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#31
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Difference between 1/4W and 1/2W resistors?
Conclusion: A 1/8W resistor is quite enough,
a 1/4W resistor is safer and conservative. *TimDaniels* |
#32
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Difference between 1/4W and 1/2W resistors?
On 3/15/2011 7:44 AM, larry moe 'n curly wrote:
Man-wai Chang wrote: It's only the resistance value of the resistor and the load that determines the current drawn, not the wattage of the resistor. Then why are resistors (in that shop at least) classified resistors by W? To prevent overheating. If you take a resistor and operate it at its rated power, it will easily reach 100 Celcius, and if it's rated for at least about 1/2W it can also burn your fingers, melt wire insulation, make circuit boards discolor or char, and even soften solder. I've been told to use resistors rated for at least twice the maximum power they'll have to dissipate (or 4x the max for hot environments, like in a car's engine compartment), and usually when I've found a cracked or burned resistor, it had been running at very close to its power rating. You've just shown the problems that have always existed when a resistor is used in a circuit which dissipates a lot of power. The biggest reason why a lot of the situations you describe happened wasn't due to the wattage rating, it was the mechanical designs used. A lot had to do with airflow. Stuffing the resistor flat to the board may have contributed to a little bit of heat-sink action, but unless there were "big around planes" underneath... all you had was an eventual board-burn. Raising the resistor off the board a ¼" cured a lot of that. Pad-burn, melted solder, gray joints were also common with just 90° lead bends. Simple thing like adding a full loop about ¼" often cured that. Just another basic heat-sink idea... As for "underhood stuff", blame the bean-counter mentality on that. -- The black flies were coming. (Alastair Mayer in "Small Penalties") |
#33
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Difference between 1/4W and 1/2W resistors?
On Thu, 17 Mar 2011 21:43:03 -0700, Timothy Daniels rearranged some
electrons to say: "david" tried again: Timothy Daniels wrote: "david" wrote: Timothy Daniels wrote: "edfair" wrote: Most of the stuff I've dealt with used 330 ohm 1/8W resistors with standard LEDs on 5V supply. Doesn't let thru enough for full brightness but doesn't lead to premature death. For peace of mind you might want to ask for flame proof resistors. Nothing like miscalulating the required value or getting a defective one and ending up with a candle. IIRC the LED drop is 1.7V which would put 3.3V across the 330 ohm resistor. Assuming that P=VI (power = voltage x current), or V**2/R, that works out to .033watts, or about 3.79 times the 1/8watt rating. The 1/4watt resistors mentioned previously in this thread should suffice quite nicely. *TimDaniels* No. 1/8W = 125 mW, not 8 mW (.125W)/(.033W) = 3.79 . Review your math. *TimDaniels* Review yours. Do I need to remind you of what you wrote? that works out to .033watts, or about 3.79 times the 1/8watt rating. 33 mW is 26% of the 125mW rating. Which says the same thing. Learn some arithmetic. No, it's not the same thing at all. Learn some arithmetic. X does not equal 1/X. You should have said: "A 1/8W resistor has a power dissipation rating that is 3.79 times greater than the power dissipated by the LED circuit, so it will be more than adequate for this application". Which is the same thing. Learn some arithmetic. *TimDaniels* No, it's not the same thing at all. Learn some English. |
#34
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Difference between 1/4W and 1/2W resistors?
On Fri, 18 Mar 2011 18:58:44 -0700, Timothy Daniels rearranged some
electrons to say: Conclusion: A 1/8W resistor is quite enough, a 1/4W resistor is safer and conservative. *TimDaniels* Of course, this isn't what you said earlier... that works out to .033watts, or about 3.79 times the 1/8watt rating. Maybe you'll finally realize that 0.033 does not equal 0.125 * 3.79, but it is equal to 0.125 / 3.79. Or, more likely, instead of admitting your mistake, which was pointed out by several, you'll continue to insist you were right all along. In either case, I could care less about anything that you say, since you have lost all real credibilty. |
#35
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Difference between 1/4W and 1/2W resistors?
"david" wrote:
[.....] In either case, I could care less about anything that you say, since you have lost all real credibilty. Promises, promises... *TimDaniels* |
#36
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Difference between 1/4W and 1/2W resistors?
On Sat, 19 Mar 2011 08:41:41 -0700, Timothy Daniels rearranged some
electrons to say: "david" wrote: [.....] In either case, I could care less about anything that you say, since you have lost all real credibilty. Promises, promises... *TimDaniels* As I suspected... "Or, more likely, instead of admitting your mistake, which was pointed out by several, you'll continue to insist you were right all along." Idiot. ....plonk... |
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