New Project - LED Array - Calculation and connection help!

Projects General Guidance
1

Alright, some of you may know that i previously tried to set up an LED array, but I didn't quite get the whole concept, but hopefully, I've now read enough, and had some help that this new design will actually work.

Let's begin with the components. In the schematic, it will not be very clear (I drafted it very quickly just now as a rough idea), but the 12V supply, is a actually from my 300W PSU Supply.

It runs at 12V @ 19Amps.

Now, this panel needed high power LED's - in this case they say 3W each - but I know they're about 2W if you actually calculate the Power=IV. Regardless, I needed them configured in a 6 X 12 array. Now, I knew I need it to be in series, because it LED was slightly different, some were 2.5V other were 3.5V, but I'll get to that in a moment. Now thanks to a bunch of very helpful Forum members, I knew that at a 12V supply, I could only do 3 LED's per series, and leave some voltage for the resistor voltage drop. So I decided to do 24 series of 3 LED's with my 12V power supply. Each series will draw a maximum of amount 500mA, and the PSU is rated at 19Amps, so all together I am only drawing about 12Amps (about 60% of the available current)

Here is where I ran into a problem. As you can see, the series all have resistors in front of them. However, after having a look, here was my problem:

I had three LED's each of which operated at about 500mA according to the data sheet, and the added voltage on average was about 8V per series, therefore (1.5X8) = 12W, that means the power going through every series is 12w, is this correct? or would it be (0.5X8)? Anyway, here is my problem. Since I would need 24 Resistors, I would need to shell out about £30. Is there anyway around this? Or am I doing something wrong.

ALSO: I need confirmation on my Resistance Calculations:

Let's say I have 3 LEDs. 2 2.5V and 1 3.5V in series each drawing 500mA (therefore constant throughout), is this how to calculate the value of the resistor I require:

R=(V1-V2)/I

*V1=Supply
*V2=Required

Therefore the resistance I need on this would be this?

(12-8.5)/0.5 = 7 Ohms?

  • I include the datasheet for LED's. I am using 3W Red and Blue leds.
    Also a print screen of my schematic.

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If no image, here is the link:

639×527 162 KB

Screen Shot 2015-10-25 at 20.54.59.png
Screen Shot 2015-10-25 at 20.54.59.png752×463 6.8 KB

2

If anyone can also tell me what wire I should use?

3

26 AWG should be fine, 2A capability

28 AWG would be okay too, 1.4A.
Your chart shows 700mA needed at the most.

4

Do all the calculations look good?

5

It would help if we know what exactly you require.

Do you need to control these leds in some way or do you just want them to be fully illuminated all the time

6

Fully illuminated all at the same time.

7

Full on all the time, then (12V - (Vf1 + Vf2 + Vf3))/500mA = R
If Vf1 +Vf2 + Vf3 = 8.5V, then
(12 - 8.5)/0.5A = 7 ohm
Watts dissipated by resistor is P = IV, 0.5A * (12V - 8.5) = 1.75W, so use 3W or 5W resistors as they will be warm.

24 strings x 500mA/string = 12A.

At the same time - high power LEDs get warm/hot, and their Vf can change (drop), allowing more current to flow, so most folks would not recommend simple resistor as current limiter in high power situations. Instead constant current drivers are used that adjust the output to keep the current flow constant and prevent the LEDs from going into thermal runaway.

Before going too much further, I would suggest just wiring up 1 string with a resistor and 12V supply and see if it is stable or not, and if you will need heatsinking (or rather, how much heatsinking), to keep things cool.

8

CrossRoads:
Full on all the time, then (12V - (Vf1 + Vf2 + Vf3))/500mA = R
If Vf1 +Vf2 + Vf3 = 8.5V, then
(12 - 8.5)/0.5A = 7 ohm
Watts dissipated by resistor is P = IV, 0.5A * (12V - 8.5) = 1.75W, so use 3W or 5W resistors as they will be warm.

24 strings x 500mA/string = 12A.

At the same time - high power LEDs get warm/hot, and their Vf can change (drop), allowing more current to flow, so most folks would not recommend simple resistor as current limiter in high power situations. Instead constant current drivers are used that adjust the output to keep the current flow constant and prevent the LEDs from going into thermal runaway.

Before going too much further, I would suggest just wiring up 1 string with a resistor and 12V supply and see if it is stable or not, and if you will need heatsinking (or rather, how much heatsinking), to keep things cool.

I've looked around buddy. The Constant current drivers aren't well documented. I saw an intractable post about it but it was not very clear. if you could show me a link to a diagram with the basic concept, or as simple explanation will do so I can proceed. Thanks.

9

Alright, I've looked into Constant Current Drivers, like this one:

This is what I should be looking at right? My original idea with the resistors won't do?

10

Resistors should wok fine with correct value selection.

Constant current drivers also work but why bother ? Extra complexity, i suspect that for full on illumination that testing/selecting individual leds will optimise.

Using constant current drivers as opposed to resistors in this application is probably a no brainer.

Both will disspate the energy as heat.

The resistors or drivers can be mounted away from the leds.
This will help with dissipation at the led end.

3 watt leds generate a lot of heat.
If not correctly cooled their lifetime will be severely reduced.

Led replacements for luminares often fail prematurley because the origional fittings do not have the required dissipation characteristics.

Leds do not like more than about 85 farenheit.

Luminaires with halogen work a several thousand degrees , they dissipate heat but have very different temperature gradient

11

Boardburner2:
Both will disspate the energy as heat.

Three LEDs with a forward voltage of ~10volt, on a 12volt supply, is almost 17% lost in heat.
Constant current drivers loose 5%.
You could have 10% more LEDs on the same power supply.
CC drivers are more expensive though.
That's why all LED strips use resistors.
Leo..