Why does LED dim when adding second LED in parallel with a single resistor

Question

If I have a led hooked up to a 12v DC supply with a single resistor it runs nicely as you would expect. If I add a second LED in parallel the brightness drops, slightly, the brightness on both is (to my eye at least equal). My antiquated primary school knowledge tells me in parallel they should draw more current and stay the same brightness.

What's more if I add any number of additional LEDs the brightness doesn't drop again, its only after the second one.

I've found some questions and answers that indicate that running multiple LEDs with a single resistor isn't a good idea because the current will be different on each LED and that makes sense. However I'm keen to understand what is actually happening to cause the LEDs to dim.

What is wrong with my understanding of bulbs in series vs in parallel and why does a second bulb cause the brightness to drop and additional bulbs not?

Answer 1

The voltage across an LED is about 2V and stays at just about 2V.

With a fixed power supply voltage it means that you have a fixed voltage across (and therefore a fixed current through) the series dropper resistor.

The resistor is acting as a constant current source and its current is shared between however many LEDs you have in parallel.

Lets say you have a 12V supply and a 1k Ohm series resistor. You then have 10mA through the resistor. With one LED there is also 10mA through this LED. Now if you add another LED there is 5mA through each LED and the brightness will drop noticibly. The current through each of the two LEDs is 50% what it was through the single LED.

Now adding a third LED the current through each LED drops to 3.333mA but the % drop in current is less than the drop when you added the second LED. The more LEDs you add the less the % drop in current.

So when adding more LEDs than two there will still be a brightness drop but it becomes less and less perceptible the more LEDS that you add.

Because the voltage which an LED 'wants' to have across itself varies slightly from LED to LED it means that you shouldn't run LEDs in parallel off a single series resistor. If you do it forces all the LEDs to have the same voltage across them which can cause differing currents between the LEDs. Some will have low currents and some will have high currents. Perhaps too high for there specification limit.

Answer 2

With the classic resistor with LED in series circuit, the current is mainly determined by the resistor. So in parallel they should draw more current and stay the same brightness. doesn't happen, the total current stays more or less the same. So when going from one LED to two LEDs, the current halves.

That current is divided between the two (or more) LEDs. When you add more LEDs they do run at lower currents, it might be that this simply isn't very visible.