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'[PIC]: Output drivers...'
2002\05\19@183554 by Pic Dude

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face
In keeping with my philosophy of minimal component-count in any circuit,
I'm wondering if it is an acceptable strategy to run multiplexed 7-segment
LED's thru a 16F872 output port w/o the use of current-limiting resistors,
when the current I need is the max rating of the output port (25ma source
or sink).

Typically I would calculate the resistor as R = (V-port - V-led) / I-led,
but if I-led is already where I want it, do I still need to insert a
resistor for the required voltage drop?

A little more in-depth, what exactly does the datasheet's "Max o/p current
sourced by any pin" mean?  The max that it can put out, or the max that I
should try to get out of it?

Cheers,
-Neil.

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2002\05\19@192426 by Olin Lathrop

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> In keeping with my philosophy of minimal component-count in any circuit,
> I'm wondering if it is an acceptable strategy to run multiplexed 7-segment
> LED's thru a 16F872 output port w/o the use of current-limiting resistors,
> when the current I need is the max rating of the output port (25ma source
> or sink).

No, bad idea.  The 25mA spec means that is the max you should allow the port
to source or sink, not the maximum it will source or sink when tied to power
or ground or whatever.


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2002\05\19@192826 by Byron A Jeff

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On Sun, May 19, 2002 at 05:33:56PM -0500, Pic Dude wrote:
> In keeping with my philosophy of minimal component-count in any circuit,
> I'm wondering if it is an acceptable strategy to run multiplexed 7-segment
> LED's thru a 16F872 output port w/o the use of current-limiting resistors,
> when the current I need is the max rating of the output port (25ma source
> or sink).

Nope. You're mistaking the given max rating with the highest possible rating.
The max rating is the maximum amount of current you should pull through an
I/O pin. However it isn't the maximum amount of current that can be pulled
through the pin, only the max current that can be pulled through the pin
without permanent damage.

If that were the case no LED would need a current limiting resistor because
it would only pull its max remommended current. But anyone who as ever POOFED
and LED because of a lack of resistor knows better. The LED will try to pull
as much current as is can, and most can easily draw several amps of current
before disintegrating.

And with LED's it's even more tricky because the max current allowed is tied
to duty cycle. Many IR LED circuits are designed to pump an amp or two into
the LEDs, however the duty cycle is held to 10 percent or less. This gives
the LED time to recover and dissapate excess heat.

>
> Typically I would calculate the resistor as R = (V-port - V-led) / I-led,
> but if I-led is already where I want it, do I still need to insert a
> resistor for the required voltage drop?

Yes. and also to limit the amount of current because the LED can only draw
the amount of current that's allowed across the resistor. So if it's a normal
LED with a 1.7V forward drop for example you'd still need (5-1.7)/.25 -> 148
ohm. So use a 150 ohm one and be happy.

>
> A little more in-depth, what exactly does the datasheet's "Max o/p current
> sourced by any pin" mean?  The max that it can put out, or the max that I
> should try to get out of it?

The latter. You do understand then. You can pull 50 or 100ma through a pin.
However you'll burn it out soon enough.

BAJ

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2002\05\19@200926 by Pic Dude

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face
So it will try to source more, pass the design spec on
the chip and destroy the port/chip?  I knew it was too
good to be true. :-(

Thanks much,
-Neil.


{Original Message removed}

2002\05\19@202356 by Pic Dude

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Yep, my LED's require 20mA at 100% duty-cycle, but I'm
operating them at 25% (4 digits).  I know I can go more
but don't have the datasheet -- I feel certain that I
won't damage them with 25ma at 25% duty-cycle.

The fun part is that I just tried it (jumpered across
the resistors to bypass them), and the intensity change
was very difficult to notice, but it seemed to work well.
(Not like I'll actually visualize the excess current on
the port :-) But oh well, I'll behave and keep the R's.

Thanks,
-Neil.



{Original Message removed}

2002\05\20@044830 by uter van ooijen & floortje hanneman

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> A little more in-depth, what exactly does the datasheet's "Max o/p current
> sourced by any pin" mean?  The max that it can put out, or the max that I
> should try to get out of it?

If it is in the 'absolute maximums' section it is the maxium that you can
draw, and afterwards still expect the chip to function within its specs.

If it is in the 'normal' sections it is one of the conditions (for you to
guarantee!) that must be met for the chip to operate within its specs at any
moment.

Wouter van Ooijen
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2002\05\20@045247 by uter van ooijen & floortje hanneman

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> In keeping with my philosophy of minimal component-count in any circuit,
> I'm wondering if it is an acceptable strategy to run multiplexed 7-segment
> LED's thru a 16F872 output port w/o the use of current-limiting resistors,
> when the current I need is the max rating of the output port (25ma source
> or sink).

Why don't you use one resistor for all segments together, and then adjust
the on-time toc achieve the desired same brightness for all displayed
values?

Wouter van Ooijen
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2002\05\20@145923 by Peter L. Peres

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On Sun, 19 May 2002, Pic Dude wrote:

>Yep, my LED's require 20mA at 100% duty-cycle, but I'm
>operating them at 25% (4 digits).  I know I can go more
>but don't have the datasheet -- I feel certain that I
>won't damage them with 25ma at 25% duty-cycle.
>
>The fun part is that I just tried it (jumpered across
>the resistors to bypass them), and the intensity change
>was very difficult to notice, but it seemed to work well.
>(Not like I'll actually visualize the excess current on
>the port :-) But oh well, I'll behave and keep the R's.

Incidentally this can break your program if it relies on reading back the
outputted values anywhere in it, as overloaded ports do not read the
proper set level. Besides exceeding the SOA of the chip and inducing
enough on-die ground and Vdd voltage drops to make other internal
operations more prone to noise and glitches.

Peter

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2002\05\20@150910 by Bob Ammerman

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----- Original Message -----
From: "wouter van ooijen & floortje hanneman" <.....wfKILLspamspam.....XS4ALL.NL>
To: <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Sunday, May 19, 2002 4:42 PM
Subject: Re: [PIC]: Output drivers...


> > In keeping with my philosophy of minimal component-count in any circuit,
> > I'm wondering if it is an acceptable strategy to run multiplexed
7-segment
> > LED's thru a 16F872 output port w/o the use of current-limiting
resistors,
> > when the current I need is the max rating of the output port (25ma
source
> > or sink).
>
> Why don't you use one resistor for all segments together, and then adjust
> the on-time toc achieve the desired same brightness for all displayed
> values?
>
> Wouter van Ooijen

This is sneaky, but in general it may not work very well because the LEDs
will not share the available current nicely. No two LEDs have the exact same
VI curve.

Bob  Ammerman
RAm Systems

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