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Low Voltage Warn on E-Bus instead of Main?

 
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scott(at)randolphs.net
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PostPosted: Tue Feb 14, 2006 4:03 pm    Post subject: Low Voltage Warn on E-Bus instead of Main? Reply with quote
The Z13-8 diagram shows the low voltage warning light connected to the main
bus. It seems to me like connecting it to the E-bus would be slightly more
useful. In that configuration, it should illuminate if the main alt drops
offline UNTIL the standby alt is engaged. At that point it stop indicating
unless/until the load exceeded the output capabiliy of the SD-8 standby
alternator. In that way it would essentially act as a "battery discharge"
warning (with the inference that if the voltage is below alternator
operating range, then the battery is discharging -- if only through the low
voltage warning circuit!) BTW, what is the current draw of the Low Voltage
Warning circuit? I presume it's very small and driven mostly by the
light/LED used.

Does anyone see a problem with this configuration? Does it miss a design
consideration that motivated it's original placement?

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deej(at)deej.net
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PostPosted: Wed Feb 15, 2006 10:44 am    Post subject: Low Voltage Warn on E-Bus instead of Main? Reply with quote
Scott wrote:

Quote:


The Z13-8 diagram shows the low voltage warning light connected to the main
bus. It seems to me like connecting it to the E-bus would be slightly more
useful. In that configuration, it should illuminate if the main alt drops
offline UNTIL the standby alt is engaged. At that point it stop indicating
unless/until the load exceeded the output capabiliy of the SD-8 standby
alternator. In that way it would essentially act as a "battery discharge"
warning (with the inference that if the voltage is below alternator
operating range, then the battery is discharging -- if only through the low
voltage warning circuit!) BTW, what is the current draw of the Low Voltage
Warning circuit? I presume it's very small and driven mostly by the
light/LED used.

Does anyone see a problem with this configuration? Does it miss a design
consideration that motivated it's original placement?




How would you know if the main buss went offline?
If you happened to be looking out the window in the brief
time that it might light until the standby alt kicked in
automatically, you might never see the warning.

I suppose you could hook the low voltage warning "sensor" to a switch,
initially monitoring the main buss, and then you manually
switch it to the E-buss and thus "acknowledge" the warning.

-Dj

--
Dj Merrill
Glastar Sportsman 2+2 Builder #7118

"Many things that are unexplainable happen during the construction of an
airplane." --Dave Prizio, 30 Aug 2005

"TSA: Totally Screwing Aviation"

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scott(at)randolphs.net
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PostPosted: Thu Feb 16, 2006 10:05 am    Post subject: Low Voltage Warn on E-Bus instead of Main? Reply with quote
Dj,

My thought was that activating the standby alternator would be a manual
process so the light would stay on until pilot action was taken. As Bob
points out, if _both_ alternators fail, I'd get a continuosly blinking
light, but I really hope both don't fail on the same day! I suppose I could
use the low voltage detection to activate the standby relay, but then it'd
kick in anytime the engine wasn't spinning (ie: before start when it'd burn
a little current in the coil). Hmm. Maybe I'll give some more thought to
that tradeoff. Automatic or manual activation? What have others done and
why?

A big question, though, is would the diode drop Bob mentioned be enough to
generate a false indication of low voltage if I were sensing the E-bus. Is
the aeroelectric part "setable" or forgiving enough to account for that? If
not I guess I'd be compelled to go with switching in front of the low
voltage warning if I want it to see the E-bus when the main is offline.

Thanks for the food for thought.

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