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fullspeceng
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Date Joined Aug 2009
Total Posts : 36

Posted 11/6/2009 3:59 PM (GMT -8)

I bought some low power leds from Digikey that use 2mA each.

Check them out.

PJ Allen
Counsel of Despair ???

Date Joined Sep 2005
Total Posts : 2807

Posted 11/6/2009 8:53 PM (GMT -8)

Rayman said...
want to drive each at their pulsed limit of ~100 mA at 1/8 duty cycle. That's only 12 mA...

I'm sorry but, PWM doesn't effect the current this way -- pulsing 100mA with 0.125 PW doesn't get you, somehow, 12mA.

You still have that same 100mA, just not continuously.

Rayman
Registered Member

Date Joined Jul 2007
Total Posts : 2408

Posted 11/7/2009 4:46 AM (GMT -8)

If you pulse fast enough, the observed LED brightness is the same between 100 mA at 1/8 duty and 12.3 mA continuous..

It's the average current that matters...

My Prop Info&Apps: http://www.rayslogic.com/propeller/propeller.htm

PJ Allen
Counsel of Despair ???

Date Joined Sep 2005
Total Posts : 2807

Posted 11/7/2009 5:11 AM (GMT -8)

The comparative brightness (dimming) from PWM is the result of an effect along the lines of persistence of vision, in this case an inability of human vision to perceive short pulses; it's not attributable to some "average current" phenomenon. If it's 100mA at 100% duty then it's 100mA at 1% duty, too.

localroger
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Date Joined Mar 2009
Total Posts : 417

Posted 11/7/2009 6:04 AM (GMT -8)

While it's true the instantaneous current is 100 mA even at 1% duty, it is also important to know why the limit exists in the first place. Semiconductors rated for 30 mA don't magically blow up the first nanosecond they see 31 mA. In the case of an output pin it's generally going to fail because the traces or switching element overheat, and that takes time. So if that's the failure mode du jour, just as you perceive limited brightness you will generate limited heat in the circuitry with PWM, and you can push the part beyond its on-paper rating.

Pretty much the same thing applies to clock speed, since the transistors only dissipate serious power while changing state and the more often you change state the more heat they produce, and people have pushed their props as high as 120 MHz by taking other measures to limit power dissipation.

Rayman
Registered Member

Date Joined Jul 2007
Total Posts : 2408

Posted 11/7/2009 6:17 AM (GMT -8)

If you look at the datasheet for the RGB matrix there is actually a pulsed rating for 1/10 duty cycle... It's 150 mA for Red&Green and 100 mA for Blue...

My Prop Info&Apps: http://www.rayslogic.com/propeller/propeller.htm

PJ Allen
Counsel of Despair ???

Date Joined Sep 2005
Total Posts : 2807

Posted 11/7/2009 6:20 AM (GMT -8)

You and I aren't in disagreement, localroger. I brought up the max current leeway in the 5th Reply to this Subject. What's unknown is how far that can be pushed. As always, when one exceeds the manufacturer's spec it's strictly his tough****. If anyone has an "argument" then it's with physics and not with me.

localroger
Registered Member

Date Joined Mar 2009
Total Posts : 417

Posted 11/7/2009 3:40 PM (GMT -8)

PJ, it seems to me that this sort of thing must be within the design intentions for the P8X32A; you're right that whenever you stray offspec you're guessing, but I'd bet Chip Gracey could give us a pretty exact idea of what is possible here (but he's probably too busy designing the P2 to weigh in on LED lighting :-( )

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