In DC, Watts = Amps x Volts. .009 x 12 = 0.108W
A 1/4 Watt resister is still big enough.
richhotrain The purpose of a resistor is to dissipate power. As Randy points out, the power to be dissipated in this instance is less than 1/10 of a watt, so a 1/4 watt resistor is adequate. Using a 1/4 watt resistor is sufficient in the vast majority of applications when it comes to HO scale decoders and LEDs.
The purpose of a resistor is to dissipate power. As Randy points out, the power to be dissipated in this instance is less than 1/10 of a watt, so a 1/4 watt resistor is adequate. Using a 1/4 watt resistor is sufficient in the vast majority of applications when it comes to HO scale decoders and LEDs.
In this case, the actual purpose of the resistor is to ballast (limit the current flow through) the LED. If the current flow through the LED wasn't limited, the LED would simply burn out. To do its job as a ballast, the resistor neccesarily dissipates power.
The rating of the power suppyl is how much it can, well, supply. Current is not "pushed" into a circuit. You could use a 10 amp power suppply, that LED with a 1k resistor is still only going to use 9ma, .009 amps. The only thing you can't do is use a power supply rated for less than 9ma, as it would be unable to supply the required 9ma.
That's just an example, you wouldn;t want to use a 10 amp power supply for this purpose, if only because in case of a short, that's 120 watts of power - a lot of heat. From a purely "will it work" point of view, you can use any power supply rated greater than the expected load, even massive overkill like this 10 amp example. From a safety and practicality point of view, a 500ma power supply is MUCH more reasonable. Rule of thumb, you don;t want to load the power supply more than 75-80% of its rating. So with 9ma per LED/resistor pair, you would not want to run more than about 40 such LEDs on the 500ma supply. That's "lit at the same time" - if those switches have 2 LEDs in them, but only one lights per a given position of the switch handle, then each switch only counts as 1 LED.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Ron Hume As the power is 12 volts and 500 ma (1/2 amp) I thought I'd have to use a resistor of at least 1/2 amp.
As the power is 12 volts and 500 ma (1/2 amp) I thought I'd have to use a resistor of at least 1/2 amp.
Rich
Alton Junction
The physically biggest wirewound one you can find, so it looks cool. (Friend has an old GIANT knife switch he uses for the main power (low voltage side, he's not THAT dumb) on his layout - totally unecessary but it looks cool.
But we're talking 9ma of current here. Power in Watts is current ^2 x resistance. .009 x .009 x 1000 is .081 watts. Less than 1/10th of a watt, so a 1/4 watt carbon or metal film resistor is PLENTY.
1/4 watt is all you need.
So having established I use 1k ohm resistors for both LEDs, the next question is; 5 watt wire wound, or 1 watt carbon film resistors?
Ron Hume.
1K on 12V is less than 10ma to teh LED< which is well within the current limit for the LED.
You would use different values because green and red LEDs tend to be of different brightness and the two values are likely an attempt to even it out, but I can't imagine much difference between 1K and 1.03K. You can't even get a 1.03K resistor, even in the highest standard set (E192), the values go from 1.02 to 1.04.
Now if they mean 1K and 1.3K, 1.3K is available in 1% tolderance resistors, and there may be some visible difference between 1K and 1.3K. But in no case with 1K be too small a resistor and fry anything.
Thanks Tom, didn't want to burn out LEDs in expensive toggles.
Just use the 1K for both. 0.03K isn't really going to make much of a difference.
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
I have bi-colour LEDs integral in toggle switches. The red LED needs 1.03k ohms and the green LED needs 1k ohm resistance with a 12 volt 500 ma DC power supply. Rather than use two separate resistors, I'd like to use just one for both LEDs. What resistance should I safely use?