Would these two schematics be equivilant iff: R4 = 1/(1/R1+1/R2+1/R3) ? Somewhere I read that this isn't equaivilant. But the sum of resistors in parallel would equal one resistor in series. And since the voltage is the same across the paths in parallel, it wouldn't affect that. The amps are divided accordingly between the seperate parallel paths and the one resistor should limit the current just like the resistors in series with each LED should limit the current. But then again, I might be wrong.
Kirchoff's 1st Law states that the current flowing into a junction in a circuit (or node) must equal the current flowing out of the junction. However the circuits are not equivelent since all the current is flowing through the resistor in the second circuit and not just per LED as in the first circuit. So you may require a higher wattage resistor than just the 0.125 watt ones you may have lying around. (Since Power=Volts Across Component x Current flowing though it) Hippo
In theory you can use the single resistor on a group of parallelled similar leds, but in practice any slight variation between the leds forward voltage will unbalance their currents, so its not a good thing to do. You'll get away with it if the leds are the same type and you're not close to the current limit for the leds.
so answer is technically yes, but circuit b produces problems in the real world that circuit a makes up for.
The other problem with the scheme is that if one of the LED's goes they all will; the more LED's you wire in parallel the lower the value of R4, if one pop's the number of LED's decreases, R4 is too small and the current supplied to the others increases... Rob.
But tbh the only time ive even seen a LED fail is when its been overdriven. Ive actually seen one explode. And yes it was the desired result ! . I was testing the maximum tolerances one day lol and crack!! LED everywhere.
But that's the point, wiring the LED's this way will likely over power one of them, which will probably fail quickly and then... Rob.
*Edit* Sorry, I just woke up when I wrote that, I'm not even sure what the heck I was doing, anyway, onto what I should have said. Say I am using 3 1.7v 20mA LED's on a 5v line. The resistor for each LED would be 165 ohms. The nearest resistor would be 180 ohms. The 15 ohm difference will offer us some leeway to ensure the LED doesn't get too much current. So R1,2,3 would be 180 ohms making R4 60 ohms. This 60 ohm resistor would limit the current to 55mA. Divided by three, each LED would get ~18mA. 2mA to spare. So I can calculate the power for the LED, which would be .1815W (181.5mW), so would a .25W resistor work? Me thinks so, but then again, what do I know?
