# Series Parallel LED Math

Discussion in 'The Projects Forum' started by Gregory6106, Jun 6, 2013.

1. ### Gregory6106 Thread Starter New Member

Jun 6, 2013
10
0
Hello!
Project: LED lighting for indoor garden

I want to build a light system that is set up with four units, each consisting of three LEDs in series. Then I want the four units wired in parallel to a single power source.

Specs:
Blue LEDs:
Forward Voltage: 3.0-3.4V
Current: 20mA
Purple LEDs:
Forward Voltage: 3.0-3.6V
Current: 20mA
Power Source:
AC to 12VDC 1A LED driver

Each series or string would be comprised of: Blue, Purple, Blue, Resistor.

So I did the math and was hoping you all could double check it and answer a couple questions.

Taken from various other threads and websites, I figured 3 LEDs and one resistor per string for a 12V supply. Then I did the math and figured out I need 120 Ohm resistors for each string. Then I calculated the power for the resistors and came up with 48mW, doubled to 96mW for reliability. Question: How do I figure out which resistor to use from this figure? 1/4, 1/2 watt...etc.

Then I looked at it logically, and I'm confused. If each string is pulling 20mA current, 4 strings total, then that would be 80mA. But my power supply generates 1A, or 1,000mA? So that would be way too much. Thought I was starting to understand this stuff, but I guess not.

Your help and explanations are always greatly appreciated.

2. ### wayneh Expert

Sep 9, 2010
15,886
5,963
I think you've got it. A load only draws as much current as it "needs". The capacity of the supply to allow more current doesn't affect that. Picture one of the LEDs and a resistor connected to your car battery. The car battery has a huge capacity but the LED doesn't care. Now, if it was a constant current supply instead of constant voltage, that would be different.

1/4 resistors will be fine.

Do you really plan to run the LEDs at 20mA? If that's what they are rated for, their life may not be great if that's where you run them. They'd still be bright at 15mA and would last a lot longer. If you need more total light, it's be better to add more strings. Your power supply has extra room.

Last edited: Jun 6, 2013
3. ### Gregory6106 Thread Starter New Member

Jun 6, 2013
10
0
So you're saying my power supply isn't actually providing a current of 1A that would burn up all my LEDs? Sorry, still confused.

Also, my question about the watt rating on the resistor. Which should I go with and how do you figure that out from 96mW?

4. ### #12 Expert

Nov 30, 2010
18,076
9,678
This is the third such question today! Look at the top of the "Chat" page and read, "Ohms Law for Noobies". It explains why a one hundred amp-hour car battery does not make every light bulb (or LED) use a hundred amps each. Yes, it starts out really simple, even stupid from one point of view, but it finishes with enough information to completely clear this up.

5. ### Shagas Active Member

May 13, 2013
802
74
You don't care how much current your power supply can generate as long as it's more than what you need.
Blue , purple blue , thats 3,4X2 + 3,6 = 10,4 volts . 12-10,4 = 1,6 volts
1,6 / 0.020 = 80 ohm resistor (for each string!) = 32 miliwatts of power dissipated by the resistor so You can use 1/4 watt resistor (use something bigger than 80 ohm for safety and longer led life ,90ohm will be better with almost insignificant light output diffrence ) .

Or you can use 1 resistor for the whole lot , then:
1,6volts/ 0.08A = 20 ohms . and 1,6x 0.08 = 0.128 Watts dissipated by resistor so you can use a 1/4 watt resistor .

What I suggest you do and this is how I roll when I'm connecting led's that I want to work at optimal power and have a long lifetime is :
Connect your string of led's and an 80ohm resistor and a potentiometer (in series) as we calculated and also an AMMETER in series . Now vary the potentiometer until you get 20 miliamps , now back it off so you have 17-18 miliamps and use a digital multimeter to see what resistance your potentiometer is at . then combine both fixed and potentiometer resistance into a fixed one and use that to get optimal power ...
unless you got the datasheets which tell you the exact current passing through the led at specific voltage drop (then you could calculate everything by theory )

Also if you don't want to go through the whole process of getting optimal current through the led's then what you can do is just add 10% to the resistance we calculated (80ohm for each string or 20ohm if using 1 resistor for everything ) and use that to be safe.

By the way , how did you get 120ohm ? :/ I'm getting 80ohms
Did you take the mid value for the drops of the led's (3,2 for the blue and 3,3 for the violet? )

Btw someone please correct my math incase it's wrong , but I use this method for my led's all the time and it works so should be fine

Ps :
20mA and say 16 or 17 mA running through the led will have an almost unnoticable diffrence in the light output so i suggest you go with a slighly lower value (higher resistance) as I said above for a longer LED life

Last edited: Jun 6, 2013
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6. ### Gregory6106 Thread Starter New Member

Jun 6, 2013
10
0
Yep, that is exactly what I did. I wasn't sure if running the LEDs at maxium voltage would lower their life span so I figured the mid values. Actually I used 3.2 for both blue and purple. Was this a mistake? Will the light intensity suffer? Also, I really appreciate everybody's input. But I still don't understand how you get 1/4 watt resistor from 32 or in my case 48 mW? And though I value your method shagas, a lot of that sounds over my head. Lol, so I'll prob stick with the strings in parallel.

7. ### #12 Expert

Nov 30, 2010
18,076
9,678
You are still typing your questions in terms of "running the LEDs at maximum voltage". You don't run LEDs at maximum voltage, you run them at maximum current and the LED chooses its own voltage, but you already know that from reading Ohm's Law for Noobies...don't you?

May 13, 2013
802
74

9. ### Shagas Active Member

May 13, 2013
802
74
Well I calculated that the resistor is going to drop 1,6 volts and will let 20ma current through so 1,6 x 0.02 = 32 Miliwatts! You offered a 1/4 watt resistor and I'm saying that you can use it because it can dissipate 250mW and you are only dissipating 32mW so it's all Cool (pun intended)
Just connect strings in parallel with a resistor for each string .
Do the potentiometer method as I described (it's simpler than it sounds and you will actually learn something) and check that each string has about 17mA flowing through it so you can be sure what's going on there and go to sleep peacefully

10. ### wayneh Expert

Sep 9, 2010
15,886
5,963
+1
As with the power supply, the ability to handle MORE power than you need is not a problem. You just have to have the minimum amount covered. A rule of thumb with resistors is that you want one rated to at least 2X the calculated power it needs to dissipate. That safety factor helps insure it won't get crazy hot and burn up. We recommended 1/4W because that is big enough and is a more-or-less standard rating that is easy to find. But you could use a 5W or a 500W resistor if that's what was in your parts box.

11. ### Gregory6106 Thread Starter New Member

Jun 6, 2013
10
0
This has all been extremely helpful! Thank you all so much. So to summarize, my strings will be: blue, purple, blue, 90 ohm resistor. And yes 12, I read and reread your post several times. And now I'm understanding that with my original design and the aforementioned strings of LEDs, I can run 4 strings with a 1A power supply. And if I wanted to add additional strings, I could. I could run 24 strings, each one pulling 20mA for a total of 480mA, well with my 1,000mA power supply. This is because, as you, 12, put it, it doesn't matter how many people drink from the pool, as long as there is enough for all of them to be able to drink at once? Am I getting it?