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 Subject: Safety factor for resistors Category: Science > Technology Asked by: skippy60-ga List Price: \$5.00 Posted: 27 Sep 2003 20:32 PDT Expires: 27 Oct 2003 19:32 PST Question ID: 260877
 ```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``` 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.``` 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```
 skippy60-ga rated this answer: and gave an additional tip of: \$1.00 `Sublime, thanks for a quick and excellent answer.`