newbie needs help :(

audio_mod

Apr 28, 2012
5
Hello,
Could someone please tell me the formula to calculate the maximum led's a power pack could handle? I have read myself into utter confusion and my brain hurts. Does it depend on wattage or ampage? I know it will need ohms law somewhere.

I have a 12volt, 3 amp power source.

I know i should only use 80% of it so as not to strain it. I will love you forever if you show me the maths clearly laid out (i'm rubbish at maths). Do i divide 3000 milliamps by 20 (the ampage of my leds). Then i read something about wattage is the key and have subsequently lost the page where that was.

Thanks if you help

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
If you only want to run the power supply at 80% load, then the current will be 2.4A.

If the LED current is 20mA, then you can have 2.4/0.02 = 120 strings of LEDs.

Each string of LEDs will have to operate from 12V, so you might be able to have three white LEDs and a resistor, or 6 red LEDs and a resistor in each string.

Alternatively (and somewhat wastefully) you could have 1 LED and a resistor as each string.

This means that you can drive something between 120 and 720 LEDs depending on how you decide to wire up the strings and the LED colour (this will determine the maximum number of LEDs can be in each string)

audio_mod

Apr 28, 2012
5

Alas my newbiness is more than you thought!

This is my 2nd go at a led light. I am wiring in series with one resistor per led, dependant on led colour (and hence voltage). I want to wire 25 led's along a conduit box.

I don't quite understand what you mean by strings (learning the jargon is half the battle). My understanding is that if i have two parallel wires (positive/ gnd), i can just 'link' the two wires with a resistor and led.

-----------------------------------
I I
R R
I I
led led
I I
--------------------------------

Is that a string?

The question then becomes, can i have between 120 and 720 led's (depending on their milliamp requirements/current usage) wired like that or does the total voltage of the led's matter in each string? It is the "each string will have to operate from 12volts" part that threw me. Can each string only maintain the accumalative voltage of the led's? So with 2 volt led's, i could have 6 on each string? And the strings are connected to the two power pack wires. So two power pack wires can lead to multiple strings of led's that add up to the voltage of the power pack; and as a total, all led's together cannot draw more current than the ampage of the adapter? I feel stupid. I hope this is somewhere near the correct understanding.

Thanks for the help. My led's arrived from china yesterday and i'm all excited, haha.

Last edited:

timothy48342

Nov 28, 2011
218
You got it. A 1-LED string is just an LED and a resistor.
+12V---LED---R---GROUND

A 4-LED string would be:
+12V---LED---LED---LED---LED---R---GROUND

A string of 4 LED's that are 2V each powered by a 12V source would leave 4V across the resister. That's the number used with Ohm's law. (resistor value to use = voltage across resistor / desired current)

If you try to put 6 2V LED's in one string, I don't think it will work. 12V - (6*2V) = 0V Even a string of 5 seems iffy since those voltage numbers are not always exact.

Of course if the voltage drop on each LED is lower, then you can have more per string.

Whatever current goes through that resister is the same amount going through each LED in the string. So how much do you want? 20mA? and you have just 1 LED?
So ( 12V - ( 2V * 1) ) / .020A = 500 ohms

How many strings can you have like these here?
Code:
12V12V12V12V12V12V12V12V12V12V12V12V12V12V
|     |     |     |     |     |
LED   LED   LED   LED   LED   LED
|     |     |     |     |     |
LED   LED   LED   LED   LED   LED
|     |     |     |     |     |
LED   LED   LED   LED   LED   LED
|     |     |     |     |     |
LED   LED   LED   LED   LED   LED
|     |     |     |     |     |
R     R     R     R     R     R
|     |     |     |     |     |
0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V
Steve covered that, but since I'm here....
3A * 80% is 2.4A
2.4A total divided by .020A per string is 120 strings.

Incidently as far as the jargon The LED's and resistor are connected in "series" They are end-to-end.
The strings are "parallel" to each other. They are...well parallel. Or side-by-side.

-t

audio_mod

Apr 28, 2012
5
You got it. A 1-LED string is just an LED and a resistor.
+12V---LED---R---GROUND

A 4-LED string would be:
+12V---LED---LED---LED---LED---R---GROUND

A string of 4 LED's that are 2V each powered by a 12V source would leave 4V across the resister. That's the number used with Ohm's law. (resistor value to use = voltage across resistor / desired current)

If you try to put 6 2V LED's in one string, I don't think it will work. 12V - (6*2V) = 0V Even a string of 5 seems iffy since those voltage numbers are not always exact.

Of course if the voltage drop on each LED is lower, then you can have more per string.

Whatever current goes through that resister is the same amount going through each LED in the string. So how much do you want? 20mA? and you have just 1 LED?
So ( 12V - ( 2V * 1) ) / .020A = 500 ohms

How many strings can you have like these here?
Code:
12V12V12V12V12V12V12V12V12V12V12V12V12V12V
|     |     |     |     |     |
LED   LED   LED   LED   LED   LED
|     |     |     |     |     |
LED   LED   LED   LED   LED   LED
|     |     |     |     |     |
LED   LED   LED   LED   LED   LED
|     |     |     |     |     |
LED   LED   LED   LED   LED   LED
|     |     |     |     |     |
R     R     R     R     R     R
|     |     |     |     |     |
0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V0V
Steve covered that, but since I'm here....
3A * 80% is 2.4A
2.4A total divided by .020A per string is 120 strings.

Incidently as far as the jargon The LED's and resistor are connected in "series" They are end-to-end.
The strings are "parallel" to each other. They are...well parallel. Or side-by-side.

-t

Absolutely wicked, eureeka moment. Thanks very much. I can now start to experiment. I kept checking back hoping someone would reply You did. Yay!

Happy kharma package sent. (to both of you).

audio_mod

Apr 28, 2012
5
Worked a treat. project 2 underway today. thanks again.

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