Need help from a Math wizzard/LED Geek

twilliard · Nov 14, 2015

The problem I am seeing in parallel led designs is the wattage of the resistor.
I have managed to get down to a 5w resistor driving the Led's at 400ma at 6 volts per channel with 3.3v Led's 4 per channel.

twilliard · Nov 14, 2015

icecool2 said:
Looks like a good layout to me. He wants 15 per strand.

You want a power supply as close to the required voltage (3.3VDC) as possible. Anything over that and you need to burn it off in the resistor and that's going to be tons of power lost. Even at 3.3VDV if we guess the voltage drop is 2.2VDC you are going to burn off 5 watts in the resistor. That's going to generate a lot of heat too. Running a 12 VDC supply will require around 50watts

Very well said!

Jimmyb819 · Nov 14, 2015

Also keep in mind that not using a resistor for each led can be problematic. With only one resistor for multiple leds if one of the leds fails the others will then have to take that current and will overload burning them all out. Even though it's more difficult, wiring each led with its own resistor is a better way to go.

twilliard · Nov 14, 2015

15 per channel at 6v would require about a 17 watt resistor at 400ma
But I think that is pushing the Led's

twilliard · Nov 14, 2015

Jimmyb819 said:
Also keep in mind that not using a resistor for each led can be problematic. With only one resistor for multiple leds if one of the leds fails the others will then have to take that current and will overload burning them all out. Even though it's more difficult, wiring each led with its own resistor is a better way to go.

That is correct but problematic with the space available

Jimmyb819 · Nov 14, 2015

Lowering the supply voltage and necessary resistance will lower the wattage of the resistors required. Using a 17w resistor is no joke

Jimmyb819 · Nov 14, 2015

I'm not sure where the 17 watts is coming from in your calculations either...

Using a 6 volt supply with a 3.3v led voltage drop leaves 2.7 volts for the resistor. Power dissipated is voltage×current which would be 2.7*.4 = 1.08 watts.

twilliard · Nov 14, 2015

Even at 4 volts pushing them at 400ma you need a 4 watt resistor which.. they don't make.
So would have to use a 5 watt which isn't too big.
I have to stay within the design of Paul's Diy so individual resistors may not fit

twilliard · Nov 14, 2015

Jimmyb819 said:
I'm not sure where the 17 watts is coming from in your calculations either...

Using a 6 volt supply with a 3.3v led voltage drop leaves 2.7 volts for the resistor. Power dissipated is voltage×current which would be 2.7*.4 = 1.08 watts.

http://ncalculators.com/electronics/led-resistor-calculator-parallel.htm

Jimmyb819 · Nov 14, 2015

Jimmyb819 said:
I'm not sure where the 17 watts is coming from in your calculations either...

Using a 6 volt supply with a 3.3v led voltage drop leaves 2.7 volts for the resistor. Power dissipated is voltage×current which would be 2.7*.4 = 1.08 watts.

I forgot you weren't using individual resistors. My bad. It would be 1.08 watts times the number of leds which I think you had 15 in the string so you are right on.

Jimmyb819 · Nov 14, 2015

Another reason to use individual resistors... we could get them down under 1 watt each.

twilliard · Nov 14, 2015

Jimmyb819 said:
Another reason to use individual resistors... we could get them down under 1 watt each.

True but where would he mount the resistors?
1 watt resistors aren't too bad but not sure how he would do that on his rail.
So far I see 2 issues, space for individual resistors
Next is the longevity of the Led's with 1 dropping resistor.

Paul B · Nov 14, 2015

icecool2 said:
Is that what you want? This is 4 parallel strings of 3 leds in series.

Are these LEDs on stars? How are they mounted? I'd like to do a quick calculation of how much heat you can realistically transfer to the copper tube.

No, these LEDs are taken from a fixture that I posted a picture at the beginning. They are about 1/4" or a little less. I realize they were not designed for this but I don't know the engineer that designed these LEDs so he won't get mad at me. I rarely use things as they were originally designed.

OK Twillard, I see that you are using 12 volts which will supply each LED with 3 volts because it is a series/parallel circuit. So how do I figure the resistors if I use 15 LEDs in that circuit instead of 4? I can also use 20 LEDs per circuit and only run 3 tubes, but it doesn't matter much.

Icecool2, remember there will be water flowing through the tubing to a radiator. I don't know how you will figure the heat load because we don't yet know the size of the tubing, the size of the pump or the size of the radiator.

twilliard · Nov 14, 2015

Just for reference here are the differences in size

Paul B · Nov 14, 2015

Are your heads exploding yet? It is a little complicated and a little engineering and math is involved. But interesting don't you think?

TheEngineer · Nov 14, 2015

Yep. That power needs somewhere to go. I support individual resistors for sure.

It is worth noting that you will need to dissipate that 60 watts of heat somewhere. That's enough to want to think about a plan for it. Especially since in this case that 60 watts is going to be purely (close enough) heat.

TheEngineer · Nov 14, 2015

Paul B said:
Are your heads exploding yet? It is a little complicated and a little engineering and math is involved. But interesting don't you think?

I miss this stuff. My undergrad is EE but I've been leading research teams for the last 5 years and have lost touch. It's fun to me!

TheEngineer · Nov 14, 2015

My comment about heat transfer is heat from the LED to the copper. If assume you can cool the water in the copper. You can only move so much energy across so much surface area.