LED Circuit Problems

[MR P]

Currently I am working on a school project that has a circuit with a 6v 7.2Ah battery and 30 Blue LEDs in series, these LEDs require a max current of 20mA and a voltage =3.3v or less than or equal to 3.8v. The Problem I am having is that iam not sure what resistor value I need that will allow my LEDs to shine at their brightest but not blow, also if I don't put a resistor in the circuit the LEDs shine bright but the wires melt due to the current I think, I am not sure what type of wires I need for this type of battery.

 

[BobK]

If you have 30 LEDs with a forward voltage of 3.3V each wired in series, you need 99V to run them at their rated specs. You can hook them up to your 6V battery and they will not even light at all.

And 20mA is not going to melt any wires.

You need to put them in parallel with each LED having it's own resistor in series with it.

Now you get 600mA at 6V. Still not enough current to melt any but the finest wires.

Do you understand series and parallel circuits? In series, voltages add, in parallel currents add.

Bob

 

[Harald Kapp]

Using 3.3 V ... 3.8 V LEds on a 6 V battery is going to be horribly ineffcient. You wast almost 50 % of the available power in the current limiting resistors.

To improve efficiency, you have at least 3 options:

1. Check the brightness of 2 LEDs in series with a small resistor (e.g. 10 Ω) on the 6 V battery.This may just work - or not, depending on the LEDs and your expectations.
2. Use a switch mode regulator to step-down the 6 V from the battery to 4 V with high efficiency. Then make the parallel connection as suggested by Bob, using a series resistor R = (4 V - 3.5 V)/20 mA for each LEd (3.5 V is the average LED voltage drop according to the data you have given).
3. Or, to reduce the number of resistors required: use a step-up regulator (instead of a step-down) to create 22 V from the 6 V battery. Then pack 6 LEDs in series with one resistor R = (22 V - 21 V)/20 mA (21 V is the average voltage drop of 6 LEDs in series). Use 5 of these 6 LED strings in parallel to complete the full 30 LED circuit.
   This is imho a very efficient simple solution (more efficient, but a bit more complex solutions exist involving constant current switch mode LED drivers which is, I think, beyond your scope).

Note that the resistor in cases 2 and 3 is 50 Ω which may not so easy to get. Use a standard 47 Ω resistor, 250 mW power rating instead. My choice of a 1 V voltage drop across the resistor is arbitrary.
You can reduce the voltage drop and accordingly use a smaller resistor to reduce waste power even more. This comes at the cost of reduced control ober the LED current as a small change in LED voltage drop will cause a comparatively higher change in LED current.
You can increase the voltage drop and accordingly use a larger resistor to improve current stabilization. A small change in LED voltage drop will then cause a comparatively smaller change in LED current. This comes at the cost of more waste power.

It's your's to make the choice.

Cheeers,
Harald

 

[MR P]

If you have 30 LEDs with a forward voltage of 3.3V each wired in series, you need 99V to run them at their rated specs. You can hook them up to your 6V battery and they will not even light at all.

And 20mA is not going to melt any wires.

You need to put them in parallel with each LED having it's own resistor in series with it.

Now you get 600mA at 6V. Still not enough current to melt any but the finest wires.

Do you understand series and parallel circuits? In series, voltages add, in parallel currents add.

Bob

I am sorry the LEDs are in parallel

 

[Harald Kapp]

Operating LEDs in parallel without each one having its own current limiting resistor is a bad idea. Have a look at our ressource on driving LEDs.

 

[Audioguru]

I have a cheap Chinese flashlight that has 24 white LEDs connected directly in parallel and they are all with the same brightness. Because the manufacturer bought millions of LEDs, tested the voltage on each one and grouped them so the LEDs in a flashlight have exactly the same voltage. The datasheet of any LED shows a range of voltages it could have. If you connect a bunch of un-grouped LEDs in parallel then the one with the lowest voltage will hog all the current, be extremely bright and quickly burn out then the next voltage one will burn out, etc.