|12-30-2012, 03:52 AM||#31|
Join Date: Oct 2012
Device: finally, a Glo!
But if not, then you have a voltage drop across PWM (either pin directly or MOSFET like you say) and the entire power saving idea of PWM is ruined :/
In the extreme case, you might as well replace PWM with a variable resistor and you'll end up with roughly the same voltage drop, only constant instead of pulsing.
Like I said initially: it's all doable, but there is a bunch of "but"s.
This is why I'd approximate the practical filtering effect by peak-to-peak measurement: if a design doesn't reduce p-p, it doesn't smoothen anything.
The initial talk of classic RC filter was better in this regard: it was leaving DC component intact, but was sinking all higher frequencies into a resistor.
The resistor is there to solve it: like you said, current running through a LED is I = (E - LEDvdrop) / R (for E >= LEDvdrop).
Without R (R=0) this formula resolves to:
- infinity for E > LEDvdrop (ie something burns)
- undefined for E = LEDvdrop
- zero otherwise.
So, this resistor is there to decide what the current is (at each duty cycle), not to protect the diode under some duty cycles but not others.
[this uses this simple approximation of a diode having constant voltage drop. The truth is exponential, so not quite like it, but not far either]
[also I would like to apologise for off-topic-ness, but geeky discussions are always fun ;p]
Last edited by sysKin; 12-30-2012 at 04:43 AM.
|12-30-2012, 03:48 PM||#32|
Join Date: Oct 2012
Device: Kobo Glo
hmmm...this sounds familiar (read page 1)
Last edited by WT Sharpe; 12-30-2012 at 06:55 PM. Reason: Hidden spam link removed from quote.
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