John Larkin wrote:
On Thu, 14 Jan 2010 12:39:35 -0500, ehsjr
wrote:


John Fields wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:





If you use 14 volts to run a 3.4 volt LED, with a dissipative system,
the efficiency is always around 24%. Different circuits just move the
heat around.

With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.


---
Yes, but if one fails shorted the second one will follow soon after if
its If(max) is exceeded, while if one fails open two will go out so
you're trading efficiency for reliability.

JF

I have a problem - my lack of knowledge - with running the
LEDs in series. I don't know aircraft rules/regs, or whether
there is ever a time you would want individual on/off control
of instrument illumination. So I want each LED to be able
to be switched on/off to accomodate that possibility, which
rules out series connection.


His posted schematic didn't have switches.

Yes, I saw that. Not knowing aircraft, I didn't/don't
know if that is an oversight or whether all instruments
are required to be illuminated all the time.




My next problem is the same that you have in mind - if one
LED fails you lose two lights. Not understanding what I'll
call "cockpit dynamics", I would be concerned about that series
design. If I'm a passenger on that plane, I want the pilot
to have 100% of his instrumentation available, 100% of
the time.


Panel backlights usually have overlapping so that any region is lit by
at least two lamps. If one goes out, a zone may be a bit dimmer but
not invisible. If led's are used in series, just locate them so that a
string failure doesn't blank any region, just dims some.

One common structure is a clear plastic plate with a white reflective
layer on both sides, then a black layer on top. Lettering is engraved
through the black on top. Multiple lamps are recessed into the back
side, and they scatter light all over the place, so no area is lit by
just one lamp.

Ah - that makes sense, and eliminates most concerns I had.
It also means that all the instruments will be lit so
switching individual LEDs on/off is not a factor. Thanks!

Ed



Actually, I'd prefer two complete systems - regulators,
PWM, whatever with two LEDs per instrument, one fed by
system A and one fed by system B. That way, it would take
two failures for an instrument's lighting to fail completely.
I'm guessing that if the 14V bus goes down, you have bigger
problems. Maybe I'm just being a nervous Nelly because
I have no knowledge of flying a plane.


A flashlight is the backup.

Do small planes still use dual magneto ignitions?

John