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tx_jayhawk
Joined: 10 Jun 2006
Posts: 9
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 Posted: Tue Nov 10, 2009 10:40 am Post subject: Preferred Method for Redundant Power Sources to Single Input |
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Bob,
I am curious as to what your preferred method is for providing redundant power sources to devices that only have a single power input.
One thought was that you could use one half of a bridge rectifier diode with fast-on tabs to provide a simple way to provide dual inputs and handle the diode backfeed protection. The only problem with that is the forward voltage drop can be significant. I noticed it is 1.7V on a Radio Shack piece that I looked at. I called B&C and they did not know (?) what the forward voltage drop was on their product (I am guessing it might be the same).
I like the simplicity of the bridge if the drop wasn't so significant. Do you tend to prefer axial diodes or some other solution?
Thanks,
Scott
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nuckollsr
Joined: 24 Mar 2009
Posts: 95
Location: Medicine Lodge, KS
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| Posted: Thu Nov 12, 2009 9:07 am Post subject: Re: Preferred Method for Redundant Power Sources to Single I |
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When flight critical devices with a single input power connection are powered from more than one source, there are not many choices. You can use manually or automatically operated "switches" to route energy from the various sources to the electrically needy accessory . . .
Manual switches are pretty obvious . . . a toggle switch transfer from NORM to ALTERNATE power source will do. Alternatively, one might install a relay that is held closed and controlled with some voltage sensing device that drops the coil and effects a transfer without pilot intervention. In the Z-Figure below . . .
http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z19-RBA1.pdf
http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z19-RBA2.pdf
. . . I've illustrated a manually operated selection of power sources that MINIMIZES common points of failure for the two power paths that source a single load. This drawing should not be interpreted as a recommended architecture for all situations; it's only illustrative of ONE recipe for success in meeting design goals for the systems that inspired the drawing.
I show a pair of diodes that are used IN THIS CASE to prevent the back-feed of power from a working to a non-working system in the even the pilot positions switches incorrectly. Obviously, a pair of diodes could be used WITHOUT switches to provide an either/or energy path from two, always-on sources. The recipe for your particular design goals may differ from that which is shown here.
Power distribution diodes come in two common flavors: Silicon Junction and Schottky Junction devices. The former is used in 99% of all power steering and rectification tasks. The later has a slight advantage in that voltage drop across the conducting junction is lower and a disadvantage of being more expensive and available in fewer user-friendly package styles.
The Maximum voltage drop ratings for any given device are stated for conditions of highest peak current sometimes stacked on top of the worst case for operating temperature. Its quite misleading because you're interested in average voltage drop under NORMAL conditions. Take a peek at this data sheet for a 2A, 40V rated Schottky device:
http://tinyurl.com/ygbolwb
Note on Figure 1 we see a plot of typical forward voltage drop characteristics. At 2A the critter nominally tosses off 0.5 volts. Now check
out:
http://tinyurl.com/y9y7fc7
In figure 1 of this data sheet we see a similarly rated standard junction device tosses off about .95 volts for the same conditions for a difference of about .5 volts.
But for reasons I've cited many times in the past, I like the modular molded bridge rectifier blocks with fast-on tabs. They're cheap. They mount right to the airframe with no need for additional heat-sinking in most cases. Finally, they're internally insulated electrically from their mounting surfaces. Check out this data sheet:
http://tinyurl.com/yzmpjbk
In figure 5 we see that depending on the internal temperature of the device, we can expect voltage drops on the order of 0.6 to 0.7 at the same 2A loads.
Further, the mono-block bridge with fast-on tabs offers a very robust, easy to wire, minimum risk for installation error solution for diode steering of energy around the ship's electrical system. The typical voltage drops experienced do not produce an observable degradation of performance. My preference for this class of diode has little to do with electrical performance . . . the little beasties are 25A plus rated devices and quite robust electrically for the way we use them. I judge them preferred for low cost, wide availability, mechanical convenience and minimum parts count.
Bob . . .
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gordonrsmith921@yahoo.com
Joined: 06 Oct 2009
Posts: 24
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| Posted: Fri Nov 13, 2009 5:09 am Post subject: Re: Preferred Method for Redundant Power Sources to Single I |
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Bob wrote:
Obviously, a pair of diodes could be used WITHOUT switches to provide an either/or energy path from two, always-on sources.
In this case when the two always-on sources differ by a volt or two, for whatever reason, Do the sources provide power proportionally to their voltage or will it be a 100% feed from the highest voltage source?
Gordon Smith
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