# LED's and Resistors How many?



## Jerry Barnes (Jan 2, 2008)

How many LED's can run off one resistor. Is it fine to use one for the whole locomotive, or do I need to use one for each bulb? I have been using 1/4 watt 1000 ohms.


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## Greg Elmassian (Jan 3, 2008)

Jerry, the easiest thing may be to point you to a LED calculator that not only tells you the resistor ohms needed, but the wattage also. 

wattage is pretty easy though P (watts) = I (current) squared times the resistance. 

You know how much current you are drawing on each LED, add them up, square it, and then multiply by the resistance in ohms... now you have watts... 

Also, you always (rule of thumb) never exceed one half of the wattage rating on a resistor... so you want to stay under 1/8 of a watt on your 1/4 watt resistor (you won't believe how hot it gets over that) 

So say you have one LED connectd to your resistor, and it is drawing 20 milliamps, or 0.02 amps... 

so I squared is 0.02 * 0.02 = 0.0004 times the resistor (times 1,000 ohms) = .4 watts... so you are already over the wattage of the resistor... period... you should be at .125 watts max... 

so put in your current and resistor value... 

the bottom line is you get a higher wattage resistor... 

Here's a nice site: http://ledcalc.com/ 

Greg


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## noelw (Jan 2, 2008)

Jerry.. Here is an quick and easy one to use. 

http://led.linear1.org/led.wiz


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## Greg Elmassian (Jan 3, 2008)

That's a nice one too Noel! 

It picks 60% of the resistor's wattage rating, rather than 50%. 

In my experience, inside a locomotive, stuff bounces around and most of the stuff is plastic, so I'm a bit more conservative on heating up the resistor. 

Greg


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## Jerry Barnes (Jan 2, 2008)

So I need a 1/2 watt resistor on each LED? It's for this dummy loco, just sitting still, getting 12 volts from the Malibu light system.


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## noelw (Jan 2, 2008)

Posted By Greg Elmassian on 29 Mar 2012 02:51 PM 
That's a nice one too Noel! 

It picks 60% of the resistor's wattage rating, rather than 50%. 

In my experience, inside a locomotive, stuff bounces around and most of the stuff is plastic, so I'm a bit more conservative on heating up the resistor. 

Greg 

I agree.... best to mount the resis. where air can get around it and not near plastic. Mainly the 1/8 watt resis... We use mostly 1/4 & 1/2 watt for our stuff and try to use one resis. for one or more LED's to get away from wasting of wattage used. 

One can do a lot of stuff with LED's if done right. 

We are changing over most of our outside building to the flat heads LED's "I call them.," and now can run lot more lighting off of our Malabo lighting sorse with less wattage used. 

On our motive power we use the Blue/white bullet head for more modern lighting due to we run at night and like to see tracks light up about 10 to 12 ft. or more. 
If don't want to shine veay far, one can use the tented/white bullet nose for more real looking lighting on modern power. 

On the older style motive power, we use the flat head tented to look like the old incendescent light use to look like, but dosen't show up very far down the tracks.

We have done a lot expermenting with LED's for night lighting around the layout and even make a night camera car that show lighting on sides of the tracks as well down the tracks..It took 5 LED's to do it right thro. Now can make night videos with no problems and still show the layout light with out interfering with the LED lighing.

We have been playing around yr's ago. with LED's when we working 8 yr's for Signetics Corp.(Sacramento Military testing facilities.) 
They test computer chips and LED's for the space projects and etc. 

.......................................................................... 

To ans. your question on 12v D/C. Malibu Trans.
Is.
Solution 0: 4 x 1 array uses 4 LEDs exactly
+12V





















R = 100 ohms
The wizard says: In solution 0: 
[*]each 100 ohm resistor dissipates 40 mW[*]the wizard thinks 1/4W resistors are fine for your application [*]together, all resistors dissipate 40 mW[*]together, the diodes dissipate 200 mW[*]total power dissipated by the array is 240 mW[*]the array draws current of 20 mA from the source.[/list]
Hope this help Jerry B. on there motive power.


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