If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #4081
Joined: Wed Aug 31 2011, 06:40PM
Location: UK
Posts: 139
Why are resistors rated in watts and not amps? Just seems a lot of extra maths to do to find the watts, when surely it is the amps that cause all the damage. Thanks.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No, it's the watts that do the damage. Heat is what kills electronic components, and it's measured in watts.
Say you buy a 1 ohm and a 1000 ohm resistor, both 1 watt. They'll be the same size, as it's the physical dimensions of the part that limit how much heat it can dissipate. But the resistive element in the 1 ohm part will be shorter and thicker, capable of handling a lot more current. About 31.6 times more.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Because there are standardised sizes of resistors which are based on the Watt dissipation. You can't have standard sizes based on the Amp rating, because you would have a different size for every ohmic value...
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
Another reason is that it may be the case that steady state amperage could be a rating in place of wattage, but average amperage is meaningless, since P=I^2*R. So if you run 1 amp through a 1 ohm resistor continuously, you generate 1 watt. But if you run 2 amps through at 50% duty cycle, the dissipation is actually 2 watts rather than the 1 you might think at a quick glance (because 1/2 * 2^2 * 1 = 2).
So there's really no way an amperage rating would actually accomplish the goal of a thermal rating.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
If you have a component that can be damaged, each method of damage will have its own rating ... so ...
Too much current will fuse the thinnest link, or cause electromigration, so there's a max amps rating. Too much voltage will cause it to break over internally, so max volts Too much pull on the leads will physically break it, so max Newtons Too much power will cause it to overheat, so max watts Too much pulse power delivered rapidly will also overheat it, so max I2t
... but the power limitation is the one that you hit first in 99% of circuits for hobbies, commerce, comms, computing, whatever, so that's the headline figure for resistors. Dig further into the data sheet, you'll find voltage, and maybe current. You'll find other limits for more expensive specialist resistors.
But perhaps the most significant reason is that for ordinary air-cooled resistors, power rating goes as size. When you are doing a board layout, size is how many components you can cram in to what space. Size is what both the resistor manufacturer and the end user need to worry about for mechanical handling, so all of the packing, tubes, reels, all use the same size chip of ceramic substrate. But once you've got that standard size, the difference between a 10ohm and a 100k ohm resistor is just the thickness, composition and wiggles of a metal ink film fired onto the surface, or turns and thickness of wire wound on to it.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Resistor Wattage
