Fault detection circuit for use in determining the existence of a fault in any one of a plurality of DC loads

A fault detection circuit for a plurality of DC loads uses a reference load as a benchmark. The voltages at the centerpoints of the loads to be monitored are monitored and compared with the voltage of the centerpoint of the reference load. The comparison is affected by faults which do not affect all loads equally (as by a short or open) and is unaffected by faults which do affect all loads equally (as by changes in voltage of the source).

BACKGROUND OF THE INVENTION 
The parent application hereto discloses a fault detection circuit for use 
with multi-phase loads, such as anti-icing or deicing heaters on aircraft 
The there-disclosed fault detection circuit operates by comparing the 
voltage at the centerpoint of a reference load with the voltages at the 
centerpoints of the loads to be monitored. 
For certain aeronautical applications, heaters are powered by 270 VDC. It 
would be advantageous to provide fault detection circuitry which would 
work in such an environment. 
One object of the invention is to provide a fault detection circuit for 
determining the existence of a fault in any one of a plurality of loads 
which are supplied by a DC source. 
Another object is to provide such a circuit which is highly sensitive to 
faults in the loads and highly insensitive to changes in the source. 
A further object is to provide such a circuit which does not require the 
use of many conductors which must be routed out of the loads. 
Still a further object is, in general, to improve on prior art circuits of 
this general type. 
In accordance with the invention, the DC loads to be monitored are 
center-tapped. A reference center-tapped load is connected across the DC 
source and the reference centerpoint of that reference load is used as a 
benchmark. The voltages at the centerpoints of all the loads to be 
monitored are compared against the voltage of the centerpoint of the 
reference load. When the voltage at any centerpoint of a monitored load is 
substantially unequal to the voltage at the centerpoint of the reference 
load, this indicates a load fault.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring first to FIG. 1, a 270 VDC source 2 in, e.g., an airplane (not 
shown) is used to power loads 4, 6 and 8. In this example, the loads 4, 6 
and 8 may be heaters such as fairing heaters or wing heaters, but it will 
be understood that the invention does not reside in the particular 
application for the load and that the named applications are only 
exemplary. It will also be understood that the invention does not reside 
in the use of three loads; any number can be monitored. It will further be 
understood that the loads 4, 6 and 8 need not be the same. 
In accordance with the invention, each of the loads 4, 6 and 8 is 
center-tapped. This divides load 4 into subloads 4A and 4B, which are 
connected at centerpoint 4CP. Likewise, load 6 is divided into subloads 6A 
and 6B that are connected at centerpoint 6CP, and load 8 is divided into 
subloads 8A and 8B that are connected at centerpoint 8CP. Each subload may 
be made up of a plurality of circuits in parallel. 
A center-tapped reference load is constructed of two like resistors 10 and 
12 which are connected across the source 2. The resistor 10 is connected 
to the resistor 12 at the reference centerpoint RCP. 
To monitor the voltages at the centerpoints 4CP, 6CP and 8CP, a network of 
diodes 14, 16, 18, 20, 22, and 24 is used. The cathodes of diodes 14, 16 
and 18 are all connected together. The anode of diode 14 is connected to 
centerpoint 4CP, the anode of diode 16 is connected to centerpoint 6CP, 
and the anode of diode 18 is connected to centerpoint 8CP. Similarly, the 
anodes of diodes 20, 22 and 24 are all connected together. The cathode of 
diode 20 is connected to centerpoint 4CP, the cathode of diode 22 is 
connected to centerpoint 6CP, and the cathode of diode 24 is connected to 
centerpoint 8CP. 
To compare the voltages at the centerpoints 4CP, 6CP and 8CP with the 
voltage at the reference centerpoint RCP, two operational amplifiers 26 
and 28 are used. These operational amplifiers 26, 28 are powered by the 
source 2 (power connections are not shown.) The inverting input of the 
operational amplifier 26 is connected to the non-inverting input of the 
operational amplifier 28, and both connected inputs are further tied to 
the reference centerpoint RCP. The non-inverting input of the operational 
amplifier 26 is connected to the cathodes of the diodes 14, 16 and 18; the 
inverting input of the operational amplifier 28 is connected to the anodes 
of the diodes 20, 22 and 24. (It will be noted that in FIG. 1, a resistor 
shown in dotted lines is in series between the non-inverting input of the 
operational amplifier 26 and the diodes 14, 16 and 18, and that a like 
resistor shown in dotted lines is in series between the inverting input of 
the operational amplifier 28 and the diodes 20, 22 and 24. For now, these 
resistors will not be discussed; the function of these resistors will be 
explained after the operation of the other circuit elements has been 
described.) 
When all the loads are operating properly, the voltages at centerpoints 
4CP, 6CP and 8CP will all be equal to the voltage at reference centerpoint 
RCP. There will be no output from either of the operational amplifiers 26, 
28 because the voltages at their inverting and non-inverting inputs will 
then be the same. This will be true even if the voltage of the source 2 
fluctuates, because the voltages at all the centerpoints 4CP, 6CP 8CP and 
RCP will then fluctuate together. 
However, if subload 4A develops a fault, i.e. is shorted or opened, the 
voltage at centerpoint 4CP will be either zero (open fault case) or 270 
VDC (short fault case). If the voltage at centerpoint 4CP is 270 VDC 
(short fault case), operational amplifier 26 will produce an output 
because 270 VDC will forward-bias the diode 14. If the voltage at 
centerpoint 4CP is zero (open fault case), operational amplifier 28 will 
produce an output because zero VDC will forward-bias the diode 20. This 
will cause operational amplifier 28 to produce a non-zero output. The same 
is true for the other subloads; when any of the subloads 4B, 6B or 8B are 
open, or when any of the subloads 4A, 6A or 8A are shorted, the 
operational amplifier 26 will produce an output. Likewise, when any of the 
subloads 4A, 6A or 8A are open, or when any of the subloads 4B, 6B or 8B 
are shorted, the operational amplifier 28 will produce an output. Thus, 
any short or open load fault will produce a non-zero output from one of 
the operational amplifiers 26, 28. (It is possible that a subload may 
change resistance without shorting or opening. If this happens, the fault 
will cause one of the operational amplifiers 26, 28 to produce a nonzero 
output, but this eventuality is unlikely; shorting or opening is much more 
common.) Advantageously although not necessarily, the outputs of the 
operational amplifiers 26, 28 may be connected together through diodes 30, 
32 respectively so that any fault is reflected in a single fault signal 
output. The diodes 30, 32 serve to isolate the outputs of the operational 
amplifiers 26, 28 from each other. Additionally, since the diodes 30, 32 
will not produce outputs unless the voltages at the outputs of operational 
amplifiers 26, 28 exceed the threshold voltage needed to render the diodes 
30, 32 conductive, the diodes 30, 32 prevent small outputs from the 
operational amplifiers 26, 28 from reaching subsequent circuitry and being 
considered as fault signals. 
Most operational amplifiers would not remain operative with 270 VDC at one 
of their inputs. Accordingly, the resistors shown in dotted lines are used 
to limit the voltages at the non-inverting input of the operational 
amplifier 26 and the inverting input of the operational amplifier 28 when 
a fault takes place. 
The invention only requires one connection to each of the loads to be 
monitored. Because of this, little real estate on the loads is required. 
This is important because the surfaces of the loads are taken up with 
resistive circuits and temperature sensors and any space which is taken up 
by the monitoring function decreases the space available for the functions 
which the loads must perform. 
The modification shown in FIG. 2 is advantageous because it establishes a 
minimum threshold below which no positive fault output signal will be 
output by the operational amplifiers 26, 28. In this modification, a small 
bias resistor R is placed in series between resistors 10 and 12. The 
inverting input of the operational amplifier 26 is connected to the common 
junction point of resistors 10 and R and the non-inverting input of the 
operational amplifier 28 is connected to the common junction point of 
resistors R and 12. 
The small bias resistor R biases the inputs of each of the operational 
amplifiers 26, 28 so that the voltage at each inverting input is slightly 
greater than the voltage at each corresponding non-inverting input. This 
in turn produces slight negative voltages at the outputs of each of the 
operational amplifiers 26, 28 when all the loads 4, 6 and 8 are operating 
correctly. As a result of this, a positive fault output signal will only 
appear at the outputs of each of the operational amplifiers 26, 28 when 
the small input differentials at their inputs have been overcome. 
The invention does not require any particular comparison scheme for the 
voltages at the centerpoints of the loads to be monitored, and the 
operational amplifiers may be provided with circuitry which causes them to 
generate fault outputs only when the inequality at their inputs exceeds a 
predetermined threshold. Alternatively, comparators may be used instead of 
operational amplifiers; persons skilled in the art can readily adapt the 
invention to particular applications by choosing appropriate comparison 
algorithms. 
Additionally, although the preferred embodiment uses center-tapped loads, 
this is merely for convenience. The taps may be off-center as long as the 
ratios of the resistances of the subloads are constant; in the example 
presented, the preferred embodiment would work properly as long as the 
ratios R(4A)/R(4B), R(6A)/R(6B), R(8A)/R(8B) were all equal to the ratio 
R(10)/R(12), R(x) indicating the resistance of the respective subload or 
resistor. The term "centerpoint" as used herein encompasses such 
equi-ratio tapping schemes. 
Although a preferred embodiment has been described above, the scope of the 
invention is limited only by the following claims: