# That how should be LEDs in series?

Discussion in 'General Electronics Chat' started by PSnetwork, Apr 3, 2013.

1. ### PSnetwork Thread Starter Active Member

Apr 1, 2013
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0
since i have opened a thread and no who answered my last question so i've opened a new one.
3 common cathodes(blue) in series with a supply of 12v thats how should be circuit with 150 ohm resistor ?
this diagram isnt really good, simply i just meant to 3 cathodes, not 4.

Last edited: Apr 3, 2013
2. ### mcgyvr AAC Fanatic!

Oct 15, 2009
5,394
1,194
+ and - is backwards.. + on LED should go to + on power supply.

For 4 x 3V (Vf) LED's you need a power supply greater than 12V. 3 would work just fine.

The resistor can be anywhere in the series circuit.. It doesn't have to be in the middle of the string.

How much current do you want each LED to receive?

3. ### gerty AAC Fanatic!

Aug 30, 2007
1,237
343
Is this what you want? You are showing 3 volt drops at each led X4= 12 volts. You show the power supply as 12v, which means you don't have enough voltage to drop across your current limiting resistor. Is the drawing what you want?
are the led voltages correct?
Does the power supply measure 12v?

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4. ### PSnetwork Thread Starter Active Member

Apr 1, 2013
30
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didnt understand you man... what does it mean " + and - is backwards, + on led should be + on psu"? sorry but my english isnt great at all im not american or british

the led voltages is 3.0 to 3.4v
but i will be use 3 cathodes, not 4
but the circuit i drew is correct ? maybe there is something wrong?

5. ### mcgyvr AAC Fanatic!

Oct 15, 2009
5,394
1,194
+ of first LED should be on + of PSU, - of last LED should be on - of PSU

Your picture shows 4 LED's.. That will not work with a 12V supply.. 3 LEDs maximum.

6. ### gerty AAC Fanatic!

Aug 30, 2007
1,237
343

And don't forget the resistor.

7. ### Tealc Member

Jun 30, 2011
140
10
The circuit you drew is not correct. You must have the anode of the LEDs on the positive side of the power supply.

Are you using RGB LEDs. It seems to be the case as you are referring to them as common cathode. Why are you only looking at using the blue?

8. ### PSnetwork Thread Starter Active Member

Apr 1, 2013
30
0
why not blue?

9. ### mcgyvr AAC Fanatic!

Oct 15, 2009
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1,194
He is asking why are you using a 3 color (RGB) LED but only using the blue section of it? What are you doing with the red and green LED's?

10. ### PSnetwork Thread Starter Active Member

Apr 1, 2013
30
0
nothing, only blue color, but why does it matter ?
maybe he is asking because i bought with 3 colors and im using only 1 and i could buy LEDs with only 1 color?

11. ### Tealc Member

Jun 30, 2011
140
10
It doesn't matter if you only plan on using one of the colours.

It's just strange why you'd buy LEDs with three colours and only ever plan on using one.