Different voltages in PWM

Using Arduino LEDs and Multiplexing
#1

Hi!

I want to power a 3W RGB led using tiny PWM drivers I've got (based on PT4115 chip). But it's PWM pin works from 0 to 2.5V. Anything beyond 2.5V just remains at 100%.

So, is there a way for arduino to output half it's 5V voltage on the PWM pins? I know it works if I analogwrite 128 instead of 255, and that's what I'm doing now, but I loose half the variation.

Also, my little driver is for 600mA and each of the colors is rated at 300mA.

So I'm already testing it in an normal 3W white led at half PWM cycle to get to the full current the driver is set. So I'm already at 128 to get this normal 3W non-rgb led to full bright. (PWM 128 to get 2.5V to get 600mA. Anything beyond 2.5V stays at 600mA).

So to apply it to my 3W RGB led (300mA for each color) I would need to go to 64, instead of 128 steps to get 300mA for each color.

Is there a (simple) way to have it 0-255 in 2.5V, so I keep at least 128 steps to choose colors from? Because the way my set-up is done, I would have 64 steps only for each color to choose from, in my 600mA driver. Changing the driver is definitely an option, but it will take maybe months for it to arrive.

Thanks for the help!

#2

Forget about voltage.

3.3volt or 5volt PWM switches the PT4115 rapidly fully on/off.
An analogWrite value of 128 means 50% of the time full on, and 50% of the time full off.
It goes so fast the we see it as a dimmer light (about 2/3 of max brightness).

A 600mA driver sends 600mA through the LED, whatever PWM value you send to it.

LED current is set by an smd resistor. About 0.15ohm for a 660mA driver (2watt per led in the string).
Replace it with a 0.3ohm resistor, and the driver is 330mA (1watt per LED in the string).
Leo..

#3

Thanks! In this driver there was two smd resistors soldered on top of each other, in paralel. I removed the top one, R330 and left the R300 and the current lowered to 350mA.

Thanks!!

#4