# Thread: Help with first build - Crystal Focus 8

1. well anyways, i went and used the formula the manual gives for calculating resistors - and i also used an online led resistor calculator - and got different answers. the manual says; R = (Vpack – Vf die) / FoC current ---my tri cree rblue diode runs 3.41v @ 1000ma/1A ----- my battery is a 7.4v 1400mAH ------ so that should be R=(7.4-3.41)/1 ---so----- R=3.99 - yet the online calculator says 4.7ohms 4.7watt resistor. Which is correct? and then the lowest i can find on TCSS is ??ohm at 5 watts.

2. The math gives you the actual values, the calculators round up to the nearest readily available resistor values, if attached to a store the calculator will round up to the nearest available resistor. Some will work in up or a 60% safety rating.

3. Your math is correct; however, finding a resistor to your exact calculation Is not always possible. Therefore, you round up to the nearest common resistor value. This is what the calculators do automatically. Although, sometimes the calculators tend to round up a bit high.

4. so then would it be safe to use the 3.9ohm 5w resistor in TCSS? Also, what is with the wattage of resistors? is the 5w just like a max rating? or will using a resistor with higher wattage like say a 10w resistor, diminish the led's brightness?

5. Ohm refers to the resistance Wattage is the rating of heat it can dissipate. Higher wattage has no effect on the LEDs brightness but does play a significant roll in the size of the resistor. The math gives you the minimum numbers you will need. Round up not down.

6. alright one other thing. in the color mixing without the Cex part, as well as the converting for single cell use - it states that converting it for single cell use allows for direct drive (pseudo color mixing) - Does this mean i will only be able to do RGrB color mixing after i convert the board to single cell use? or can i still have full color mixing, without the Cex, and using a double cell like the one i already have, i.e. 7.4v 1400mAH 18500

7. It is possible to do pseudo colour mixing using a 7.4V power supply as well. The only prominent disadvantage is that you will probably have to use external resistors because SMD resistors won't have a high enough wattage rating.

8. yeah, thats what the last few comments were about - is figuring what resistors to use. See it doesnt say that in the manual. It just says about doing it with a single cell, and doenst specify if it is possible to still do it with a dual cell and no Cex.

9. okay so i just test wired my led, and it wasnt as bright as i had expected. there was a huge dead spot in the blade. So, let me ask this: to test it, i wired the green and rblue of my RGrB, up to the battery. I connected both led (-) to the battery (-), and both (+) to the battery (+) using a 4.7ohm resistor. So does the fact that both led's were wired up to the same 7.4v source with a resistor that is higher than necessary for the green led - mean that they were both being under driven? like was the voltage split between them, or was it just both equally 7.4v - 4.7ohms?

10. Theoretically both were equally resistored equally. Though I would heavily caution using a single resistor for multiple Dice unless they are the same color, manufacture, and "POT"...basically identical dice from the exact same production run. Otherwise you risk a fatal (to the LEDs) runaway condition.