Powering LEDs, dim LED in parallel

Question

I want to build an light fixture for my aquarium and decided to use LEDs, almost everywhere it's suggested to wire the LEDs in series. but that will need a high voltage for many high power LEDs. also if one of the LEDs in series for whatever reason burnt, all LED's will turn off.

The main reason to wire LEDs in series (if I understand correctly) is because of the manufacturing variation in LEDs, that may cause one LED pull more current than the others if wired in parallel.

Let's assume that we've 5 LEDs, each needs 1 V / 10 mA and it's connected to a power source which provides 1 V and 60 mA constant current:

^{simulate this circuit – Schematic created using CircuitLab}

In above diagram each LED can't pull more than 10 mA and resistors will not get hot either because the power source have a current limit.

But what if the little variation in LEDs cause one LED to pull less current than the others? what's the solution for that?

Answer 1

The solution for this is to use a higher supply voltage.

If there is a large voltage drop across the resistor, then small variations in the forward voltage of the LED will cause relatively small variation in the voltage across the resistor, and therefore small variations in LED current.

With a supply voltage close to the LED forward voltage there must be a very small voltage drop across the resistor, so that small changes in LED forward voltage cause large changes in resistor voltage, and large changes in current. So, LEDs that have a bit higher forward voltage get significantly less current and appear dim.

Answer 2

But what if the little variation in LEDs cause one LED to pull less current than the others? what's the solution for that?

For the combination you have provided, the current will be less than 10mA per LED resistor combination because of voltage drop of the LED.

If one of the LED pulls lesser current than the others, it will be simply less brighter due to a higher forward voltlage drop compared to the rest of the LEDs. This will not have any influence on the other LEDs as such because all of them are in parallel.

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1. Even if the LEDs are in series, the little variation in the LEDs will be there as a variation in the luminous intensity but they will be barely palpable.
2. Choosing LEDs from a reputed supplier from a same batch and bin will yield the best results.
3. You are wasting about 40% of the power considering forward voltage drop of 3V. With a 5 V supply. It is my assumption but in any case, it is a huge waste in power and there are better ways to drive the LED.
4. Opt for a constant current LED driver if possible

Answer 3

LEDs cannot be made the same even if they have the same part number. The datasheet shows the range of forward voltage. You can buy many LEDs, test them all and group them into piles that have the same forward voltage then throw away the ones that cannot be in any group. You can buy some LEDs that are already grouped into what they say are binned.

Answer 4

I want to build an light fixture for my aquarium and decided to use LEDs, almost everywhere it's suggested to wire the LEDs in series.

The reason you see this suggestion is that you should run them in series. Parallel like you are doing is more difficult, less efficient, produces worse results and is more dangerous. There is no reason to do it.

also if one of the LEDs in series for whatever reason burnt, all LED's will turn off.

Being fail safe is a very good thing. If you circuit is burning, you want the power disconnected. Dumping power into an overheating circuit is a fire hazard.