Hi, I'm using a PCA9685 with a pinout board like Adafruit's. I'm trying to run a bunch of LEDs at 20 mA, and the chip is supposed to be able to source 10 mA and sink 25 mA. I understand that to have it Source, you connect the LED between the PWM and GND pins and to Sink you connect it between V+ and PWM.
I've tried both ways and using a multimeter and a red LED I get 12.5 mA. Why is it not 25 in Sink? Also, why do I get 12.5 on Source mode as well?
Another question: Using an LED of a different wavelength gives me a different current output, say a green LED gives me around 9 mA. Why are these LEDs not reaching 10/25?
Last question: I know the board has 220 Ohm resistors with each LED output. Does the original chip without the board also have these? Are these what limit the current, and if so, can I get around that?
I've tried both ways and using a multimeter and a red LED I get 12.5 mA. Why is it not 25 in Sink? Also, why do I get 12.5 on Source mode as well?
Because those figures are limits in the way that you should not exceed them. The current in either mode is determined by the resistance of the load. So when you source current you are overloading your chip and will damage it if you continue.
Another question: Using an LED of a different wavelength gives me a different current output, say a green LED gives me around 9 mA. Why are these LEDs not reaching 10/25?
Because an LED has a voltage drop that is dependent on the colour. You subtract this voltage drop from the output voltage and use what you have left to calculate the current by using ohms law and the seriese resistance.
I know the board has 220 Ohm resistors with each LED output. Does the original chip without the board also have these
No.
Are these what limit the current, and if so, can I get around that?
Yes that and the voltage drop of the LEDs.
You can get more current by using the output to drive another FET, which you then use to control your LED. Note this will invert the output.
At these voltages the multimeter will not give you an accurate reading, it measures current by the voltage drop of its internal resistor. The best way is to measure the voltage drop across your led resistor and with Oh,'s law you can easily calculate the current. You may want to also measure the resistance of the resistor to be sure of its value.
Thanks for all the responses guys, they've been super helpful!
My intention for this project was to use the datasheets' values at 20 mA as that's their typical test point. So without getting extra FETs, this is the highest current I can get from each pin? It sounds like I should be sure at least to run on current sink mode to not damage my chip then.
So the chip on itself has no resistors which I need for my current project, but it's too small to actually prototype with and I don't know how to code with I2C, so I got the Adafruit version that's a breakout board with libraries. But this one already has resistors on it and won't let me reach above a certain current depending on that resistor. Would it be a totally stupid idea to try and put solder over the resistors on the breakout to create a short, and put my own resistor on the prototype board to achieve the current I'm looking for? Or how would I go about using FETs to increase current, please? Sorry if these are stupid questions.
So the chip on itself has no resistors which I need for my current project,
Correct.
Would it be a totally stupid idea to try and put solder over the resistors on the breakout to create a short, and put my own resistor on the prototype board to achieve the current I'm looking for?
Not totally stupid but a better idea is to remove the resistors and replace them with your own value.
Or how would I go about using FETs to increase current, please
You add a pull up resistor of about 1K from output to the supply voltage. Then connect the gate of a logic level FET to the output pin, the source of the FET to ground, and finally the drain to the LED and resistor going up to the supply voltage.
Clearly the board is designed with current limiting resistors chosen to give a sensible and conservative output current to LEDs.
If you are achieving 10 to 12 mA per LED, why do you feel the need for a trifle more brightness? Are you using the LEDs in outside daylight or to light a room?
@Grumpy_Mike: thank you for that explanation! Could I possibly ask if you know a link for an illustration or example schematic?
@Paul__B: It's not for lighting per se, rather I've selected a specific set of LEDs to measure their individual and combined light spectrum in reference to other standards. I want to achieve high currents (obviously having checked these LEDs datasheets intimately to not exceed any maximum ratings) as I require a certain irradiance. That means I need to run the LEDs at higher currents than visual needs require.
I've selected a specific set of LEDs to measure their individual and combined light spectrum in reference to other standards.
The combined spectrum will just be a simple addition of the individual spectrums.
We did have someone some years ago that thought you could get other spectrum lines when you combined the RGB LEDs and some how do a spectrum sweep, but that is only a visual illusion, in practice you just get the three emission peaks in different combinations of intensity.
