LED Connection

Discussion in 'The Projects Forum' started by tapan6415, Mar 11, 2011.

1. tapan6415 Thread Starter New Member

Mar 11, 2011
9
0
Hii,

I had connected white led 5v with input power supply 5v , but when ever i connect red LED with serial then the white led becomes off.

2. Wendy Moderator

Mar 24, 2008
20,772
2,540
Different colors drop different voltages. LEDs, once the dropping voltage is reached, are current controlled devices. You do have to reach the dropping voltage, and it is a really bad idea to put LEDs in parallel to each other, each needs its own dropping resistor.

I have a tutorial for beginners on LEDs, Chapters 1 and the 1st half of Chapter 2 cover the basics.

LEDs, 555s, Flashers, and Light Chasers

3. tapan6415 Thread Starter New Member

Mar 11, 2011
9
0
i made electric model train, but total number of leds,

14 white leds

2 red leds

2 green

now can u telll me plz they ll work with 12v dc power supply or not?

i have to also connect 12v 60rpm dc motor with it.

will it carry load of train (5kg)?

4. jmdejoanelli New Member

Mar 13, 2011
10
1
All LEDs must be connected in series with a current limiting resistor.

Assuming you aren't using high powered LEDs, we can say that you require around 15 to 20mA running through each LED.
Each colour LED has a different voltage drop which comes built into the type of semiconductor that is used to make each of those colours. This means you will need up to 3 separate value resistors.

For the white LEDs:
Voltage drop for white LEDs is generally around 2.7V, so with a 12V supply, you'll need a resistor with value (12 - 2.7)/0.020 = 465ohms, so round up to 470ohms (standard value).

For the green LEDs:
Voltage drop is around 2.2V, so the resistor value required is: (12 - 2.2)/0.020 = 490ohms, so round up to 560ohms(standard value).

For the red LEDs:
Voltage drop is about 1.7V, so you'll need a resistor with value: (12 - 1.7)/0.020 = 515ohms, so again round up to 560ohms(standard value).

So in total you'll need 14 x 470ohm resistors and 4 x 560ohm resistors.

Each LED will have one end connected to ground and the other end connected to 12V through its appropriate resistor.

In regards to your motor, it isn't possible to say if it will operate your train or not without more details. I'm not an expert on motors, but it really depends on the size or power rating of the motor. For a 5kg train I would say you need a pretty powerful motor or a highly geared motor maybe.

Good luck pal.

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5. tapan6415 Thread Starter New Member

Mar 11, 2011
9
0
so,

as u said me i have to connect all leds in serial connection but should i need to drop down the voltage from 12v to 3v or it will works with 12v , i means i have to stepdown it or its work with 12v?

6. Wendy Moderator

Mar 24, 2008
20,772
2,540
I disagree a bit on the voltages. Most modern white LEDs are 3.4-3.6, green are around 3.0, while red tend to be 2.4-2.6. It does vary a lot with the make of LEDs (and color), and older generations that aren't made anymore drop a lot less.

I have a tutorial on LEDs in general, just read chapters 1 and the 1st part of chapter 2.

LEDs, 555s, Flashers, and Light Chasers

Just realized this is the second time I've posted the link. It really will answer your questions, you might at least try to give it a read.

The power supply voltage sets the number of LEDs you can have in a chain. You need at least a volt or 2 for headroom, to set up the current you will have going through them. So at 3.6VDC forward voltage drop (Vf) you can only have 3 white LEDs in a chain, the total voltage will be 10.8V. You can have as many chains as the power supply can handle though.

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7. Audioguru New Member

Dec 20, 2007
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896
No.
White LEDs are about 3.5V.