Quick LED question...ok, not that quick

[Grumpy Popeye]

Ok, I have a two string of LEDs, two LEDs in series per string. My source is 9 VDC.

Being lazy, I punched the numbers into an online resistance calculator, which gave me 150 Ohms. No problem.

Out of curiosity, I connected my ammeter to the circuit, and I am getting 16-18 milliamps.

Then I got to thinking,"If I parallel my two strings, I'll save on the number of resistors I need and I can get away with using a lower resistor. Since a single string uses 150 Ohms, halving that would naturally be 75 Ohms, with the closest to that being 82 Ohms.

wire everything up, put the battery on and the ammeter reads 23-26 milliamps. seemed a touch high, so for kicks I stick the 150 Ohm resistor back in the circuit. Now it's 22-24 milliamps. not much of a difference. There is no excess heat build up on any of the components

So, Should I go ahead and run the lights n series- Parallel, or should I go back to the individual series strings?

[tetsujin]

I'm not entirely sure what the various layouts are that you're describing. It's a bit hard to convey a circuit layout without pictures.

Basically, what's best when you're wiring up multiple LEDs and regulating the current with resistors is to have a resistor for each individual series strand. There's a trade-off between how much power you waste, and how many resistors you use, and how sensitive the circuit becomes to minor changes in supply voltage and temperature.

So let's say you start with a circuit like this:

(A)----/\/\/\----|>|----|>|----(B)
(A)----/\/\/\----|>|----|>|----(B)

That's two strings, each one with a resistor and two LEDs. The ends of the two strings are tied together at points A and B, so you have two identical series circuits in parallel.

It's important to understand how voltage and current work here: the voltage difference between nodes A and B is the same for both branches. The voltage across the ends of the branch, along with the characteristics of the resistor and the two LEDs, determines how much current flows through the branch.

When you rearrange the circuit like this, to save a resistor:

(A)----/\/\/\----(C)
(C)----|>|----|>|----(B)
(C)----|>|----|>|----(B)

you save a resistor but the power usage of the circuit stays the same. But the current through each of those branches between (C) and (B) is now determined just by LEDs. You still have the resistor there stabilizing the whole set-up with respect to the power source, but differences in the LEDs can potentially make one branch starve the other one. (If one of your LEDs takes a little bit less voltage, it lowers the voltage between (C) and (B), which in turn cuts down the current running through the other branch.) Putting a resistor on each branch makes the branches less sensitive to the influence of other branches.

[Grumpy Popeye]

Probably safer in the long run to boot!

Looks like I may have to add another battery in parallel, a full light test gobbled up a single nine volt in no time flat!