Understanding Ohms law.

[vsutherland01]

Hello!

I trust this forum the most and I need to ask the good folks on here about Ohms law. I have been making sabers for a few months now and I may have a pretty massive flaw in my understanding of calculating resistors, this would be really bad.

As far as I know (Vsupply-Vled)/amps=ohms

Now this is simple for parallel LED where each diode gets its own resistor, but in the effort to build with fewer wires I ended up following Tim's style of wiring parallel die from one of his instructional videos. Positive of Die1 to positive of Die 2, and the same for the negatives. So there is only one + and - wire running up to the MCPCB.

Using this method I wire one resistor for all LED in parallel, I know this is not the most recommended way to wire but Tim told me he hasn't had any problems.

So here comes my potential fatal flaw. Using ohms law to calculate I have been adding amps of all LED in parallel. So:

(Vsupply-Vled(because voltage does not add in parallel))/amps of all LEDs in parallel.

Is this wrong and if so please help me understand. I searched through electronics forums and everyone was calculating resistance this way and it made sense to me since in parallel amps adds up, but I just had someone tell me this is wrong. I need to know, I can't keep second guessing my math.

[Forgetful Jedi Knight]

There is a tutorial I wrote on the subject floating around (it is stickied). However I teach putting an individual resistor on each LED.

[vsutherland01]

I read your tutorial and it's very helpful, I am just wanting to know that the same way you added the voltages of both die in series for calculating resistance, you treat amps the same way when calculating resistance for LED in parallel right?

[Silver Serpent]

That is correct. You add up voltages when you wire in series, you add up current (amps) when you wire in parallel.

[vsutherland01]

I just had someone on Facebook try and help but he just scared the snot out of me when he told me I can't add amps when calculating resistance.

I am verifying that some LEDs I have are bad, I am doing everything in my power to make sure it wasn't my fault before contacting LED supply.com.

I wired correctly and won't even get a blink out of the twin reds, I am calling all my methods into question even the way I do resistors. I used two 1.5Ohm resistors in parallel because when you calculate resistance for two reds in parallel you get.

(3.7-2.5)/2amps=.6ohms

If this is wrong then I need to know now. I added the amps for each cree up (1 amp for each=2 total) for the calculation.

[FenixFire]

What type of LEDs?

[vsutherland01]

Twin cree XPE photo reds, the warm white Cree XPE2 works just fine.