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Topic: Methods to bias/power 3W LEDs (Read 2180 times) previous topic - next topic


I did a quick search for the PT4115 chip and it looks like if you add a 100k pot with the wiper arm going into the "DIM" pin of the 4115 with the PWM signal from the PCA board....and also a capacitor to isolate the DCs from the PCA and the 4115....or is there more?


Dec 13, 2017, 10:13 pm Last Edit: Dec 13, 2017, 10:42 pm by Wawa
Still I would like to know more about this
Both boards are basically the same.
The small board has the PT4115 in a smaller package, and it has a bridge rectifier added because it is meant to go inside a LED relacement for a 12volt AC powered halogen bulb.
If you want to use them, you need to remove the four diodes in a row (the bridge rectifier), and solder two power leads (or pins) directly to the capacitor + and -.
The two "halogen light" prongs can also be removed.
The last thing is adding a dim wire to the dim pin of the PT4115 (the dim wire goes to the PCA9685 PWM output).
PCA9685 ground has to be common with LED power supply (you can also use a ground wire between LED driver ground and the PCA ground pin strip).
This Youtube video shows how to use pins and vero board instead of wires.

Another option is the PicoBuck LED Driver from Sparkfun (three drivers in one, with 330/660mA selection).
"IN1 IN2 IN3 GND" goes to three PWM outputs and the ground strip of the  PCA9685.
VIN is the supply, and the OUTs go to the LEDs.


Thanks Leo!  You are a great resource for me...I will try the mods to the existing boards I have.


The board layout is different on the boards that I have (see pics), from the ones in the video.

I will try to figure out the layout on my boards to make the mods they describe in the video....Thanks



So I looked at the attached data sheet on the 4115 chip.  In the PDF document there is a "typical application" schematic that is probably the same circuit, just different board layouts.  If this is so, then I don't think it will be necessary to remove the bridge rectifier diodes, because with the DC power applied, they will be reversed-biased all the time.

.....or maybe I am missing something......


Dec 15, 2017, 08:08 am Last Edit: Dec 15, 2017, 08:12 am by Wawa
Yes, you could leave the diodes and connect LED power directly to the big capacitor.
Easier soldering wires on with the diodes and pins removed though.
Measure before you solder wires on. There are different versions of those boards.

The diodes have some volt drop, and that can be problematic if you want to use strings of three LEDs and a 12volt supply. There won't be much left for the chip to do it's thing.
So don't connect power to the diode bridge (the big pins).

You can test the LED driver by just connecting power and the LED.
The driver will be fully on with nothing connected to the dim pin.

I think I did mention that the low value current limiting resistor sets LED current.
Formula is 100/value (in mA). 0.3ohm is common (333mA)

Another option is an old 19volt laptop supply to power the LEDs.
You can use strings of five LEDs per driver.
I use 24volt/7or10Amp supplies from ebay (strings of 6 LEDs).


Yes I see now that it is cheaper to use 24 VDC supplies when using these ICs. 

In your experience are the DC power supplies that are everywhere on eBay reliable?


Dec 15, 2017, 08:50 am Last Edit: Dec 15, 2017, 08:51 am by Wawa
I have two cheap/ebay 24volt/7Amp metal frame supplies powering my basement lighting (20+ 12watt lights).
Supplies are on 24/7 for almost two years now. Lights are not always on though, except for the security ones.
7Amp, because they are fanless. Bit of coil whine, but that's normal with switching supplies.
Can't hear that, because they are inside a case.

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