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Thread: Tutorial - Calculating Resistor Values

  1. #1
    Owner of the Custom Saber shop Strydur's Avatar
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    Default Tutorial - Calculating Resistor Values

    The Custom Saber Shop Tutorial – Calculating Resistor Values




    One of the key things that all good saber builders understand is how to calculate the resistors they need to use in a build. Below I have listed important terminology, calculations with examples and links to free online calculators that will do the work for you if you can’t get your head around this process. After the calculations section we explore LED drivers as better alternative to using resistors for your main LED.

    Important Terms
    VS = Voltage Supply: Nominal power from your battery solution
    VL= LED Voltage: Voltage rating for your LED (Commonly referred to as Vf)
    I = LED Current: The amount of power you will be driving to your LED (A)
    R = Resistor: The value of the resistor needed in ohms (Ω). Resistors are wired to the anode of your LED prior to your positive power lead.
    = Ohm: Unit of resistance
    A = Amps: 1,000 milliamps
    mA = Milliamps: 1/1,000 of an amp
    P = Power, in watts
    IF = Maximum forward current. The max amount of power you can safely drive to your LED.
    VF typ = Typical forward voltage. VL in our calculation.
    VF max = Maximum forward voltage.
    Positive power lead = Generally, a red wire is used from your power supply’s positive output
    Negative power lead = Generally, a black wire is used from your power supply’s negative output
    Series = A, generally, preferred method of wiring multiple LEDs at the same time by wiring them end to end. If we assume that the first LED’s anode is wired first to a resistor and then to your positive power lead, then the LED’s cathode would be wired to the next LED’s anode. This would continue down the line with the final LED’s cathode wired to your negative power lead. The advantage of series wiring is that the voltage is distributed across each LED which allows you to use only one resistor and only minimal wiring.
    Parallel = A method of wiring multiple LEDs where all of the anodes are wired together with each LED getting its own resistor* then wired to the positive power lead and all of the cathodes wired together to the negative power lead. The good thing about wiring in parallel is that the full power from your power supply is delivered to each LED. The bad thing is that this presents a serious power-drain. It also leaves more room for error because of the additional amount of wiring required. [*NOTE* - It is possible to wire in parallel with only ONE resistor for all of the LEDs, however, this is generally not a best practice and will not be covered in this tutorial]
    Anode = When referring to LEDs, this is the positive (+) pad, or, generally, the longer lead on small LEDs. The anode is also on the round side of a small LED. Can also be determined via multimeter.
    Cathode = When referring to LEDs, this is the negative (-) pad, or, generally, the shorter lead on small LEDs. The cathode is also on the flat side of a small LED. Can also be determined via multimeter
    Multimeter = A very important tool for any saber builder! Good multimeters are capable of reading voltage, current and resistance. If you do not have a multimeter or do not know how to use one, pick one up and learn it prior to moving on in this tutorial.

    Online Resistor Calculators
    Single LED resistor calculator: www.led.linear1.org/1led.wiz
    Series/parallel LED resistor calculator (with wiring output): www.led.linear1.org/led.wiz
    Another calculator: www.ledz.com/?p=zz.led.resistor.calculator

    Manual Calculations

    Single LED: R=( VS- VL)/I
    P=Vs*I
    Ex.
    AccLED3 @TCSS – 2.2V 25mA
    Power Supply – 9V
    R =(9-2.2)/0.025
    R=272: Next highest resistor value =330Ω
    P=9*.025
    P=0.225: Standard value current-limiting resistor wattage = W
    Resistor to use = W 330Ω






    LEDs in Series: R=( VS- (VL1+ VL2+ VL3…))/I
    P=Vs*I
    Ex.
    3 AccLED3 @TCSS – 2.2V 25mA, wired in series
    Power Supply – 9V
    R =(9-(2.2+2.2+2.2))/0.025
    R=96: Next highest resistor value =100Ω
    P=9*.025
    P=0.225: Standard value current-limiting resistor wattage = Watt
    Resistor to use = W 100Ω








    LEDs in Parallel: R=( VS- VL)/I
    P=Vs*I
    Ex.
    3 AccLED3 @TCSS – 2.2V 25mA, wired in parallel
    Power Supply – 9V
    R =(9-2.2)/0.025
    R=272: Next highest resistor value =330Ω
    P=9*.025
    P=0.225: Standard value current-limiting resistor wattage = Watt
    Resistor to use = W 330Ω FOR EACH LED!!



    **NOTE** Don’t forget that you need THREE RESISTORS in this example, one for EACH LED!!!



    Power Extender Resistor Calculation for FoC™: R=( VS- VL)/I
    P=Vs*I
    Ex.
    LZ4-20MA00 RGBA 10W LEDengin @TCSS – 2.5V 700mA (amber die used for FoC™)
    Power Supply – 7.4V (recommended solution for Petit Crouton and Crystal Focus boards)
    R=(7.4-2.5)/0.7
    R=7: Next highest resistor value = 8.2Ω
    P=7.4*0.7
    P=5.18: Standard value current-limiting resistor wattage = 4 Watt (this is ok with a PeX for FoC™)
    Resistor to use = 4 W 8.2Ω




    **NOTE**
    The part code listed @TCSS is LZ4-20MA00 but the data sheet we want is for LZ4-00MA00. The “00” means that the specs are specific to the LED. The “20” in the code @TCSS tells us that this LED is mounted to a standard MCPCB (LED star) where each die can be wired individually.










    Where to Buy





    Drivers
    Now that we’ve covered resistor calculations, it is time to talk about using a driver in place of a resistor for the main LED in your custom saber. We’ll skip over the technical jargon and simply state that drivers are a much more efficient option for driving your LED. Your options for drivers @TCSS are…
    700mA BuckPuck –
    http://www.thecustomsabershop.com/Bu...6-wire-P9.aspx
    http://www.thecustomsabershop.com/Bu...wire-P363.aspx

    1000mA BuckPuck –
    http://www.thecustomsabershop.com/Bu...-wire-P10.aspx
    http://www.thecustomsabershop.com/Bu...wire-P364.aspx

    1000mA/1,200mA/1,500mA Constant Current Driver –
    http://www.thecustomsabershop.com/Co...iver-P609.aspx

    700mA/800mA/1,000mA/1,200mA/1,400mA Adjustable LED Driver –
    http://www.thecustomsabershop.com/Ad...iver-P497.aspx

    Petit Crouton™ -
    http://www.thecustomsabershop.com/Pe...-v16-P612.aspx

    With every choice except for the Petit Crouton, you need to know, before you purchase the driver, how much power (mA, milliamps, 1/1000 of an amp) that you are going to drive to your LED. That means you need to be able to read the specs on TCSS or the LED’s data sheet. The Petit Crouton is excluded because the user configures how much power is driven when setting up the SD card that comes with the board. The Petit Crouton can drive anything up to 1,500mA (1.5A). Even with a Petit Crouton, resistors must be used on accent LEDs and when setting up FoC™ with a PeX.




    In this example, the Luxeon Rebel LEDs are rate at 700mA. It is possible to overdrive many LEDs but not all. For instance, red and amber LEDs should not be driven any higher than 700mA while green, cyan, blue and royal blue can be driven at 1,000mA (1A). In the case where a data sheet is not provided, you should research the LED’s specs independently.





    Summary

    After reading this tutorial you should have an understanding of the terminology related to accent and high-power LEDs, be able to calculate the resistor needed for your application and know how to use online resources to calculate your resistor. Furthermore, it is highly recommended that you use an LED driver for your main, high-power LED. Additional resources are available on TCSS Forums: http://forums.thecustomsabershop.com/
    Last edited by Forgetful Jedi Knight; 05-29-2014 at 05:29 PM.
    Tim
    The Custom Saber Shop

  2. #2

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    I am working on a project to light 4 LEDs each with a different value on a single power source, I'm wanting the sequence to go (Red LED switch-6V=220ohm 1/4 watt resistor, Blue LED-6V=150ohm 1/4 watt resistor, Red LED 2V=20mA, Green LED 2.2V=25mA) the trouble I'm having is trying to find the right voltage battery or batteries for this and what resistors to use for the right voltage battery.
    I have a circuit board coming in the mail.

  3. #3

    Default

    I am currently figuring out the power and wiring etc and I know that I have a 3.7v battery and 2v taken away for the speaker but the led requires 0.7 or 700 mA if you prefer what sort of resistor would be needed. Or would a 3 w resistor be needed to keep the current away.

    Nevermind but I will post the answer for others baisicly. The speaker has nothing to do with the voltage. Look up the LEDs voltage as well as mA and enter on the led calculator the answer would be entered on the calculator which would be 2w
    Last edited by bionicmad; 05-28-2014 at 10:35 PM.

  4. #4

    Default

    Hello.
    I'm a kid, I don't know any better.

  5. #5

    Default Yes, another resister question. Hey, I researched first!!

    Hello, and thanks in advance! Ok, I watched Tim's videos several times on Youtube, watched this one, read, read, and read. Been up all night and my brain hurts... wanted to check in before I hit the "buy" button on Ebay.

    I bought the PAN18650P 3.7 volt battery from TCSS. Bought the main LED from Saber Forge and the website says it will come with the resisters. I want to buy the 12 mm purple lighted switch from TCSS [12MRBP]. The specs say its 3v and 20 MA. I entered these three figures in the led linear 1 site. 3.7 source, 3 forward, 20 MA. It gave me 1/8W or greater 39 ohm resistor. Can't find this on TCSS. Ebay has a pack of "39 Ohm 1/2w Watt 5% Tolerance Metal Film Resistor (20 Pieces)" 1/2w is *greater* than 1/8, so would this work? ** I assume all risk for whatever I install** just need a check-in.

    After I pull this splitter from my brain it's on to study series vs parallel when wiring! So much more confusing than I expected. So much variance!

    Thanks again!
    -LR

  6. #6

    Default

    Quote Originally Posted by Lord Rudolph View Post
    1/2w is *greater* than 1/8, so would this work?
    The watt value of a resistor represents the maximum amount of power it is able to dissipate, so higher is fine. It generally means the resistor is physically larger.

    The DynaOhm is a suitable resistor for your application, and should be about the same price as your lot of 20 resistors.

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