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Topic: PWM cable lenght (Read 9388 times) previous topic - next topic

MrGlasspoole

I'm using PT4115 drivers for the LEDs in my shower wall and dim them with PWM:
http://www.micro-bridge.com/data/CRpowtech/PT4115E.pdf

The thing is the location for the Arduino for accessing him easy.
From the driver to the LED i guess its best to make the distance as short as possible
because the driver goes up to 1MHz (datasheet).

Is it a problem to make the PWM cable from the Arduino to the drivers ~3 meters (10ft) long?
The PWM pin will be split into 3...

Then i have 18 3.2V/20mA LEDs i also need to PWM.
4 connected directly to the Arduino and 14 connected to a PCA9685.
Here i need 4-5 meters (13-16ft) and CAT cable would be best - problem?


DVDdoug

I'm going to guess that you'll be fine..   The 1MHz switching frequency is filtered through an inductor anyway, and the PWM from the Arduino isn't running anywhere near that fast.

cmiyc

because the driver goes up to 1MHz (datasheet).
The key here is "up to."

That's the speed the chip supports. The actual frequency is whatever you drive it with. If you're using then Arduino's default PWM, its in the range of 400-600Hz.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

MorganS

Put a freewheel diode close to the driver chip. (Not close to the LED load.) This will help avoid the problems of inductive voltage spikes in the long cables.

Diode D in the aplication circuit shown on the first page of the datasheet.
"The problem is in the code you didn't post."

MrGlasspoole

The actual frequency is whatever you drive it with.
Thats something i could not figure out from the datasheet.
That means if i use 200Hz PWM on the DIM pin the output is the same?

But the sheet says you can use up to 20KHz and output max is 1MHz...

Put a freewheel diode close to the driver chip.
The diode is there. This are kit's for self soldering:
http://led-stuebchen.de/index.php?page=product&info=7

So you guys think it is save to use 3 meters (10ft) from driver to LED without killing my neighbours radio or my phone?

What about the other thing: 4-5 meters (13-16ft) from Arduino (PWM) through CAT to the 20mA LEDs?

cmiyc

Thats something i could not figure out from the datasheet.
That means if i use 200Hz PWM on the DIM pin the output is the same?

But the sheet says you can use up to 20KHz and output max is 1MHz...
Oh, I misread it.

So the buck converter is running at 1MHz. However, the LEDs are still switched ON and OFF by the DIM pin, which has a max PWM frequency of 50KHz. So you're still going to see mostly the PWM frequency across your wires.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

Wawa

I have made custom boards with a PCA9685 and 16x PT4115E LED drivers.
The boards run on 24volt and can drive upto 16 downlights each. Most LED downlights are 12watts@~19volt.
There is ~7.5meters of Cat-6 between the Arduino and two of these boards (I2C protocol and supply).
And the "PT4115E to LED" wiring varies between 3 and 10meters.
I hear nothing on FM radio, and very little on AM radio.
Leo..

MrGlasspoole

7.5 meters I2C - do you use the P82B96?

And little is not nothing :)

Wawa

#8
Dec 22, 2015, 08:43 am Last Edit: Dec 22, 2015, 09:33 am by Wawa
Yes, ~7.5meters of Cat-6.
Just a Mega connected to two PCA9685 chips. Nothing else.
One wire pair for clock and ground, another pair for data and +5volt.
System runs 24/7 without any problems.


Little as in a few birdies on AM radio when dimming, but nothing when the lights are on full, because the LED drivers output almost pure DC without PWM.
I have used a 100n ceramic cap across the LED outputs on the board to try to limit radiation.
Leo..

MrGlasspoole

Ok i did forget to ask about at how much Kbaud?

Wawa


MrGlasspoole

Thats cool.

Since you know the PCA9685 maybe you can answer this:
http://forum.arduino.cc/index.php?topic=367229.0

Wawa

The source/sink part?

Sourcing is when the PCA powers the LED (LED connected to ground).
Sinking is when the PCA switches a powered LED to ground (LED connected to supply).

Arduino's 10-bit PWM sucks at dimming home lighting LEDs to a low level (course steps).
But the 12-bit PCA9685 has smooth dimming to very low light levels.
I use a library with 255 lineair brightness steps (12-bit CIE conversion) .
https://github.com/brunnels/PCA9685
I think Adafruit also has a library.
Leo..

MrGlasspoole

Ok after clarifying that long cables from drivers to LEDs and also I2C can work out of spec...
What about PWM from the Arduino directly to 20mA LEDs or the drivers?

If this also works I'm free with the Arduino placement and can layout everything in a way that i can save money on cables.

Wawa

Ok after clarifying that long cables from drivers to LEDs and also I2C can work out of spec...
What do you mean with out of spec.

From the datasheet:
The PCA9685 is in the new Fast-mode Plus (Fm+) family. Fm+ devices offer higher
frequency (up to 1 MHz) and more densely populated bus operation (up to 4000 pF).

Cat-6 is 53pf/meter.
Leo..


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