Smooth LED fading via PWM

[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)

[Ioannis]

For this specifically LED it might work and as long as the voltage is absolutely stable.

But I insist on using a current source. You will be able to control the brightness for a really 0 to 100%.

Even for a test it is interesting.

I have a client that for more than a year is refering to his power (chinese) LEDs as 24Volts... And I reply with the current of your LEDs is....

Ioannis

[HankMcSpank]

Just going off tack a little (still intend breadboarding this up so I can easily inster leds to suit - but for now stuck with my PCB).

I want the PIC pins to sink surrent, therefore I rejigged everything to have all the LED cathodes pointing at the pin, with all the anodes commoned and a P channel mosfet switching 3.2V to the anodes (via HPWM on the mosfet gate).

Something took me aback a little, with a duty value of 255, my leds were still *just* lit (remember we are talking reverse now, so a value of 255 is essentially 0), I changed the duty value to 256 & then the leds switched off ...therefore for all I thought I was using 8 bit/256 values (PR2=63 @8Mhhz, no pre/post ) I was actually using 257 values (and therefore a value 0f 255 sorta equalled 1)...this sucks cos obviously 257th value needs 9 bits! Now I know 255 is almost 256 so as not to worry that much....but is there any way of setting up the HPWM so that 100% duty = 255 exactly?

(BTW I've since inverted the PIC's HPWM ouput so now a value of 0 = the supply voltage & 255 = 0, which has solved my problem of the LEDs not totally extinguishing at a value of 255)

[HenrikOlsson]

Hi Hank,
I would think PR2=64 would make the resolution an even 8 bits, have you tried that?

/Henrik.

[HankMcSpank]

No I haven't tried it, but according to Mr E's multicalc, a PWM PR2 value of 64 yields @8Mhz gives 259 steps, with a value of 258 yielding a duty of 99.23%

Unless I'm missing something obvious (highly likely!), I'd have thought it'd be simpler to set a HPWM PR2 number that gives exactly 256 steps (therefore a max duty value of 255)

[HenrikOlsson]

Hi Hank,
Hmm, you might be right. It just seems that a PR2 of 64 would be more "even" since 64*4=256. On the other hand, 63 is 111111 in binary and if we stick the two lower counter bits to that we 11111111 which is 255 and then a dutycycle value of 255 "should" be 100%.

You are "splitting" the dutycycle value, putting the 6 high bits in CCPR1L and the two low bits in CCP1CON.5 and .4 right?

/Henrik.

[HankMcSpank]

Hi Hank,

You are "splitting" the dutycycle value, putting the 6 high bits in CCPR1L and the two low bits in CCP1CON.5 and .4 right?

Yes I am. Here's what I'm seeing...

PR2 = 62 ....... 253 is final value (giving 254 steps ....0-253 etc)
PR2 = 63 ....... 256 is final value (giving 257 steps)
PR2 = 64 ....... 260 is final value (giving 261 steps)

I'm rather surprised that there's no obvious HPWM setting which would give exactly 256 steps (0-255), but I guess for most, 255 is as near 256 so as not to worry so a setting of PR2=63 should suffice.