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Can you split voltage?

In the above will the bottom LEDs experience 3v whilst the top tastes 12v?

Time for some elec!

Yes, in short.

Long answer begin:
This is a prime example of Kirchoff's voltage law. In simple terms "The total sum of voltages in a closed loop must equal zero"

Take your circuit - imagine the bottom LEDs are removed. Easy to see what's going on, right?
3v+3v+3v+3v = 12v going through your LED.
Now, slightly more complicated bit.
Remove your top LED and put your lower LEDs back in.
3v+3v+3v+3v = 12v going through all your LEDs total.
But, with the extra "loops" you've made each one is only seeing a change of voltage of 3v.
12-9, 9-6, 6-3, 3-0 all equal 3.
The problem most people have here is that it is more correct to think of voltage as a difference from a point. (A potential difference) By way of an example, if you drop a ball from 1m or 3m which height would make it hit the earth faster? The 3m one, UNLESS you caught it 2m from the earth, when it would hit at the same speed.

Going back to your circuit, taking them a row at a time and assuming the LEDs are all the same and capable of taking 3-12v which row would be brighter? An LED from the bottom, or the one from the top?

Thanks Twibz. In that case the one at the top would be the brightest (probably for a very short while Smile ) as it would get the 12v of the batteries in series with it. The 4 below would be around 1/4 brightness (all dependant on relative resistance on each circuit), I think.

The real life application I was thinking of was 4 coin cells spaced around a blaster running equal numbers of 3v LEDs in parallel (so I don't have to bother resistoring them) but give the full 12v grunt to strips of LED tape that (i think) is typically resistored to 12v.

If you want that many LEDs in a springer then using 12v strip is absolutely the WRONG way to do it. Strip is really useful in 3s flywheelers, but in this instance I would go with a single IMR and individual leds.

You can get 5V LED strips which will work just fine on 3.7-4.2V from an single lithium cell and you can just use your own resistors to bring down the voltage for the individual LEDs.

It's worth noting that you would likely still need a resistor in series with each LED when powering them from a single 3V cell, it'll just be a lot lower value than the resistor you'd need to power it from a 12V battery (forward voltage for an individual LED can be anywhere between 1.8-3.3V depending on a few things including it's colour). The forward voltage is practically constant across a wide range of current so it doesn't take much additional voltage across the LED to result in a lot of current flowing through it.

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