Smooth LED fading via PWM

[HenrikOlsson]

So why not turn it off (0% dutycyle) when you want it to be off? Why do want to drive current thru it and NOT have it light up at all?

[HankMcSpank]

So why not turn it off (0% dutycyle) when you want it to be off? Why do want to drive current thru it and NOT have it light up at all?

Because I seek a smooth fade from max brightness through completely off.

What I'm observing is that a duty cycle of 1 seems too discernibly 'on' still (I'd have thought you'd have to get up real close to it at such a duty cycle to see if it is on or not?), the 'jump between 0 & 1 from a brightness perspective, whilst small, is still too big which is why I'm thinking more resolution may help?

(before you think why don't I just try it myself ...well, firstly I want to ask others if when using 256 steps of pwm, whether a duty value of 1 normally easily visibly shows the LED to be on.....and secondly, I've got to wrap my head around 10 bit HPWM bit mapping - the pesky registers are spread a little).

[HankMcSpank]

Just to give you a flavour, this is a duty setting of 1...



Now LED3 & LED6 happen to be faulty (I can't be bothered to swap them out right now - they're way duller than others and clearly have an issue), but I reckon with a duty cycle value of 1 the led brightness should actually be more their level!

Now LEDS are notoriously difficult to photograph...the LEDS don't actually as bright as they do in the photo 'in the flesh', and these being high brightness have a lens that accutely magnifies any LED light but nevertheless I think they're too bright for a duty value of 1!

[HenrikOlsson]

Ah, I misunderstood - again...
Yes, more resoultion might help but I honestly think the problem is the lack of current limiting resistors. Can you just give it a try, something lowish like 47ohms or so.

/Henrik.

[HankMcSpank]

Hi Henrik,

As you can see, the board I'm working on a pcb I knocked up ....so a bit of hacking involved, but I'll give your suggestion a go tomorrow....I've just tried 9 bit PWM which was better but a duty value of 1 is still too visible.

To my other question - does anyone know how to turn 512 linear values into log values (so I can put them in a LUT)

[cncmachineguy]

Hank, how fast is the "1" pulse hitting the LED? think of it this way, what is the duration of that single pulse? at what value does the LED max out to the eye? I don't think the issue is number of steps, but rather the duration of the step.

[HankMcSpank]

Hi Bert,

I'm using a HPWM frequency of 31.25KHZ (8Mhz clock & HPWM PR2 value of 63), so I'm figuring a duty value of 1 should only result in a pulse width of approx 125ns ?