nd August 2008 10:39 AM
Positioning loudspeakers XXX Multi-Way 0 28th May 2007 03:14 PM
amp rating for fuse and fuse holder? riotubes Parts 5 27th August 2006 03:10 AM


New To Site? Need Help?

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