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'[EE] another LED drive question'
2011\12\21@153439 by John Gardner

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Is driving a LED from more than one pin acceptable practice?

Assuming pin & device specs are respected, of course, and
with current-limiting resistors in both legs.

App is low duty-cycle pulses.

Jac

2011\12\21@185745 by IVP

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> Is driving a LED from more than one pin acceptable practice?

Don't see why not. I've a product which has three pins paralleled
to supply Vcc for a little RF transmitter when the PIC comes out
of sleep. Best to turn the pins on/off all together with a MOVWF
rather than a series of BSF/BC

2011\12\21@192351 by John Gardner

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Thanks, Joe. That's what I had in mind

2011\12\21@192657 by John Gardner

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Out of curiousity, does your design use resistors in the
three legs

2011\12\21@193938 by IVP

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> Out of curiousity, does your design use resistors in the
> three legs?

No. I probably should if I was concerned about a failure that
could fry the pins, but I need the voltage up to get transmission
range. If I was driving a LED I would though, as a voltage drop
wouldn't matter. Do LEDs ever fail short ?

Jo

2011\12\21@195626 by John Gardner

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Makes sense. I was thinking about equalization resistors in
battery packs, but it's a completely different kind of load.

Don't remember ever seeing a shorted LED. Russell has,
I bet...  :

2011\12\21@202406 by IVP

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> Makes sense. I was thinking about equalization resistors in
> battery packs, but it's a completely different kind of load.

If you wanted to share the load as equally as possible wouldn't
that entail constant-current limiters

2011\12\21@204204 by John Gardner

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We're talking < 10 uS pulses; I figured ignorance would prevail,
but asked anyway...  :

2011\12\21@214114 by RussellMc

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> wouldn't matter. Do LEDs ever fail short ?

I see JG anticipated my response.
Yes, LEDs do sometimes fail short.
And on occasion a failed 5mm LED will fail hard-short  enough to fry a switch.
A manufacturer was meant to put series resistors in series with 6 LEDs
that were in parallel.
They instead used a common series resistor. Very naughty.

Occasionally a single LED would go short and occasionally this
resulted in magic smoke from the switch. Which suggests that it was
more closely rated than it ought to hve been.



Russel

2011\12\22@002046 by Justin Richards

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> Yes, LEDs do sometimes fail short.

I cant recall every replacing a failed LED or noticing a failed LED in
any equipment.  The only LEDs I have know to be failed are those that
I cruel to when I was young

2011\12\22@003706 by Sean Breheny

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At work we used to have some LEDs fail in a product and it ultimately
turned out to be ESD-related. However, before we knew that, we did a
test where we connected 50 of the same model of LED in parallel but
with individual series resistors (one per LED). The resistors were
sized so that one batch of 10 was running at max rated current, the
next batch of 10 was 25% over, the next 50% over, the next 75% over,
and the last batch of 10 100% over (200% of rated current). We ran
this test for 2 years continuously. None of the LEDs run at max rated
current failed. All of the twice rated current ones failed - and I
think they failed within the first year with some failing within a
couple of months. I don't think that any of the 25% over batch failed,
but some of both the 50% over and 75% over failed.

If I recall correctly, some failures were open circuit. Some of the
LEDs just got much dimmer than normal. Others would flicker on and off
and it seemed to be related to a loose connection internally to the
LED because you could affect it by putting stress on the leads of the
LEDs.

These were large (about 1 cm diameter) blue LEDs with a max continuous
rated current of, I think, 25mA.

So, the moral of the story is that even low-power indicator LEDs will
fail eventually when run at even 1.5x their max rated current.

Sean


On Thu, Dec 22, 2011 at 12:20 AM, Justin Richards
<spam_OUTjustin.richardsTakeThisOuTspamgmail.com> wrote:
>> Yes, LEDs do sometimes fail short.
>
> I cant recall every replacing a failed LED or noticing a failed LED in
> any equipment.  The only LEDs I have know to be failed are those that
> I cruel to when I was young.
>

2011\12\22@005444 by Justin Richards

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> So, the moral of the story is that even low-power indicator LEDs will
> fail eventually when run at even 1.5x their max rated current.
>
So it appears they rarely fail if run at 1.0x or less of their rated
max current

2011\12\22@012409 by Sean Breheny

face picon face
Yes, I agree, as long as there is no ESD problem or other abuse.

On Thu, Dec 22, 2011 at 12:54 AM, Justin Richards
<.....justin.richardsKILLspamspam@spam@gmail.com> wrote:
>> So, the moral of the story is that even low-power indicator LEDs will
>> fail eventually when run at even 1.5x their max rated current.
>>
> So it appears they rarely fail if run at 1.0x or less of their rated
> max current.
>

2011\12\22@021609 by RussellMc

face picon face
> > Yes, LEDs do sometimes fail short.
>
> I cant recall every replacing a failed LED or noticing a failed LED in
> any equipment.  The only LEDs I have know to be failed are those that
> I cruel to when I was young.

I've had a sample of 1 million +++  to observe :-).

As these have passed through small Chinese factories the chances of
people having been unkind to them is high.

NOTE: That is not a racist statement in any form. I'ts a matter of
happenstance). Some Chinese factories are about as good as you can
get. Some are very poor indeed. There are a very very large number in
the very widely spread middle.



      Russell

2011\12\22@022151 by RussellMc

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>> So, the moral of the story is that even low-power indicator LEDs will
>> fail eventually when run at even 1.5x their max rated current.

> So it appears they rarely fail if run at 1.0x or less of their rated
> max current.

That MAY be true but be VERY ware of extending specific examples to
general conclusions.

Chinese made LEDs are a very variable feat - agin, world class through
utter junk.

Most failures occurred when 6 LEDS operated in parallel with a single
shared series resistor (very naughty) were fed with 6 x rated current
notionally shared amongst 6 LEDS.

It's easy to imagine some LEDs getting 2 x rated current in some cases
with this arrangement.

BUT I think I have seen short circuit LEs in a cinstant current fed
series string.


    Russel

2011\12\22@041345 by Forrest Christian

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On 12/22/2011 12:21 AM, RussellMc wrote:
> Most failures occurred when 6 LEDS operated in parallel with a single
> shared series resistor (very naughty) were fed with 6 x rated current
> notionally shared amongst 6 LEDS. It's easy to imagine some LEDs
> getting 2 x rated current in some cases with this arrangement. BUT I
> think I have seen short circuit LEs in a cinstant current fed series
> string. Russell
Wouldn't the LED with the lowest Vf "win" (or I guess perhaps lose if you think about it), and take all the current, at least until the increased forward current caused the Vf to increase to the amount of the next LED?   That sounds like seriously a recipe for disaster.   (I'm assuming LED's behave like other diodes as far as current vs Vf).   With LEDs with poor quality control, I'd imagine the Vf could vary widely, even making this worse.

I know when paralleling normal diodes, you make the same 'fix' - that is you want to add a series resistor on each diode ... to help equalize the differences between the diodes.

-forres

2011\12\22@045107 by RussellMc

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> Wouldn't the LED with the lowest Vf "win" (or I guess perhaps lose if
> you think about it), and take all the current, at least until the
> increased forward current caused the Vf to increase to the amount of the
> next LED?   That sounds like seriously a recipe for disaster.   (I'm
> assuming LED's behave like other diodes as far as current vs Vf).

Lower Vf LEDs take more current but not all current.

At any given voltage across a group of LEDs in parallel each will be
at the same voltage on its Vf curve but the currents at that Vf will
vary by LED. If the sumed Ifs is what is available at that Vf via the
common resistor from the Vsupply which is > Vf then the situation is
(somewhat stabl*e). If not then the operating point will shift until
stability occurs.
Is IF=f can vary substantially with small shifts in Vf it is not hard
to get very high spreads between individual in If this way.

On a solar powered light the effect can be seen easily by removing the
battery, turning o the light and illuminating the PV panel. LEDs light
at different brightnesses and by changing the panel illumination you
can push them up and down their Vf curve and observe the effect. You
can't do this test with 'my' SN2 light as it has a "turn off in
sunlight" feature. but if you could you'd see closer matching at the 6
tight beam LEDs are in series and constant current fed. As are the 3
RoomLight LEDs.



   Russell McMahon

* I say somewhat stable as an LED will heat due to current drain which
will affect its operating point and so affect its relationships with
the other LEDs. Aslow dance can be expected but so slow and so small
in most cases as to not be overly noticeable or important.

2011\12\22@065524 by Chris McSweeny

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On Thu, Dec 22, 2011 at 9:50 AM, RussellMc <apptechnzspamKILLspamgmail.com> wrote:
> * I say somewhat stable as an LED will heat due to current drain which
> will affect its operating point and so affect its relationships with
> the other LEDs. Aslow dance can be expected but so slow and so small
> in most cases as to not be overly noticeable or important.

Though the change in operating point due to heat actually makes things
worse - the Vf of an LED decreases as it heats up, so the LED with the
lowest initial Vf will take more of the current, heat up more, and
decreases its Vf even more, making for a bit of a viscous cycle.
Shouldn't make things hugely worse with paralleled LEDs, but it's
something to bear in mind (and can be a big issue if attempting to
drive LEDs from a fixed voltage with no series resistor).

(I appreciate you know this, Russell, but it's as well to make others
aware who might think that heating would decrease the current as is
typical with a resistor).

Chri

2011\12\22@073712 by RussellMc

face picon face
> Though the change in operating point due to heat actually makes things
> worse - the Vf of an LED decreases as it heats up, so the LED with the
> lowest initial Vf will take more of the current, heat up more, and
> decreases its Vf even more, making for a bit of a viscous cycle.

Yes. (A vicious one as well :-) ).

I see my text was ambiguous.

>> * I say somewhat stable as an LED will heat due to current drain which
>> will affect its operating point and so affect its relationships with
>> the other LEDs. Aslow dance can be expected but so slow and so small
>> in most cases as to not be overly noticeable or important.

In the context I meant "somewhat stable as opposed to stable" but I see
that that could be taken as eg "not as unstable as it otherwise might have
been" :-).

So, yes.
Effectively you y=tend to get thermal runaway, saved mainly by the fact
that when lots of paralleling is done LEDs are usually small eg 5mmso not
too much power per case size. AND wgen people parallel large numbers they
are often somewhat thermally linked either on a pcb or within an enclosure.

In cheap junk lights with a common series resistor OR even no resistor at
all !!! you often see a much brighter LED. This will soon fade and/or die
and it's olace be taken by the next victim in the chain, but usually worse
as there are less LEs to share the same current amongst.



         Russell.






{Quote hidden}

>

2011\12\22@102900 by Mark Hanchey

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On 12/22/2011 2:21 AM, RussellMc wrote:
> Chinese made LEDs are a very variable feat - agin, world class through
> utter junk. Most failures occurred when 6 LEDS operated in parallel
> with a single shared series resistor (very naughty) were fed with 6 x
> rated current notionally shared amongst 6 LEDS.

I went through the chinese LED nightmare last week. These were boards of LED with 12 LED on each board and 6 LED in series  connected to a common connection point on each board. The boards were supplied power by a driver that monitored the current and voltage.  The nightmare began when one LED failed in a shorted state,  .3 ohms measured across it, then another failed in the same way.  I began to replace the failed LED and I did that for 4 of them, then one failed on another board and before I knew it there were more failing than working. They were spec'd at 25ma and the driver fed them 22ma and they still failed. Guess those 100 LED for $5 bargains aren't bargains after all.
Mark

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