Circuit arrangement for testing the correct functioning of circuit(s)

There is disclosed a circuit arrangement including a load (7) and a current source (1) for supplying current to the load via a current path with a winding (3). A core of magnetizable material (2) is coupled inductively with the winding and a Hall effect transducer (4) detects magnetic flux in the core for use in producing an output indication dependant on the magnitude of the load current. For testing the correct functioning of circuit(s), the circuit arrangement is further provided with an additional winding (6) on the magnetizable core, through which a further current source supply a further current, in which the core is also inductively coupled so that the transducer also detects flux in the core due to the further current.

The present invention relates to testing a circuit arrangement, for example 
a circuit arrangement in a railway signalling system whose correct 
operation is vital to the safety of the system. 
According to the present invention, there is provided a circuit arrangement 
comprising: 
(a) a load; 
(b) supply means for supplying current to the load via a current path; 
(c) magnetic circuit means which is coupled inductively with the said 
current path; 
(d) magnetic flux detecting means for detecting magnetic flux in the 
magnetic circuit means, for use in producing an output indication 
dependent on the said current; and 
(e) testing means, comprising further supply means for supplying a further 
current via a further current path, the magnetic circuit means also being 
coupled inductively with the further current path so that the detecting 
means also detects magnetic flux in the magnetic circuit means due to the 
further current.

Referring to FIG. 1, a current I through a conductor 1 is passed through a 
load (not shown). A known form of detection means for detecting the 
current I comprises a core 2 of magnetisable material in the shape of a 
toroid and defining a magnetic circuit, the conductor 1 being wound as a 
winding 3 on the core 2, there being in a gap in the latter a linear 
output Hall effect transducer 4 whose output is coupled to an amplifier 5. 
Current I produces a flux in the magnetic circuit formed by the core 2, 
the magnitude of which is proportional to the magnitude of the current I 
The transducer 4 produces an electrical signal proportional to the 
magnitude of flux passing through it, which signal is thus proportional to 
the current I. The amplifier 5 then increases the amplitude of this signal 
to produce an indication of the magnitude of the current I. 
In a non-safety critical environment, such an arrangement is acceptable. 
If, however, the current measurement is vital for safety purposes, then it 
is unacceptable for the following reasons 
(1) The winding 3 may be short-circuited, thus giving a zero measurement 
when a current is actually flowing. 
(2) The winding 3 may develop a shorted turn, resulting in less flux being 
produced and thus giving a measurement of less current than is actually 
flowing 
(3) The core 2 may fracture, increasing the reluctance and decreasing the 
flux. This would result in a measurement of less current than is actually 
flowing. 
(4) The transducer 4 may move out of the gap and consequently out of the 
flux path. This would result in a measurement of less current than is 
actually flowing. 
(5) The characteristics of the transducer 4 may change and result in an 
increased or decreased measurement. 
(6) The gain of the amplifier 5 may alter and result in an increased or 
decreased measurement. 
At least some of the above shortcomings may be overcome by adding a further 
winding 6 on to the core 2, the winding 6 carrying a "test" current for 
testing the circuit arrangement comprising conductor 1, winding 3, core 2, 
transducer 4 and amplifier 5--it will be known if there is a fault in this 
circuit arrangement if the output of amplifier 5 is not what would be 
expected from the test current. 
FIG. 2 is a diagram of an embodiment using such a further winding, items in 
FIG. 2 which correspond with items in FIG. 1 having been given the same 
reference numerals as in FIG. 1. In FIG. 2, direct current I in conductor 
1 is passed through a load 7 by closing a switch 8 under the control of a 
microprocessor 9, the switch 8, the winding 3 and the load 7 being 
connected in series across a voltage supply. The output of the amplifier 5 
is connected to a detector 10 whose output is connected to the 
microprocessor 9. A low resistance proof resistor 11 is connected across 
the switch 8. A direct test current of known magnitude is passed through 
the winding 6, so that the output signal of the detector 10 will be of a 
known, expected magnitude if there is no fault as regards the core 2, the 
transducer 4, the amplifier 5 and the detector 10, but not of this 
magnitude if there is such a fault. The circuit arrangement of FIG. 2 
guards against the above shortcomings in the following manner: 
(1) The microprocessor 9 expects an output signal from the detector 10 that 
corresponds to the state of the switch 8. If there is a zero output even 
though switch 8 is closed, then the microprocessor detects that there is a 
fault. 
(2) If a shorted turn should occur in winding 3, then the current flowing 
via the low resistance proof resistor 11 when the switch 8 is open will be 
below the expected magnitude. The low resistance proof resistor 11 cannot 
reduce in resistance to compensate for the shorted turn. 
(3) Should the core 2 fracture, then the reluctance will increase and flux 
will decrease, resulting in a response of less than the expected 
magnitude. 
(4) Also, core fracture can be detected by virtue of a lower output signal 
from the detector 10 than expected as a result of the test current and 
this can be detected by the microprocessor 9. Similarly, if there is a 
positional fault in the transducer 4, a change in the latter's 
characteristics, a change in the characteristics of the amplifier 5 or a 
fault in the detector 10, the output signal from the latter resulting from 
the test current will not be as expected and this can be detected by the 
microprocessor 9. 
Referring to FIG. 3, this is a diagram of an arrangement for controlling 
the energisation of a load which is the filament of a lamp, which could, 
for example, be a railway signalling lamp. In a railway signalling 
environment, it is, of course, essential that the filament of a signalling 
lamp only be energised when it should be and it must also be known if the 
filament is not being energised to the correct amount when it should be. 
In FIG. 3, items which are the same as items in FIG. 2 have been given the 
same reference numerals as in FIG. 2. The arrangement of FIG. 3 allows for 
the measurement of the current flowing in the filament and also allows for 
the testing of the measurement function without affecting the filament 
current magnitude. The filament of the lamp (11) is connected in series 
with a pair of switches SW1 and SW2 and a fuse 12 across a voltage supply. 
Connected in parallel across switch SW1 is a low resistance proof resistor 
13 and connected in parallel across switch SW2 is a low resistance proof 
resistor 14. The opening and closing of switches SW1 and SW2 is effected 
under the control of the microprocessor 9 via respective enabling circuits 
15 and 16. Each of circuits 15 and 16 comprises an opto-isolator and an 
AND gate. The microprocessor 9 carries out self-checking routines and if 
it is deemed to be operating correctly, then a dynamic signal 17 occurs on 
a line 18 as a "health signal". The signal 17 is passed through a 
band-pass filter 19 (designed to fail to safety, i.e. produce no output, 
if it becomes faulty) whose output signal is an enabling input for the AND 
gates of circuits 15 and 16. The function of the detector 10 in FIG. 2 is 
achieved by a sample and hold circuit 20 whose output feeds an analogue to 
digital converter 21, the output of the latter being connected to the 
microprocessor 9. If each of switches SW1 and SW2 is open, then a low 
current flows through the winding 3 and the filament of the lamp 11, but 
not enough to cause illumination of the latter, by virtue of resistors 13 
and 14. If one of the switches SW1 and SW2 closes when it should be open, 
then twice the low value of current will occur and this will be detected 
by the circuitry comprising the winding 3, core 2, transducer 4, amplifier 
5, sample and hold circuit 20 and converter 21 and the microprocessor 9 
will accordingly detect a fault. The reason why there are two switches 
(SW1 and SW2) in series with the filament of the lamp 11 is that if one of 
them does fail to a closed, then the other switch should still be open, 
for safety purposes. 
For testing the circuitry comprising the core 2, the transducer 4, the 
amplifier 5, the sample and hold circuit 20 and the converter 21, the 
additional winding 6 is provided, functioning as in FIG. 2 by receiving an 
appropriate direct test current (supplied via supply means 22). 
If the microprocessor 9 receives an output signal from the converter 21 
which indicates that there is a fault condition somewhere in the overall 
arrangement, then a direct current output is provided on a line 23 to an 
opto-isolator circuit 24 which provides an output to close a switch SW3, 
as a result of which a short-circuit is placed across the voltage supply 
and the fuse 12 is blown to isolate the filament of lamp 11 for safety 
purposes 
With reference to FIG. 4 there will now be described an alternative to the 
arrangement shown in FIG. 3 whereby tests may be carried out to establish 
whether: means for detection of the current flowing when the filament of 
the lamp 11 is to be energised (the "hot filament" current) is operating 
correctly; whether means for detection of the existence of a fleeting or 
intermittent current when there should not be a current through the 
filament is operating correctly; and whether means for detection of the 
low current through the filament when each of the switches SW1 and SW2 is 
open (the "cold filament" current) is operating correctly. The arrangement 
of FIG. 3 is altered as shown in FIG. 4. More particularly, the circuit 20 
and detector 21 are replaced by three detectors 25, 26 and 27 (which could 
be Schmitt trigger operational-amplifiers). The detector 25 is adapted to 
trigger if the output of amplifier 5 is indicative of a "hot filament" 
current; the detector 26 is adapted to trigger if the output of amplifier 
5 is indicative of a "cold filament" current; and the detector 27 is 
adapted to trigger if the output of the amplifier 5 is indicative of a 
fleeting or intermittent current. There are two test windings 6a and 6b on 
the core 2, wound as shown, the winding 6a being energised by closure of a 
switch SW4 and the winding 6b being energised by closure of a switch SW5. 
Reference numerals 28 and 29 denote low resistance proof resistors. As 
regards the detector 25, this is set to trigger if the current through the 
filament of lamp 11 is above a threshold slightly less than the "hot 
filament" current. Should the current decrease too much, then the output 
of detector 25 will go "high". To test the detector 25, the switch SW4 is 
closed to allow current to flow through the winding 6a, which is wound so 
that it produces a flux that opposes that created by the filament current 
This reduces the flux in the transducer 4 and causes the detector 25 to 
change state. If it does not change state, then it is known that it is 
faulty It is to be noted that the lamp brightness will not diminish during 
this test since the test current through the winding 6a is a direct 
current. 
When the lamp 11 is expected to be off, it is necessary to make frequent 
checks to ensure that it is off. Should a fleeting or intermittent current 
through its filament occur, then the current causes the threshold of the 
detector 27 to be exceeded, so that its output goes "high". To test the 
detector 27, the switch SW5 is closed to allow current to flow in the 
winding 6b. This current is set to a level which produces enough flux to 
cause the detector 27 to be triggered. If it does not trigger in this 
event, then it is known that it is faulty. During the testing of the 
detector 27, the filament of the lamp 11 should not be receiving any 
current. 
To test the detector 26, with switches SW1 and SW2 open and the resistors 
13 and 14 allowing the "cold filament" current to pass through the 
filament of the lamp 11, the switch SW4 is closed to allow a current to 
flow through the winding 6a whose flux opposes that of the winding 3. This 
means, that, if the detector 26 is operating correctly it should trigger 
and its output go "high", as if the "cold filament" current had dropped If 
it does not do so, then it is known that it is faulty. 
Incidentally, should either of switches SW1 and SW2 fail to open when 
required, then the current allowed to flow by resistor 13 and 14 will 
result in the detector 27 being triggered. 
Instead of using three detectors 25, 26 and 27, they could be replaced by a 
single analogue to digital converter whose output is connected to the 
microprocessor. 
The above testing operations by appropriate closure of switch SW4 or SW5 
under a certain condition may be effected under the control of the 
microprocessor 9.