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TOPIC: 3W 6500k LED Lighting Project - Need for some Analog Electricity Help

3W 6500k LED Lighting Project - Need for some Analog Electricity Help 2 months 2 weeks ago #17964

  • hop
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Greets Everyone! (Especially you Jon! )
And FOR Jon… I placed tags to allow you and others to move to the meat of this question without suffering through my long-winded preface. Lol
{BEGIN LONG WINDED PREFACE}
I have a project I am working on to help me with something that has ALWAYS plagued me with my projects. Proper lighting, and especially SPOT lighting. What comes from this project later is holiday lighting effects, accent lighting for my home, and even plant supplemental lighting.
I would LOVE to have a lab. A long bench with shelves, cabinets, endless test equipment, a long beautiful white surface covered with a sizeable anti-static mat. And of course, proper bright white lighting that allows my tired aging eyes to read all the small device print. But as reality has it, I am not an engineer but more of a hobbyist, drunk with the constant feed of the science behind my ideas and creations. Self-fulfilled with satisfaction by applying what little I know to furnish my house and family with something you cannot buy online, and cannot be done by anyone else in my neighborhood. I am stuck between the profitable inventor and production and the prototype.
It’s not as sad as I make it, but it does limit me somewhat. Without an electrical engineering degree, and most of the complex math forgotten with the faded years of my high school youth, I am left with endless searches via google, and posting my inquiries here and a few places elsewhere… hoping someone with the talent I wish I had will throw me a nugget. All I really need is a breadcrumb trail though most times.
So my lab is a place in our third bedroom, populated with minimal but at least somewhat respectable equipment. Not the lab I dream of but it’s a place to pursue my passion.
{END LONG WINDED PREFACE}
So to make it more productive for me, I need to tackle each item on my list of goals, and the one I will go after today is lighting!
I devised on paper an idea that IS available online but more expensive than I think it should be, and certainly not the exact design I want, so I went after it as a project.
The 3 watt LED in various spectrums of white has finally hit the market in large volumes and cheap prices. I found a place where I can get 50 of these in a snap-apart grid for $10. Certainly enough of them and cheap enough to prototype with. They are probably crap, but again, it’s a prototype.

Forward voltage is 3.6-3.8vdc with a max current of 700ma. They are supposedly 6500k wavelength and 180-200lm. This wavelength is great for vegetable production of indoor plants, but that is another project.
I am great with the digital aspect of things. MCU’s, C and C++, Linux, software development platforms, but when it comes to analog electricity, or better known as ohm’s law… well it was said a few times here. Ω’s Law + Hop = FIRE!
{BEGIN ACTUAL QUESTION}
I have countless strips of WS2812B RGB LED’s and the means to control them. But when it comes to these white light strips and obviously individual LED’s like what I am working with here, individual control of brightness in an array is not possible with the existing design. An engineering friend of mine suggested that I look to getting small SMD WS2811 chips, adding P-Channel MOSFETS, a couple of resistors and a cap, and control each 3W LED using that scheme. I LOVE IT! It seems viable.
No I really do not care about controlling each LED in a 3x array individually, but the WS2811 is 3 channel because of the RGB aspect, so why not develop 3x LED arrays and just control the brightness of all of them in 3x LED pods? The goal is to be able to send digital data to control the pod’s brightness, and send more data to control other pods in a larger array. The device conditions the data signal and passes on additional data to the next node once the locally needed data is stripped off. In this case, 24 bits, or 3x 8 bit bytes for each channel’s brightness value. PWM is also supplied. I just need to use the mosfets to drive the LED’s.
The individual control of each 3w LED in the pod can be folded out into a straight line, allowing a string of these to need only one WS2811 for each group of 3. This is important for my lightning simulator effects for Halloween, and, that’s another project too for another thread.
For now? Spot lighting pods. I even found a nice diffuser for the project! I also found some nice gooseneck hardware with M8/M10 machined male and female ends for mounting. And, the end-point mounting hardware. I can equip one of these assemblies with a cheap MCU, capacitive touch controls for intensity, and I have my adjustable high-intensity spot lighting! WOOHOO!
Enter the brick wall I slammed into. Selecting components. The designs I have reviewed favor a P-Channel MOSFET with low RDS(ON) to minimize heat generated by the driver, especially during PWM control. I am absolutely lost picking the correct small SMD device to meet my needs and that is what I need help with. With drivers needed every 3 LED’s, cost is an issue. The funny thing is, the hardest part for me use to be designing and having the pod boards made. With all that Jon Chandler has offered here, I am confident I can do that pretty easily.
But I need to pick a P-Channel MOSFET.
Power I plan to supply is 5v 6a (or more depending on the application). Obviously, if I have 3x LED’s at max 700ma, I am looking at 2.1a per pod at full on.
The WS2811 power requirements are a bit of a mystery and need to research more. The datasheet shows a 100Ω resistor between 5vdc and VDD in, and not sure why. The datasheet says 6-7vdc with an output voltage of 12vdc. I will test this with my scope. My limited knowledge of analog has me asking… is that too high for a low RDS(ON) device?
3W LED forward voltage is between 3.6vdc and 3.8vdc. I need to make sure I do not exceed that and many have stated I should use at LEAST a 1w resistor of a value of about 1.4Ω to 4Ω.
I need a circuit and P-Channel MOSFET recommendations.
I hope someone can help me here. Jon? Wink Wink.
Thank you for your time!
{END ACTUAL QUESTION}

3W 6500k LED Lighting Project - Need for some Analog Electricity Help 1 month 3 weeks ago #17978

  • Jon Chandler
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I will help you throw a dart when I get home today. My experiece with MOSFETS is limited, but everything I have tried works so I am doing something right.

Many more comnents later.

3W 6500k LED Lighting Project - Need for some Analog Electricity Help 1 month 2 weeks ago #17979

  • jmessina
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Hop,
I hate these chinese datasheets. They're hard to decipher and they're constantly filled with misleading/conflicting/incomplete info.

I had a look through the thread over on electronicspoint.com, and most of the comments there seem good. Here's my take on it-

According to the WS2811 datasheet, the "max VDD voltage" is 6V ~ 7V.
In the typical application sections, with 5V VDD:
"We require, in order to prevent power spikes phenomenon and power reverse polarity, series a not more than 100ohms resistor at the power supply pin(VDD)"

and at 12V VDD (yes, 12V is > "max 6V-7V" called out above):
"In this circuit, R1 is used as the IC internal LDO divider resistance and the value is 2.7K"

From that, I would assume that the IC has an internal regulator (of unknown voltage and current), and that input resistor is used to limit the current into the VDD pin so that the regulator sees a lower voltage and dissipates less power. Along with the bypass cap it would also form an RC low-pass filter that would help filter out some of the switching noise that you'll see on the VDD line when these things are PWM'ing.

Switching that much current around (3 x 700mA = 2.1A), you'll likely see huge spikes in the VDD rails unless you add a boatload of capacitance to the VDD line on each board.
...with an output voltage of 12vdc
The WS2811 outputs are "constant current" outputs, not voltage outputs. It won't generate 12V on the output... that "12V output" spec is meant as the max voltage the outputs will handle.

Think of the OUTR, OUTG, and OUTB pins as open-collector transistors... all they do is sink current to ground.
To use them to drive a MOSFET you need voltage outputs. That's why you need to replace the LEDs with resistors, effectively giving you an open-collector with pullup output. The pullup is what will supply the voltage. I think BobK recommended 270 ohms... that sounds about right.

Using a P-CHAN as a high-side switch would keep the same logic as the original ckt... when the PWM output is on (sinking current) the output voltage will be 0V, turning on the output MOSFET. You could use an N-CHAN device to switch the low-side of the LED, but you'd have to program it "backwards"... 100% duty-cycle -> full off, while 0% -> full on.


For the output MOSFETS:
Assuming you don't drop any voltage across the MOSFET, with a 5V supply the series dropping resistor has to drop:
5-3.6 = 1.4V
5-3.8 = 1.2V

I'd pick the higher of the two so that the LED voltage does not exceed 3.6V, so:
E=I*R -> R=E/I:
R = 1.4V/700mA = 2ohms

and the power in that resistor would be:
P=I*E -> P=I*(I*R):
P = 700mA^2 * 2ohms = 0.7^2 * 2 = 0.98W

So, you're going to chew up almost 1W of power in the dropping resistor, and it's going to get HOT when you have the LED on at a high pwm duty cycle. As you lower the duty cycle (to reduce the brightness) the average power will drop so it'll run cooler.

For a 1W SMD resistor you're probably looking at a 2512 case size. The heat dissipated by the resistor depends a lot on the PCB, but even with that package size you might see as much as an 80-100degC/W temperature rise. It likely won't be that bad, but just to give you an idea of the amount of heat you could potentially be dealing with.

A P-CHAN MOSFET that switches with a low Vgs like the DMG2307L suggested by BobK sounds like a reasonable choice. They're relatively low-capacitance so that's good too. The power dissipated switching the load depends on the gate charge as well as the Rds output resistance, since you have to charge and discharge the gate with that 18mA from the WS2811 output. The slower you do that, the more time is spent in the "linear region" and the output Rds is higher until the MOSFET is fully on.

Are you really tied to using these things? It sounds workable, but there's bound to be better choices for driving 3W LEDs.
I would think you'd be a lot better off with a constant current driver... that way the LED drive wouldn't change with temperature
as the LEDs heat up.

Jerry
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3W 6500k LED Lighting Project - Need for some Analog Electricity Help 1 month 2 weeks ago #17980

  • hop
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I hate Chinese data sheets and product descriptions also. There seems to be more self gratification than usable or reliable data.

I am not locked into using WS2811 chips for this project at all. I just picked them because I have not found a device that does what they do on the data side of things. At least not in a self contained package. I already have all the software and data driver systems in place to use these, but with these 3w LED's instead of small 20ma RGB types.

Your reply is a giant wealth of information though that will help me with this project. I appreciate and thank you so much for that Jerry!! I was not aware though that I would have dissipation/heat issues other than the LED's themselves and the only way I will understand all that is to build a single 3xLED prototype and run it through the paces. Something I plan on doing rather soon, using my scope to see what is happening. The intriguing part of this of course is to have a bus of spot lighting where individual nodes can be controlled as far as brightness using a stream of data from an MCU.
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