Re: DC Voltages

From:         ehahn@bass.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         20 May 96 10:13:17 
References:   1
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In article <airliners.1996.668@ohare.Chicago.COM> rwbaker@airmail.net ( ) writes:

> I have a question about the 28VDC used on commercial aircraft.  Are
> there any special or unusual characteristics about this current?  I
> have a need to take the 28VDC and convert it to 12VDC in an aircraft,
> any ideas?

The 28VDC is pretty clean for the most part, with some minor ripple
voltage.  However, it's not uncommon for it to fluctuate slowly
anywhere between 24 VDC to 32 VDC, depending on the type of aircraft,
source of the power, ambient conditions, etc.

Also, depending on the aircraft, you may expect some transient spikes
when aircraft power is switched around from various sources - e.g.
from ground power, to APU-driven generators, to engine-driven
generators.  Also, depending on which bus you draw power from, this
effect may be somewhat mitigated - the battery (essential DC) bus, for
example, will be pretty steady because of the battery system
stabilizing the circuit.

A simple voltage / regulator circuit ought to be enough if you just
want simple 12V output.  However, it also depends on your application
- will this be a certified piece of hardware?  If so, depending on the
criticality (e.g. flight critical vs. essential vs. non-essential),
you may need to demonstrate different levels of robustness and
reliability in the design, to mitigate potential failure modes.

BTW, most modern avionics typically generate their own internal DC for
TRANSISTOR applications from the ship's 400Hz 115VAC (via an internal
power supply).  These systems tend to only use the 28 VDC power for
lighting and annunciation, and some power relay circuits.

Hope this helps,
ed

(ex-airline avionics engineer)

--------   Ed Hahn | ehahn@mitre.org | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.