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Hi,
I am trying to put 3 LED's ,same type white , along with an SMD into the warp Nacelle of a Bandai Voyager model . The available space inside the nacelle is quite tight . I am using a 360ohm resistor on a 9 volt source . I hooked up all 4 LED's and it seems to work fine but as a beginner in electronics I want to know if I should put a resistor on each LED ? Any advice ,please .

Thanks in advance . l
 

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A resistor on each LED would be ideal, but really the resistor is there to drop the voltage and amperage to the LED. If you want to hook up a single resistor in Series to a set of LEDs, you can do so. Just as long as the resistor can drop the voltage and amperage for the number of LEDs in that circuit. Ohm's Law is V=IxR, Voltage= Amps (called 'I' here) x Resistance. The other variants are I=V/R and R=V/A ( the one you should use for this need). (see https://www.rapidtables.com/electric/ohms-law.html} To get the number of milliamps available on your 360Ω resistor is I=V/R or I=9V/360Ω. Handy calculator shows 0.025A=9V/360Ω. That is 25mA total capacity for that Voltage supply and Resistor.

I should point out that if you had the room, putting the LED/resistor circuits in Parallel, rather than Series would give you the advantage of allowing the LEDs to keep lighting the engine if one of them fails. In Series, like a strand of Christmas tree lights, if one LED fails, all of the LEDs in that circuit would stop lighting up. The size of the Voyager nacelle is limited on the Revell Voyager, less so in the Bandai. I would recommend putting the resistor in the body of the ship and run the wire from it to the LEDs through the pylon, rather than putting the resistor in the nacelle itself. The reason is the resistor will be putting out heat, so a larger space to dissipate heat is useful. But, that does depend on how long you light the model up. The main thing I have read is that heat in a confined space can cause problems for the LEDs. I am not sure how true this is, but it kind of makes sense since overvoltage degrades the LED light source internally. Just unsure as to whether the resistor could get hot enough to degrade the LED through the transparent casing at that voltage and resistance.

LED Basics: Gaining an understanding of how to work with 3 and 5mm LEDs gives you ratings for different LEDs. "Assume 3.4 volts for bright white, bright non-yellowish green, and most blue types." And, "Design for 12 milliamps for the 3.4 volt types....", "You can design for higher current if you are adventurous or you know you will have a good lack of heat buildup. In such a case, design for 25 ma for the types with voltage near 2 volts, 18 ma for the 3.4 volt types, and 15 ma for the 430 NM blue."

So, what this is saying is that for Maximum brightness without degrading the LED, your 360Ω resistor and 9V should only power 2 white LEDs (each 3.4 volts and 12mA). You can put more LEDs on the circuit, but the brightness for each will drop. Your using 4 white LEDs on a 360Ω resistor can be done. You will notice that the brightness is less compared to using 2 LEDs.

For example, let's figure how many Volts you need for your 4 LEDs: V=0.012(4)mA x 360Ω = 0.048mAx360Ω. Answer V=17.28V! If you drop it to 3 LEDs, V=0.036A x 360Ω = 12.96V. So IF you were to use a 12V DC source, you would have all 3 LEDs just a hair under max brightness without causing them to degrade. 2 LEDs would be overdriven for 9V at 12mA each, for the voltage is 8.64V at 360Ω and you would have to increase the resistance to compensate. Ideally, using 9V and 0.024A (2x 0.012A LEDs), R=V/A, R=9V/0.024A, R=375Ω.

If you know that your LEDs have a different voltage and milliamp rating, then plug those figures into the equation to see the proper resistor rating.
 

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I never saw anyone use "I" for amps, either. I corrected it for ease of understanding. The use of "E", short for electromagnetic force, in place of Voltage has been shown also as "V" for voltage. Since many people are familiar with V*A=W (volts times amps = watts), I figured for the use of LED circuitry, standardizing the V and A would be advantageous here.
 

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E, I, and R are the engineering units taught. You can search images that show the EIR triangle and the PIE triangle. It may help the novice.
 

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Yes, I understand. This may be the engineering approach, but I think VxA=W and variants are more known to the general public. I know I found it confusing when I discovered the I, E, and R used for Ohm's Law. If one has no interest in electronics beyond putting leds in a model kit, I think the translation to the same terms that a modeler likely knows is justified. A greater education in electronics can correct that, but we are talking basic concepts at the moment. Otherwise, we should be using a consistent terminology. Like ExI=W, which we don't.
 

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You might want to 'know' the formulas E=IXR and P=IXE if you desire to go past simple LED lighting and adventure further into control electronics.

As far as LED lighting the precise calculations are unnecessary. In 'model' lighting it is more about the scale of intensity. With typical LED's using a 470 ohm resistor is sufficient. You can use a value that will result in little to no heat and then be able use smaller 1/8w resistor. In some models resistor size can be an issue. In forty years I've never calculated any LED circuits. The purpose of resistors is to limit current flow or create certain voltage drops and is more important in transistor circuits and integrated circuits, for example. I also use LED driver IC's instead of direct, and for strobes I use the LM3909.
 
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