Power supply device with unbalance monitoring circuit

A power supply device with a monitoring circuit for detecting an unbalance in a direct voltage transmitted between two line conductors and which is normally balanced with respect to ground potential. A voltage divider circuit is arranged between the two line conductors and consists of components which are identical in pairs. The center tap of the voltage divider is connected to ground potential and at further taps thereof in each case positive and negative part-voltages are produced. A first positive part voltage is compared with a second negative part voltage and a second positive part voltage is compared with a first negative part-voltage so that the balance of the direct voltage with respect to ground potential is simply and reliably monitored.

BACKGROUND OF THE INVENTION 
This invention relates to a power supply device including a unbalance 
monitoring circuit for a direct voltage transmitted between two line 
conductors, and which is balanced with respect to ground potential. 
In optical communication transmission systems, two metallic line conductors 
are provided in parallel with the fibre glass lines transmitting the 
communication signals. The metallic line conductors supply a direct 
voltage, generated by a power supply device, to the line devices of the 
transmission link, for example, amplifiers (power feeding). For this 
purpose, the first line conductor is connected to the pole of the direct 
voltage which is positive with respect to ground potential and the second 
line conductor is connected to the pole which is negative with respect to 
the ground potential. The amplitude of the potential existing at the 
positive pole corresponds to the amplitude of the potential occurring at 
the negative pole. Thus, a direct voltage is present between the two line 
conductors which is balanced with respect to ground potential. The line 
devices connected to the transmission system are grounded in each case. If 
a line conductor is damaged, for example, due to digging work, and, as a 
result, this line conductor is connected to the ground potential 
(unilateral ground fault), this unbalance of the direct voltage results in 
a disturbance or a failure of the line devices that are energized by the 
direct voltage. The purpose of an unbalance monitoring circuit is to 
monitor this operating condition. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a power supply device of the 
type initially mentioned which, in a simple manner, reliably monitors the 
balance of the direct voltage with respect to the ground potential. 
In a power supply device of the type initially mentioned, this object is 
achieved in that between the two line conductors, a voltage divider 
circuit is arranged which consists of components which are in each case 
identical in pairs, the centre tap of which is connected to the ground 
potential and at the further taps of which positive and negative 
part-voltages referred to ground potential can be picked up, in which 
arrangement, in each case, a first positive part-voltage is compared with 
a second negative part-voltage and a second positive part-voltage is 
compared with a first negative part-voltage. 
The balance of the direct voltage with respect to ground potential is 
reliably monitored by comparing the first positive part-voltage with the 
second negative part-voltage and by comparing the second positive 
part-voltage with the first negative part-voltage, respectively. 
In one embodiment, in each case one tap for a positive and a negative 
part-voltage is connected to a comparator. In this arrangement, a direct 
voltage which is needed in any case in the power supply device, for 
example, for a control circuit, is used as the voltage supply for the 
comparators of the unbalance monitoring circuit. Due to the fact that no 
individual separate supply voltage, which, in particular, is floating with 
respect to the direct voltage, is needed for operating the unbalance 
monitoring circuit, the unbalance monitoring circuit can also be simply 
and inexpensively retrofitted to power supply devices already installed. 
In a further embodiment, the comparators for the part-voltages are 
constructed as operational amplifiers, the inverting input of the first 
operational amplifier being connected via in each case one component of 
the voltage divider circuit to the taps for the second positive and the 
first negative part-voltage and the non-inverting input of the second 
operational amplifier being connected via in each case one component to 
the taps for the first positive and the second negative part-voltage, and 
the non-inverting input of the first and the inverting input of the second 
operational amplifier being connected to ground potential. In this 
arrangement, the output voltage of the two operational amplifiers in each 
case becomes zero with balance, whearas a disturbance is signalled when an 
unbalance of the direct voltage occurs with respect to the ground 
potential. 
In one embodiment, the components of the voltage divider circuits are 
constructed as resistors, which results in an operating threshold of the 
operational amplifiers dependent on the respective value of the direct 
voltage. 
By constructing the components between the taps as diodes, the operating 
threshold of the operational amplifiers can be made constant and thus 
independent of the respective value of the direct voltage. 
Further advantageous embodiments are set forth in the dependent claims.

In the illustrative embodiment shown in the FIGURE, the unbalance 
monitoring circuit is located between a positive pole E+ and a negative 
pole E- of a direct voltage U which is balanced with respect to the ground 
potential, 0 V. The direct voltage U is supplied from a feeds power supply 
device 3 and power to two line devices 4, for example, of an optical 
communication transmission system, via two line conductors a, b. The line 
drawn dashed between the two line devices 4 indicates that further line 
devices can also be connected. The first line conductor a is connected to 
the positive pole E+ and the second line conductor b is connected to the 
negative pole E- of the direct voltage U. Between the two line conductors 
a, b and the ground potential 0 V there is located, in each case, a series 
circuit of six resistors R1a, R2a, R3a, R1b, R2b, R3b, which are in each 
case identical in pairs and at the taps E1a, E2a, E1b, E2b of which in 
each case two positive U1a, U2a and negative U1b, U2b, part-voltages 
referred to earth potential can be picked up. In this arrangement, the 
resistors R1a, R2a, R3a, R1b, R2b, R3b are dimensioned in a manner such 
that the value of the resistor R1a corresponds to the value of the 
resistor R1b, the value of the resistor R2a corresponds to the value of 
the resistor R2b and the value of the resistor R3a corresponds to the 
value of the resistor R3b. Thus, the first positive part-voltage U1a 
appears between the tap E1a and ground potential 0 V, and the second 
positive part-voltage U2a is derived across the series circuit of the 
resistors R1a, R2a between the tap E2a and ground potential 0 V. 
Correspondingly, a first negative part-voltage U1b is produced across the 
resistor R1b between the tap E1b and ground potential 0 V and a second 
negative part-voltage U2b is developed across the series circuit of the 
resistors R1b, R2b between the tap E2b and ground potential 0 V. The 
common tap E2a of the resistors R2a, R3a is connected via a resistor R4a 
to the inverting input of a first operational amplifier OP1, the 
non-inverting input of which is connected to ground potential 0 V. 
Furthermore, the inverting input of the first operational amplifier OP1 is 
connected via a resistor R5b to the common tap E1b of the resistors R1b, 
R2b. Correspondingly, the common tap E2b of the resistors R2b, R3b is 
connected via a resistor R4b to the non-inverting input of a second 
operational amplifier OP2, the inverting input of which is connected to 
ground potential 0 V via a resistor R8. The common tap E1a of the 
resistors R1a, R2a is likewise connected to the non-inverting input of the 
second operational amplifier OP2 via a resistor R5a. The two operational 
amplifiers OP1, OP2 in each case have feedback via a respective resistor 
R6, R7. Between the inverting and the non-inverting input of the 
operational amplifiers OP1, OP2, "anti-parallel-connected" diodes D1, D2, 
D3, D4 are in each case arranged. The outputs of the operational 
amplifiers OP1, OP2 are connected to a signalling connection S via diodes 
D5, D6, respectively. The operational amplifiers OP1, OP2 are operated 
with an auxiliary voltage referred to ground potential which can be 
supplied from the power supply device 3 and is in any case needed for a 
control circuit provided there. 
In normal operation, that is to say when there is no disturbance in the 
supply of power of the direct voltage U via the line conductors 1, 2, the 
direct voltage U is balanced with respect to ground potential 0 V between 
the line conductor a and the line conductor b, that is to say the 
amplitude of the first positive part-voltage U1a is equal to the amplitude 
of the first negative part-voltage U1b and the value of the second 
positive part-voltage U2a is equal to the value of the second negative 
part-voltage U2b. This results in a voltage at the output of the first 
operational amplifier OP1 which results from the product of the feedback 
resistor R6 by the sum of the quotient of the second positive part-voltage 
U2a and of the resistor R4a and the quotient of the first negative 
part-voltage U1b and the resistor R5b. In this connection, the resistors 
R4a, R5b are dimensioned in a manner such that U2a/U1b=R4a/R5b. As a 
result, the ratio U2a/R4a becomes equal to the ratio U1b/R5b and the 
output voltage of the first operational amplifier OP1 becomes zero, that 
is to say there is no signalling of a disturbance via the signalling 
connection S. Resistors R4b, R5a are correspondingly dimensioned, that is 
to say the value of the resistor R4b corresponds to the value of the 
resistor R4a and the value of the resistor R5a corresponds to the value of 
the resistor R5b. Thus, the output voltage of the second operational 
amplifier OP2 also becomes zero in normal operation. 
If, however, there is an unbalance of the direct voltage U with respect to 
ground potential 0 V, and thus also an unbalance of the positive and 
negative part-voltages, for example, due to a disturbance caused by 
digging work, the ground of the quotient U2a/R4a is no longer equal to the 
value of the quotient U1b/R5b. As a result, the output voltage of the 
first operational amplifier OP1 also assumes a value which differs from 
zero and which causes, for example, a light-emitting diode to be operated 
via the signalling connection S and thus signals the disturbance. The 
signalling connection S is correspondingly activated via the second 
operational amplifier OP2. 
The operating threshold of the operational amplifiers OP1, OP2, is 
determined via the resistors R1a, R2a and R1b, R2b respectively. In this 
connection, the operating threshold in the illustrative embodiment shown 
in the FIGURE is selected to be proportional to the direct voltage U which 
can assume, for example, values of between 30 and 1,200 volts in a 
practical circuit arrangement. In a further embodiment, not shown in the 
FIGURE, the resistors R2a, R2b are replaced, for example, by Zener diodes. 
Thus, the operating threshold can be selected to be constant, that is to 
say independent of the current value of the direct voltage U. Instead of 
the operational amplifiers OP1, OP2, for example, comparators can also be 
used, the output of which can be evaluated with the aid of a 
microprocessor.