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[Reply #30 on:]

3. You shouldn't need heatsinks, because they are rated for 3A and you are only drawing 350mA from them.

He;s looking at putting the LED's in parallel and therefore drawing 2-2.5A

I missed that bit. But it's a bad idea to connect LEDs in parallel - especially high power ones - because they are won't share the current evenly. The hotter ones have a lower forward voltage at a given current than the cooler ones, which causes the hotter ones to take more current, which makes them hotter...

[Reply #31 on:]

I missed that bit. But it's a bad idea to connect LEDs in parallel - especially high power ones - because they are won't share the current evenly. The hotter ones have a lower forward voltage at a given current than the cooler ones, which causes the hotter ones to take more current, which makes them hotter...

You're right, what would you recommend then?

Is a good idea buying 15 of these?



http://dx.com/p/mr16-1-3w-650-700ma-constant-current-regulated-led-driver-8-40v-input-13557

Based on the PT4115

http://www.micro-bridge.com/data/CRpowtech/PT4115E.pdf

Code:

FEATURES
z Simple low parts count
z Wide input voltage range: 6V to 30V
z Up to 1.2A output current
z Single pin on/off and brightness control using DC
voltage or PWM
z Up to 1MHz switching frequency
z Typical 5% output current accuracy
z Inherent open-circuit LED protection
z High efficiency (up to 97%)
z High-Side Current Sense
z Hysteretic Control: No Compensatio
z Adjustable Constant LED Current
z ESOP8 package for large output power application

I would have to change the capacitor for one of 40-50v and a resistor to set the output at 550mA.
Also to step down the vin from 32v to 30v but won't be a problem using voltage regulator that can handle 10A.

The construction of the pcb looks really poor but it's also really cheap ($1,49 for more than 10).


PD: I may need some help to change the resistor to get an output of 550mA.

That's the formula in the datasheet: Iout = 100mv/Rs therefore Rs = 100mv/550

Then, to get 550mA output, Rs need to be 0,181ohm (or the closest value) . I tough there was something wrong with the units (because for me less than a ohm is weird) but the resistor that comes in the pcb is R160  (0.16ohm) so it make sense.

[Reply #32 on:]

Looks like that little driver would work, and certainly cheap enough.

> ...and a resistor to set the output at 550mA.
> DC Forward current (IF): 300MA

I'm missing why you want to drive the 300mA LED with 550mA.

[Reply #33 on:]

rather than modifying the reference resistor, why not go with these ..? http://dx.com/p/mr16-1-1w-320-350ma-constant-current-regulated-led-driver-8-40v-input-13553

[Reply #34 on:]

Looks like that little driver would work, and certainly cheap enough.

> ...and a resistor to set the output at 550mA.
> DC Forward current (IF): 300MA

I'm missing why you want to drive the 300mA LED with 550mA.

rather than modifying the reference resistor, why not go with these ..? http://dx.com/p/mr16-1-1w-320-350ma-constant-current-regulated-led-driver-8-40v-input-13553

Because I don't think that the forward current is 300mA, it's suppose to be a bit less than the double.

If you look at the picture, the 50w version es almost 2 of 30w in parallel.

30w  > 10leds in serie (300mA)
50w  > 16leds in 8s2p  (? mA)

[Reply #35 on:]

rather than modifying the reference resistor, why not go with these ..? http://dx.com/p/mr16-1-1w-320-350ma-constant-current-regulated-led-driver-8-40v-input-13553

Because I don't think that the forward current is 300mA, it's suppose to be a bit less than the double.

If you look at the picture, the 50w version es almost 2 of 30w in parallel.

30w  > 10leds in serie (300mA)
50w  > 16leds in 8s2p  (? mA)

16 + 16 + 16 = 48

[Reply #36 on:]

rather than modifying the reference resistor, why not go with these ..? http://dx.com/p/mr16-1-1w-320-350ma-constant-current-regulated-led-driver-8-40v-input-13553

Because I don't think that the forward current is 300mA, it's suppose to be a bit less than the double.

If you look at the picture, the 50w version es almost 2 of 30w in parallel.

30w  > 10leds in serie (300mA)
50w  > 16leds in 8s2p  (? mA)

16 + 16 + 16 = 48

That's the total number of leds in the matrix , but there is only 16 per color. Each CCsupply will drive a single color of each matrix (3channels per matrix = 15 CCsupply in total) and that's why I'm calculating the maximum current for every color and not for the whole matrix.

[Reply #37 on:]

The current does not increase with the number of LEDs in series. That is why it is called a constant current power supply. What changes is the forward voltage so...

If you've got 10 LEDs in series with a 300mA current rating and 2.5 Vf/ LED then you still need to supply 300mA to the LEDs but 10 x 2.5 + X Volt for it to work.

[Reply #38 on:]

I have a few 10watt of these models, same thing only smaller.

To compare, I used a cree ssc p7 (12watt) the ebay ones use double
the power and give off a lot less light than anything i've seen from
Cree or SSC and even NXP (Phillips)

Because of the high wattage, a switching regulator for power, then use
PWM through an n mosfet, tie the base to gnd with a 10k resistor
and you should be good to go.

with these 10watts i have i'm just going to use power transistors, via pwm
with a 200ohm resistor on base, roughly 20ma per pin vs next to no current
on a mosfet.

[Reply #39 on:]

I can see the point about each color drawing 500ma...
16 LED per color so with 48W total (48 LEDs), power for each color is around 16W. Using P = I*E, or I = P/E, I = 16W/32v or 500mA.

[Reply #40 on:]

The current does not increase with the number of LEDs in series. That is why it is called a constant current power supply. What changes is the forward voltage so...

If you've got 10 LEDs in series with a 300mA current rating and 2.5 Vf/ LED then you still need to supply 300mA to the LEDs but 10 x 2.5 + X Volt for it to work.

Yes, but in this case we have 2 rows of 8LEDs in series for each color so the current will be the double.

I have a few 10watt of these models, same thing only smaller.

To compare, I used a cree ssc p7 (12watt) the ebay ones use double
the power and give off a lot less light than anything i've seen from
Cree or SSC and even NXP (Phillips)

Because of the high wattage, a switching regulator for power, then use
PWM through an n mosfet, tie the base to gnd with a 10k resistor
and you should be good to go.

with these 10watts i have i'm just going to use power transistors, via pwm
with a 200ohm resistor on base, roughly 20ma per pin vs next to no current
on a mosfet.

I know the ebay LEDs are crap but the can be easily changed for a better ones if required almost without changing too much the main circuit.

About using a mosfet... That was the first idea but the people who commented made me learn about the advantages of using constant current supply and I think it's worth it.

I can see the point about each color drawing 500ma...
16 LED per color so with 48W total (48 LEDs), power for each color is around 16W. Using P = I*E, or I = P/E, I = 16W/32v or 500mA.

Exactly, and the supply's will give up to 700mA so I have to set a new limit.


By the way, I didn't want to multiplex but I will have to because the ethernet shield that I'm going to use will take a couple of precious PWM pins and I won't enough.

I was thinking on using the famous TLC5940 (current sinking) between the arduino and the dim pin of the PT4115 pulling up the line. Will this IC be able to PWM 15 channels without flickering?

[Reply #41 on:]

While you may be able to use the TLC... I'd suggest a much more versatile solution with 16 channels right out of the box http://www.adafruit.com/products/815
This uses the PCA9685 which is a 16 channel, 12 bit PWM chip. It's advertised as a servo motor driver, but what you use the PWM output for is up to you ;-)
We use it on our LED shield and it works by well https://ledshield.wordpress.com/
The nice thing with this chip is that it uses the I2C bus interface of the Arduino and does not use any of the PWM pins. Also this provides you with much more flexibility in terms of PWM frequency. Hard to beat deal at about $15.

I have not looked at both chips and compared if inputs/outputs are compatible but the PT4115 should be a good chip to pair with the PCA9685 as it has a 5000:1 dimming ratio ( alas only at 100hz PWM frequency, and decreasing with increasing PWM frequency )