fuse voltage rating? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 February 2012, 04:36 AM   #1
diyAudio Member
 
Join Date: Feb 2012
Default fuse voltage rating?

i am working on an old alamo electra tube amp. on inspection before installing a 3 prong i noticed that the fuse is rated at 250 V. is it safe to just switch this out for 120? wouldn't that change the ampere rating also? any tips on finding the correct and safe fuse would be appreciated.

chris
  Reply With Quote
Old 23rd February 2012, 05:18 AM   #2
diyAudio Member
 
sofaspud's Avatar
 
Join Date: Nov 2005
Location: San Antonio
The voltage rating is independent of the current rating. Do you have a replacement in hand? Daring to go from memory, I recall the usual AGC glass fuse to be either 125V or 250V. This rating is an "arc-over" rating like that for resistors, and must be respected in the same way.
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
  Reply With Quote
Old 23rd February 2012, 08:59 AM   #3
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
Fuses are specified in terms of current for your unit. The fuse voltage has no effect on that as sofa points out. What you don;t want to do is put 32v fuses in 120v circuits. 250v fuse in a 120v circuit is perfectly fine.

Remember, when a fuse is intact, there is almost zero volts ACROSS the fuse. The only time there is any voltage from one end to the other is when the fuse is open. There will be a tiny voltage drop across a good fuse due to the tiny resistance of the fuse, but we can ignore that. SO the fuse voltage rating is about how many volts can safely be across a blown fuse. If a fuse that can handle 120v would be OK, then certainly a fuse that can handle twice that is OK too.
  Reply With Quote
Old 23rd February 2012, 04:38 PM   #4
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Just for interest,

When a fuse blows the wire " Element" melts, This causes a small gap and an arc forms that conducts until the gap in the melting fuse wire gets big enough to stop the arc. This is very fast however max fault current will flow for this time and do damage. Some fuses have sand inside so that the gap gets filled and the arc is stopped "quenched" faster. There are Time lag, to take into account magnetisation current of Tx's, quick blow to protect electronics and diodes.

The voltage rating is as already posted to ensure that the arc is quenched during a fault "element gap after current is exceded".

Fun link:

Electrical fuse burning with increasing current - YouTube


Regards
M. Gregg
__________________
What is the sound of one hand clapping?

Last edited by M Gregg; 23rd February 2012 at 04:54 PM.
  Reply With Quote
Old 23rd February 2012, 05:14 PM   #5
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Off topic,

Just a thought a 10A fuse does not blow at 10A..

Some fuses blow at 1.5x the rated value.. and can sit at rated value for some time..relative to fault clearance time.

Regards
M. Gregg
__________________
What is the sound of one hand clapping?
  Reply With Quote
Old 24th February 2012, 12:20 AM   #6
WILD1 is offline WILD1  United States
diyAudio Member
 
Join Date: Feb 2012
Location: Nacogdoches Texas [not really part of U.S.]
When you measure voltage across any resistance aren't you measuring the decrease in voltage? So when you measure a fuse in a 120 volt circuit you get 0 or close to because there is no decrease you have 120 volts both sides. However when that fuse blows you interrupt the circuit, you have no flow and your meter shows a decrease of 120 volts So when the fuse is intact you have 120 volts across it and it is only when the fuse blows that you have zero volts. Just a thought,I do agree that a 250 volt fuse is fine in replacing a 120 volt fuse.

Take care,
WILD1
  Reply With Quote
Old 24th February 2012, 06:55 AM   #7
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Quote:
Originally Posted by WILD1 View Post
So when the fuse is intact you have 120 volts across it and it is only when the fuse blows that you have zero volts. Just a thought,I do agree that a 250 volt fuse is fine in replacing a 120 volt fuse.

Take care,
WILD1
The point realy is not resistance.

Think spark plug..When a fault occurs the current drawn will only be limited by the resistance/inductance of the mains supply. So only the resistance of the cables etc limit the current. Now how long it takes for the fuse element to go open circuit is dependant on the heating effect caused by the fault. So forget the 10A fuse what current flows until the element opens is full current. The fuse does not limit the fault current untill it opens.The speed of operation is very important for damage limitation.

When the fuse element opens an arc is drawn this can be greater depending on any back EMF produced by transformer windings etc the (higher the voltage the greater must be the gap to break the arc). The arc will continue to jump the gap in the fuse element until it burns away to a distance great enough to stop the arc. This has nothing to do with fuse element resistance. The speed of operation of the fuse is dependant on the current drawn. Looking at Auto disconnection of supply if the fault to earth does not draw enough current the fuse will heat slower and the damage will be greater. Thats why no resistance is allowed in a safety earth connection.

Regards
M. Gregg
__________________
What is the sound of one hand clapping?

Last edited by M Gregg; 24th February 2012 at 06:58 AM.
  Reply With Quote
Old 24th February 2012, 07:10 AM   #8
diyAudio Member
 
sofaspud's Avatar
 
Join Date: Nov 2005
Location: San Antonio
Wild1 had it right to begin with, then changed his answer at the end.
I just don't want anyone thinking an open fuse has 0 volts across it.
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
  Reply With Quote
Old 24th February 2012, 07:12 AM   #9
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Quote:
Originally Posted by sofaspud View Post
Wild1 had it right to begin with, then changed his answer at the end.
I just don't want anyone thinking an open fuse has 0 volts across it.
Yep I think Volt drop is a bit confused in this context..Obviously there will be a volt drop across the fuse under fault conditions linked to resistance of the fuse element or it would not heat, however this has nothing to do with voltage rating and the gap required for fault clearance.


Regards
M. Gregg
__________________
What is the sound of one hand clapping?

Last edited by M Gregg; 24th February 2012 at 07:20 AM.
  Reply With Quote
Old 24th February 2012, 08:32 AM   #10
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Final comment,

Regards voltage being non relevant and current being the only issue for fuse operation. Volts X Amps =Watts = Heat.

So here is food for though..
would the fuse element heat faster with higher voltage or operate at the same time at the same current rating..would a higher voltage fuse used on lower voltage with the same current blow slower..

So why have different voltage rating fuses at all except for arc gap.
(Operation time)?

For our purposes as long as the fuse is above the working voltage of the circuit it will work. There is always more ..LOL

Regards
M. Gregg
__________________
What is the sound of one hand clapping?
  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
CM LABS 914a original fuse rating? IFixAudio Solid State 1 5th January 2011 06:56 PM
amp rating for fuse and fuse holder? riotubes Parts 5 27th August 2006 03:10 AM
Mystery Amp Rating on Fuse DreadPirate Solid State 5 24th April 2006 01:10 PM
Help - fuse rating (USA style) TNT Parts 2 7th March 2004 03:04 PM
Fuse Rating jwatts Multi-Way 3 6th January 2004 08:37 PM


New To Site? Need Help?

All times are GMT. The time now is 05:47 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