Message-ID: <6746aa2e@news.ausics.net>
From: Computer Nerd Kev <not@telling.you.invalid>
Subject: Re: Anybody Seen a Simple LED "Fail-Over" Circuit ?
Newsgroups: comp.os.linux.misc
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Rich <rich@example.invalid> wrote:
> Computer Nerd Kev <not@telling.you.invalid> wrote:
>> Rich <rich@example.invalid> wrote:
>>> 
>>>          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".

You can get "fusible resistors" that are supposed to fail
open-circuit if overloaded.

>> 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.

Depends on the details. Say you have flashing warning lights
driven by a microcontroller which also has spare remapable ADC
inputs: You could add a capacitor in parallel with the LED+resistor
and switch the micro's pin from HIGH digital output into ADC input
mode to turn the LED off. While the light fades from on to off,
measure the discharge of the capacitor - too fast means a short,
too slow means open-circuit. Yet there's only one more component
per LED if you already have a suitably capable microcontroller
there.

For traffic lights to look normal, you could flash so quickly that
it's not noticable to the eye (if you've got surplus brightness).

Now the problem is that capacitors tend to fail short-circuit more
often than most other common components including LEDs. So you can
detect the failure, but the failure is now more likely.

> 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).

Flashing to indicate a warning instead of turning permanently off
would help there. Need to retrain everyone to use traffic lights
which always have two lights on if applied to that example
application though, so probably not a solution there.

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