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

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?

• Please draw a schematic explaining what you're doing. Commented Jan 16, 2020 at 12:06

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.

• "you shouldn't run LEDs in parallel off a single series resistor" does this hold true if they are the same model of LED? or does small variations of components in the real world mean that they are still effectively "different LEDs" Commented Jan 16, 2020 at 13:00
• If you put binned LEDs that are measured to be within some tolerance, this technique may be used, and is used on some systems like LCD backlights and some LED "fairy lights" etc. But in general, this is not a way to light up random LEDs if they are not binned or if they are not from the same batch of LEDs, or different color. Commented Jan 16, 2020 at 14:14
• @msaspence Parallel strings of series of LEDs are sometimes used. If you have ten LEDs in series the variation between them averages out. Putting individual diodes in parallel is more risky because the odds of getting one with a high/low voltage drop are greater than the odds of getting ten extreme values in a row. Commented Jan 16, 2020 at 14:31
• It might be worth noting that the human perception of brightness from an LED is non-linear vs. the forward current in the LED; this also contributes to the minimal viewed brightness difference with a decrease in forward current. Commented Jan 16, 2020 at 16:26

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.