Smooth LED fading via PWM

[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 ?

[Ioannis]

1. Bright LEDs light even with nA current. I had a 5 meter LED stripe that some of the LEDs were ON while not connected to any power source!!! (Yeah, from static charge).

2. LEDs are not like lamps, so you cannot control them by a simple PWM idea. LEDs are current dependant devices so, you have to control them by current source and NOT by voltage as you do now.

My suggestion is not to bother with more resolution an try to control, by PWM, the current of the LEDs. Then you will have the exact same effect like simple filament lamps.

Ioannis

[HankMcSpank]

1. Bright LEDs light even with nA current. I had a 5 meter LED stripe that some of the LEDs were ON while not connected to any power source!!! (Yeah, from static charge).

I reckon this is the issue here....high brightness leds, with a lens that intensifies even the smallest amount of light = looks too 'on' even when almost off!

LEDs are not like lamps, so you cannot control them by a simple PWM idea. LEDs are current dependant devices so, you have to control them by current source and NOT by voltage as you do now.

Yeah, I know led's are current driven devices, but what gives...if the rated forward voltage is say 3.4V, but I run them at 3.1V regulated (because they yield sufficient brightness even at that voltage level)..then PWM principles ought to work fine (in fact, PWM does work just fine without the resistor...I'm running it here now!)....& there's always the internal series resistance of the PIC OP pin (5 ohms? not a lot, but resistance nevertheless)

[Charlie]

1. Bright LEDs light even with nA current. I had a 5 meter LED stripe that some of the LEDs were ON while not connected to any power source!!! (Yeah, from static charge).

Static charge is by nature static and won't power LEDs. Do you have a wireless router? There is a known issue with early LED Christmas lights illuminating by acting as rectifying antennae. I have a set - turn off my wireless router and they extinguish. Of course, you might live next door to a ham operator as well... but walking across the carpet won't do it.