linderman
Posts: 14
Joined: Thu Dec 03, 2015 4:08 pm

LEDs in parallel

Fri Dec 11, 2015 1:58 pm

Hello,

I want to connect 2 leds in parallel. Each of them have forward v. of 2V and current draw at 20mA. I want to use single resistor.

Image

So the calculation is:

(3.3V - 2V)/0.04A = ~32ohms resistor i need to protect the LEDS?

But i also have to protect the Pi which should output max of 16mA per 3.3 GPIO pin. With ~32ohms resistor pin will be forced to give 40mA, which out of a question is bad.

So to protect the pin also i will need another calculation for the parallel circuit. I will have to give up of some mA (led brightness) and lets say i can live with 4mA per LED.

So the new calculation is: (3.3V - 2V)/0.008A = ~162ohms right?

And since i have few 330ohm resitors, then (3.3V - 2V)/330 = ~4mA , devided by two for each LED = ~2mA per LED under this configuration.

Is that right?

Please, look the attached image and tell me, am i on the right direction?

Thank you and sorry if i am too "can smell it from distance" type of NOOB :)
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joan
Posts: 14468
Joined: Thu Jul 05, 2012 5:09 pm
Location: UK

Re: LEDs in parallel

Fri Dec 11, 2015 2:33 pm

With a single resistor all you can do is protect the Pi. That may limit the current enough to protect the LEDs. Remember that LEDs do not load balance, when in parallel one LED could take 90% of the current.

linderman
Posts: 14
Joined: Thu Dec 03, 2015 4:08 pm

Re: LEDs in parallel

Fri Dec 11, 2015 2:40 pm

Yes , but since the (330ohm) resistor is limiting the overall current to 4mA and single led can draw as much as 20mA , then both LEDs seems to be safe (even if one of the leds is using 100% of the amps left after the resistor). So isn`t this schema protecting both Pi and LEDS :?:

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joan
Posts: 14468
Joined: Thu Jul 05, 2012 5:09 pm
Location: UK

Re: LEDs in parallel

Fri Dec 11, 2015 2:48 pm

linderman wrote:Yes , but since the (330ohm) resistor is limiting the overall current to 4mA and single led can draw as much as 20mA , then both LEDs seems to be safe (even if one of the leds is using 100% of the amps left after the resistor). So isn`t this schema protecting both Pi and LEDS :?:
Yes, in this situation, as the Raspberry Pi can't safely sink enough current to be a danger to a LED. Your assumptions wouldn't have worked with a component capable of safely providing more power.

linderman
Posts: 14
Joined: Thu Dec 03, 2015 4:08 pm

Re: LEDs in parallel

Fri Dec 11, 2015 8:05 pm

Ok , i tried parallel circuit with 2 leds using 1 resistor for both (330ohm) - nor LEDs or Pi got damage. Both LEDs produced visible light.

But then i tried another thing , the result of which i dont understand:

2 Leds connected in serial circuit

3.3 pin -> resistor 330 ohm -> LED -> LED -> Ground

I didnt expect the LEDs to produce any light , but they did. No matter it was dimmer and barely visible i still dont get it , since the LEDs are 2v and in serial circuit they summed draw 4v but pin outputs only 3.3v. Why both LEDs even lighted?

Thanks

gregeric
Posts: 1509
Joined: Mon Nov 28, 2011 10:08 am

Re: LEDs in parallel

Fri Dec 11, 2015 8:23 pm

Difficult to explain without the aid of a graph of current on the y axis and voltage across the diode on the x. In fact the best I can do is think up an analogy which describes the shape of that graph:

Think of an aeroplane sat on a runway. It's given he OK for take off, so sets off picking up speed from zero. It gets faster and faster until at some speed the wheels lift off the ground ever so slightly. The plane gets a little higher as the speed continues to pick up, until it's clear enough of the runway that the pilot can pull the stick right back and allow the plane to go almost vertical without the tail hitting the tarmac. The plane continues to soar into the sky.

Picture that sideways on, speed vs altitude. That will be volts vs current. With the LED, it will emit light at the point the wheels left the ground - not much, & therefore not much light, but more & more as the speed picks up. At the point the pilot pulls the stick back to go vertical, that's your quoted "2V" hockey-stick upwards thrust. So the LED emits light with less than 2V, but not much, and becomes saturated around the ~2V mark.

linderman
Posts: 14
Joined: Thu Dec 03, 2015 4:08 pm

Re: LEDs in parallel

Sat Dec 12, 2015 5:39 pm

Thank you for the good explanation!

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