Re: Anybody Seen a Simple LED "Fail-Over" Circuit ?

Computer Nerd Kev <not@telling.you.invalid> wrote:

Rich <rich@example.invalid> wrote:

186282@ud0s4.net <186283@ud0s4.net> wrote:
LED's are, at a low level, 'current' responsive lights.  Driving them
with a current source is the best way to drive them.
 

Searches really don't bring up much here.
 
Yea, there are more complex solutions ...  but what can be done with
the fewest, simplest, most robust parts ?

 
fewest, simplest, most robust -- you get to pick two....
 
The simplest (if you can assume the upstream power supply will be
functional [1]) is to drive each in parallel with their own current source
(fixed current driver). I.e.:
 
         PSU
          |
  +-------+-------+
  |               |
driver          driver
  |               |
 LED             LED
  |               |
  +-------+-------+
          |
         Gnd
 
 
Then if one led (or its driver) fails, the other continues to operate,
because it does not depend upon the first one.

 
Unless the driver chip fails short-circuit, causing the PSU to shut
down power to both drivers.

fewest, simplest, most robust -- you get to pick two.....

But this is far from 'fewest' parts, as you need one driver per led. 
While some driver chips can be had for pennies each in 1K quantities,
that still adds to the BOM cost in the end.

 
If the PSU has regulated voltage output, or LED brightness can vary
with the supply voltage (such as from a battery), then a resistor
would do instead of the LED drivers.

Yes, and you still have the same potential for a possible "fail short"
with a current limiting resistor, which would then drive that led with
too much current.  And if it happens to fail short when overdriven too
much, you are back to your 'fail short' for the "drivers".

Not strictly Linux, but we DO sometimes wanna drive external
displays.  Usenet electronics groups ...  dismal at this point.
 
LEDs are great, but never "forever".  They DO fail - but for some
safety apps you can't just HAVE things go black.

 
Most LED's that fail do so because they are being driven hard [2]
(right at the limits that they are rated for, if not well beyond
sometimes).  If you derate your drive by a fair amount you'll find they
do, in fact, appear to last nearly forever.  But then you will need
more LED's for an equivalent amount of lumens of light output.

 
Or use high-brightness LEDs where low-brightness ones would
normally be sufficient at their maximum current.

Or, the meaning of "derate" when doing component selection.

But unless there's something specific that makes these LEDs more
likely to go wrong,

Over driving them too hard, and insufficient heat dissipation for
whatever drive level you choose for them, are by far the big two
reasons for them to go wrong.  Must every other failure will either be
infant mortality or very very rare occurrence possibilities. 

And, note, there is a huge overlap in "over drive" and "insufficient
heat sinking" such that over driven LED's are often also insufficiently
heat sinked for the actual amount of heat the driving will generate. 

If you don't try to squeeze every last ounce of "performance" out of a
given LED, and you adequately remove the waste heat, they will (at
least everything but Shenzen junk) last nearly forever.

I'd expect the drive circuitry and wiring to be as common a point of
failure as the LEDs themselves.  To detect
open-circuit/short-circuit, you could pass a small current through
them and use that to tell whether the LED is OK (current is correct
for the LED's forward voltage drop specification), triggering a
single bulb-failure warning if it's not (possibly simpler in practice
than duplicating every LED on a display panel, even if the total
number of components is similar).

Yes, you could design a "detector" that could detect open/short for the
LED and/or its driver.  But then that means you've excluded "fewest
parts" (at least) from the design selection criteria.  And, depending
upon how 'robust' you really need to be, you'd need to detect failures
of the detection circuitry itself as well.

Another commenter's statement of inverting the indicator, where "on"
means "situation normal" and "off" means "abnormal" is probably the
absolute simplest way to go.  But then the "LED indicator" fights human
psychology that senses a new stimuli appearing in the environment (lamp
turning on) far more readily and quickly than noticing that a continual
low level stimuli has disappeared (light has gone out).