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Thread: Inconsistant resistor calculators

  1. #1

    Default Inconsistant resistor calculators

    Hey guys,

    Returning newbie with a question about some differing results I get with resistor calculators. The values I'm using are Vs (3.7V), Vf (3.4V), and a current of 1A (1000mA). When plugging these into various calculators I get these results: (.5ohm 3.2W) (1ohm 2W)
    saber shop calculator (.3ohm .3W) which the .3ohm 1W is recommended as the closest they sell in the shop

    So my question is are these merely coming up with different values based on commonly available sized resistors or am I missing something here? The large discrepancy with the wattage has me confused. I just ordered some new resistors from the shop for my tri cree (BBW) and a 5mm blue accent led. I went with the .3 ohm 1 W for the blues, a 1ohm 2W for the white, and an 18 ohm .25W for the accent. The math really should be pretty straight forward but now I'm wondering if I did something wrong in my selections. I just want to make sure I've got the right parts prior to going through with the build.

  2. #2


    While going “over” the specs that you need doesn’t hurt, the best (and easiest) way to figure out what you need is to do the math yourself. Read this:
    All n00bs READ these first (PLEASE)!!!:
    1. Forum Guidelines
    2. FJK’s “Down and Dirty” guide to Ohm’s Law

    "Yeah, yeah, I've heard it all before... you want blindingly bright, super loud, running 1138 blinkies off of the cheapest sound card you can find AND you want all of it to run on a battery the size of a dime, and run for a very, VERY long time. That one cracks me up every time..."
    My email:

  3. #3


    The difference in those online calculators lies in how they round their results. I know for a fact that the 2nd one on your list will never give a result of less than 1 ohm.

    Once you've figured out how the math works (see FJK's post above), you can do your own calculations pretty easily. I only use the online calculators to double-check my math and make sure I haven't made any major errors.
    We all have to start somewhere. The journey is all the more impressive by our humble beginnings. for the lazy man's resistor calculator! for getting resistor values the right way!

  4. #4


    Thanks for the responses. I have pretty much no education or experience with electrical engineering so I have spent a lot of time trying to at least understand the math behind these very simple calculations. I had been generating the resistance values using the forumla and felt confident with my results, it was the wattage rating that had me unsure. Using the manual from the nano biscotti it gives the formula for resistor wattage as P = (Vsupply - Vled) * led current. I don't remember seeing the P = I^2 * R formula. Using a formula I somehow remembered from waaaay back in high school (P=I*V) and (V=I*R) I was able to algebraically come up with the wattage formula you provided in the link. Now I feel a little better about calculating out what I need. Thanks again!


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