01-31-2011, 10:48 PM
For DC, watts = volts times amps. In the event of knowing only the resistor value in ohms and the current in amps, you cal also use current squared times resistance.
For the typical use of a 1K resistor with a white LED, at 12 volts that's about 9ma, or .009 amp. Or .108 watts - so a 1/8 watt resistor will be ok, although I usually just use 1/4 watt. For those SMD LEDs in a palce like a number board, you don;t want a real bright spot in the middle of the numbers so you may have to go with a bigger resistor, 1.8k or 2.2k. 2.2k will be about 5ma, which works out be about .055 watt, so the 1/8 or even 1/16 watt resistor will work (smaller than 1/8 will almost always be a SMD resistor - the smaller the wattacge generally is a physically smaller resistor).
Thus ends the electronics lesson for tonight.
--Randy
For the typical use of a 1K resistor with a white LED, at 12 volts that's about 9ma, or .009 amp. Or .108 watts - so a 1/8 watt resistor will be ok, although I usually just use 1/4 watt. For those SMD LEDs in a palce like a number board, you don;t want a real bright spot in the middle of the numbers so you may have to go with a bigger resistor, 1.8k or 2.2k. 2.2k will be about 5ma, which works out be about .055 watt, so the 1/8 or even 1/16 watt resistor will work (smaller than 1/8 will almost always be a SMD resistor - the smaller the wattacge generally is a physically smaller resistor).
Thus ends the electronics lesson for tonight.
--Randy
Modeling the Reading Railroad of the 1950's in HO
Visit my web site to see layout progress and other information:
http://www.readingeastpenn.com
Visit my web site to see layout progress and other information:
http://www.readingeastpenn.com