LED Resistor Calculator vs. My Primitive Math

I found a great online calculator for LED resistors:
http://www.luxeonstar.com/resistor-calculator.php

But I'm puzzled: when I do the math with the same inputs, I get slightly different resistor values.

My calculation is simply:
Vsupply - Vforward (total) / current as spec'd by mfg. = resistor value.

Obviously I'm missing some part of the calculation. Might as well get the math right before I make crispy critters. Any words of wisdom?

(Or if there's an existing thread, just point me to it. Thx.)

Doug

Nothing wrong with your formula. The resistor value the calculator gives you is the nearest larger standard size resistor.

At the bottom of the calculator it also shows the exact caculated resistance needed. However. this doesn't necessarily come out to a standard size which is why they "round up" to the next larger size resistor. Make sure you get either the recommended wattage resistor or one with a higher wattage value.

Weldon

I'm totally new to any kind of modding, but am really lost on resistance calculation. I dropped a Seoul 't' bin star into my old Golston 7w. Measured the resistor (1.2 ohms). The calculator, assuming vforward of the Seoul star to be 3.7v, vsupply at 6v, and current at 1000 ma, should have used a 2.3 ohm (2.7 ohm nearest common value) resistor.

Yet, when I check current by unsoldering the resistor lead from the + of the Seoul star and measuring between the resistor lead and the + pad of the star, I get 1.01 a. The star hasn't fried yet, but is bright as all-getout.

Why these results?

John F

A resistor calculator is a good place to start BUT I use a multimeter to see what the LED is really drawing and find the best resistor from there.

Good luck with your project.

Thanks for the link!

jtice coded the "LEDPro" software a while back - it's really easy to use and does a bunch of calculations for you. Do a quick search or PM him for a copy.

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