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

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.

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.

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.

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

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