A resistor has a power rating, which is the maximum allowable power in watts
without damaging the resistor. How much safety factor should
one use when designing circuits with resistors?

skippy60...

As an ex-Navy electronics technician, I'll show my predjudice
by pointing you to some Navy training materials.

Typically, a safety factor of 50% is used in circuit design,
when it comes to resistors. So, if the circuit has a combination
of amperage and voltage which would cause a resistor to use
2 watts, a resistor with a rating of 2 + 50%, or 3 watts, would
be used. This is borne out by the discussion of power ratings
on this page from the Navy Electricity and Electronics Training
Series site on Tpub.com:
http://www.tpub.com/neets/book1/chapter3/1-8.htm

The NEETS site, which is a veritable goldmine of information
related to electricity and electronics, starts here:
http://www.tpub.com/neets/

Perhaps you will find a use for this site in regard to related
electronic circuit questions.


Please do not rate this answer until you are satisfied that
the answer cannot be improved upon by means of a dialog
established through the "Request for Clarification" process.

sublime1-ga


Searches done, via Google:

resistor "power rating"
://www.google.com/search?q=resistor+%22power+rating%22

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Request for Answer Clarification by skippy60-ga on 28 Sep 2003 06:00 PDT

sublime,

Can you come up with a second source? I don't like relying on only one
source for the answer to safety factor.
Thanks.

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Clarification of Answer by sublime1-ga on 28 Sep 2003 13:21 PDT

skippy...

Certainly! The recommendation of a 50% safety factor is also to be
found on the following webpage, in Chapter 3 of a manual discussing
'Marine Electricity, circuits, and theory', under 'Power Rating':

"In most circuits, the actual power a resistor uses is considerably
 less than the power rating of the resistor because a 50 percent
 safety factor is used."
http://myweb.tiscali.co.uk/montecarlo/marine-electrical/Ch3.htm


However, it is easy to find citations which recommend utilizing
a resistor with a power rating 2 or more times the expected
wattage. Given that the resistor is often the least expensive
component in the circuit, it can't hurt, and it allows for 
atypical fluctuations in the circuitry, such as might be
found, e.g., during power-up:

Play-Hookey.com is another goldmine of information on several
topics. On a page about resistors, from their discussion of DC
theory, the following is noted:

"A general rule of thumb is to always select a resistor whose
 power rating is at least double the amount of power it will
 be expected to handle. That way, it will be able to dissipate
 any heat it generates very quickly, and will operate at normal
 temperatures." (about the middle of the page):
http://www.play-hookey.com/dc_theory/components_resistors.html


Likewise, at the site of Power Technology Incorporated, who
design and manufacture high-quality lasers, they have a 
technical library which includes a page on ballast resistors:

"...we recommend de-rating the resistor wattage by 50% or more."
http://www.powertechnology.com/Techlib/ballast.asp

Which is to say, if a resistor is rated at 2 watts, use it in
a circuit calling for a 1 watt resistor.


And, for an even 3, here is a page from the Staco Switch site,
titled 'StacoSwitch's rugged duty Keypads' Lighting and
Switching Overview' which also recommends doubling:

"It is good idea to at least double the power rating of the
 resistor."
http://www.stacoswitch.com/m2ltsw.html


If anything is unclear, or the links don't work, please
feel free to post another Request for Clarification.

sublime1-ga


Search results were obtained from the original search:

resistor "power rating" 
://www.google.com/search?q=resistor+%22power+rating%22

Sublime, thanks for a quick and excellent answer.