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Could use some guidance with resistors

Discussion in 'General Electronics Discussion' started by sergioq, Jul 17, 2014.

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  1. sergioq


    Oct 29, 2013
    Hello All,

    Was hoping you could give me a hand with a resistor problem, as I am new to this.

    I used an LED calculator to find out what "resistors" to get.

    Now I am pretty sure I entered my LED info correctly. And the results it gave me back below***

    So I go to Radio Shack. Yes, Radio Shack, am new at this, so just want to build a prototype edition. but they don't have the EXACT resistors. I assume there's an obvious solution. I just don't know whether or not to go above or below.

    I guess I am asking how to "Round Up or Down when it comes to resistors?

    Many thanks ahead,


    Solution 0: 1 x 2 array uses 2 LEDs exactly

    +6V R = 22 ohms R = 22 ohmsThe wizard says: In solution 0:

    each 22 ohm resistor dissipates 316.8 mW

    the wizard thinks 1W resistors are needed for your application

    together, all resistors dissipate 633.6 mW

    together, the diodes dissipate 816 mW

    total power dissipated by the array is 1449.6 mW

    the array draws current of 240 mA from the source.
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    22 ohms is a standard ("preferred") value for resistors. If Radio Shack doesn't have it, that's because they're crap.

    If you use a higher resistor value, the LED current will be lower than you intended. This is a good idea if you were planning to run the LEDs near their maximum current.

    If you use a lower resistor value, the LED current will be higher than you intended.

    There is a fair amount of tolerance in the LED forward voltage, which affects the current as well, so there's not normally any point being TOO fussy about resistor values. If you're running LEDs at their absolute limit, you should be using a current regulator instead of resistors to set the current.

    See Steve's resource at It explains how to calculate resistance from current and vice versa, so you can check the effect of using a different resistor value.
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