making a small ciurcuit with 20 led´s

[Audioguru]

Since your LEDs have different brightnesses then their currents are probably different. The brightest ones might burn out if you use a 27 ohm resistor.

Since your LEDs are soldered in parallel then you cannot measure their individual currents.

Cheap LEDs are made to look bright by making their case focus them in a narrow angle.

 

[theelectronicsfan]

those im using (gonna buy more so all are same type) costs about 0.5 USD each
but how about i use a 18Ω resistor?

Added after 1 minutes:

Led im using:
Specifications
case: 5mm
emitting colour + material: GaAsP/GaP
wave length: 625nm
luminous intensity (If = 10mA): 8-30mcd
viewing angle: 60°

Added after 20 minutes:

Ok how about this:

What do you guys think=P

Added after 1 minutes:

you have to open it in full zize and zoom it or it will look like crap

 

[betwixt]

This thread is getting increasingly silly. 'theelectronicsfan' clearly wants to ignore the fundamentals rules of electronics in the hope it will save some time to unsolder all the LEDs.

I'll give it one more try:

If you join them in series, that means in a chain with the + end of one diode connected to the - of the next in the chain, they will all pass the same current and be roughly the same brightness. The voltage you need to operate them is a little higher that the total of all the LED voltages added together. You need one resistor to limit the current for the whole chain and it's value is (Supply voltage - total LED voltage) / LED current. The power the resistor will dissipate is (LED current * LED current) * resistor value.

In parallel, as stated many times already, the diode with the lowest voltage drop will decide the voltage across all the other diodes, it will 'hog' the current at the expense of all the others. All the LEDs have the same voltage across them (they are wired directly across each other so that must be the case) so suppose the LED with the lowest voltage is 2V and another is 2.2V, the voltage will be held at 2V by the the lowest voltage LED and the other one will be dim at best, it doesn't have enough voltage to fully light up. If you increase the current by dropping the resistor, the voltage will eventually rise and the dimmer LEDs will get brighter, but only because the lowest voltage LEDs are now burning out.

You have two options to fix your problem, playing with one resistor value is never going to work, whatever its value and whatever it's power rating.

Option 1. (this is best)
Split your LEDs into several smaller series chains. You are starting with 12V and your LEDs have a maximum forward drop of 2.2V so you can safely wire 5 LEDs in one chain. That gives a maximum requirement of 5 x 2.2 = 11V. In each chain, add a resistor of (12-11)/0.02 = 50Ω. It will dissipate (1*1)*0.01 Watts so even a tiny resistor will do. Wire each of the chains in parallel.

Option 2.
Wire all the - ends of the LEDs together and connect the back to the power source, add a resistor in the + end of every diode and connect the resistor back to the power source. The resistor needs to be (12-2.2)/0.02 = 490Ω and it will dissipate 0.2W, still quite small.

Option 1 will only need one resistor per 5 LEDs, option 2 needs the same number of resistors as LEDs and produces more heat. The choice is yours but I (an all the other contributors) assure you there is no magical way of making your present circuit work as you intend. You will have to rewire it.

Note that I have used 20mA (0.02A) in my calculations above.

Brian.

 

[theelectronicsfan]

ok thanks

 

[Audioguru]

Make and post a video of your 20 paralleled LEDs burning out. Measure the voltage across them all and watch the voltage increase as each LED burns out.

A manufacturer does not make LEDs to be the same. That is why they specify a range of forward voltages. LEDs made on different dates will have different forward voltages and you don't know when they were made.

 

[theelectronicsfan]

No!!

Added after 14 seconds:

i dont wanna burn them

Added after 3 hours 42 minutes:

Thanks you guys i solved my problem now! i cant thank you enought! Tyvm!!!!!

 

[betwixt]

I saw flashes in the sky - thought it was a thunderstorm but it might have been all those LEDs

 

[Audioguru]

The problem was solved but we will never know how.

 

[hameeds01]

Driving LEDs or Elephat .. . . . . .

 

[theelectronicsfan]

I saw flashes in the sky - thought it was a thunderstorm but it might have been all those LEDs

Haha god damn funny

 

[simonmorkel1]

Quality Christmas tree light bulbs have rating between 150 and 200mA. Low rating bulbs can easily burn out. Artificial Christmas Tree should have 90 to 100 light bulbs per foot.