Sensing circuit detects when the output from a matrix is present and when it should be present. The circuit provides short circuit protection for a power distribution system where the selection of the driven load is accomplished by digital logic

Owen, J. R., III

English

NASA Technical Reports Server

.December 1968	 Brief 69-104,43 
NASA TECH BRIEF 
I	 /& 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Short Circuit Protection for a Power Distribution System 
Inhibit Signal
New Circuit 
EJ Conventional Circuit 
The problem: 
To provide short circuit protection for a power dis-
tribution system where the selection of the driven load 
is accomplished by digital logic. 
The solution: 
Provide a sensing circuit that will detect when the 
output from a matrix is present and compare the out-
put with a reference signal which determines when 
the matrix output should be present. 
How it's done: 
The illustration shows Power Distribution Matrix 
driving circuits that are selected and enabled by X and 
Y Decoders. An execute command activates X and Y 
Select Execute Gates which provide correct timing sig-
nals to X and Y Decoders. The Y Select Execute Gate 
also supplies a reference voltage to Comparator. The
n and m decoder input lines define the matrix output 
to be selected. When an execute command is given, the 
X and Y Decoder outputs are routed to the Power 
Distribution Matrix. The Yn and Xm combination 
selects and activates one of the output drivers in the 
matrix. Each output driver signal is applied to an OR 
Gate and the resulting sense voltage is compared in 
magnitude with the reference voltage. If the sense 
voltage is more positive than the reference voltage (the 
normal situation), there is no Comparator output. If 
there is a short circuit (the abnormal situation) on 
one of the outputs, the sense voltage will be more neg-
ative than the reference voltage. The Comparator will 
generate an inhibit signal that will deactivate the X 
Select Execute Gate. This overload protection circuit 
generates the inhibit control, therefore, to disable the 
output driver of the Power Distribution Matrix with-
out affecting the execute command. The importance of 
(continued overleaf) 
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this point is that as long as the execute command is 
present, the output is inhibited because of the pres-
ence of the resulting reference voltage without the 
presence of a more positive sense voltage. 
When the execute command is removed, the short-
protection circuitry is automatically reset and the pre-
viously shorted output can be reselected. This reset 
handles those situations where the short is transient 
in nature and proper operation is attainable after the 
short is removed. 
Notes: 
1. A disadvantage of this system, which can only be 
evaluated as a system attribute, is that the output 
will not return to normal operation until the ex-
ecute command signal is removed and reapplied. 
This means that if the overload is removed while 
the command is present, normal operation will 
continue to be inhibited. Where this feature is ob-
jectionable, additional logic control can be instru-
mented to feed the inhibit signal back to the
execute command generation circuitry. When this 
occurs, the execute command can be reinitiated, 
either selectively or automatically, to attempt to 
activate the' output again. The "hunting" effect on 
the system would determine the incorporation of 
this feedback feature. 
No further documentation is available for this 
innovation. 
Technical questions concerning this innovation 
may be directed to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B,68-104,43 
Patent status: 
No patent action is contemplated by NASA. 
Source: J. R. Owen, III 
of International Business Machines 
under contract to 
Marshall Space Flight Center 
(MFS-14993) 
Brief 68-104,43	 Category 01


Short circuit protection for a power distribution system

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