GB1576096A - Load protection circuit arrangements - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO D.C. LOAD PROTECTION, CIRCUIT ARRANGEMENTS (71) We, SIEMENS AKTIENGESELL- SCHAFT, a German Company of Berlin and Munich, German Federal Republic, do hereby declare the invention, for which we pray, that a patent may be granted to us; and the method by which it is to be performed, to be particularly described in and by the following statement: - The invention relates to d.c. load protection circuit arrangements for protecting electrical load circuits against the inadvertant connection of inverted supply polarity.

A known means of protecting electrical load circuits from an incorrect connection to a d.c. supply source utilises a diode ar- ranged in series with the load which is to be protected. However, in high power de- vices, and especially if at low operating voltages, this measure leads to a consider-: able power loss. For example, a series blocking diode of this kind can virtuall double the power losses of an inverter current supply for a teleprinter which is to be operated at a low voltage.

A protection circuit is described in the German Patent Specification No.. 956,524 wherein a mains rectifier is so arranged in parallel with a "switch that during operation with a correctly poled d.c: voltage, the.

switch is closed by a relay connected inparallel to the load across the mains terminals. However, as the relay, in series with a resistor, is directly connected to the mains terminals, additional measures, such as a parallel diode, must be provided to prevent this relay responding in the case of incorrect poling.

One object of the present invention is to provide an improved circuit arrangement for protecting d.c. loads when connected to supply source terminals incorrectly, in such manner that the relay responds only in the case of correct polarity, and additional measures are not required.

The invention consists in a d.c. load protection circuit arrangement for protecting electrical load circuits in the event of a connection being made to a supply source in the wrong polarity, a series diode being 'pr'Q' vided that is conductive when the correct polarity is applied,' and said' did,de . being connected in parallel with a ma'ke,-c'6iita?c't' of a relay that is connected in a shunt arm on the side of said diode remote from the supply source terminal connections.

The proposed arrangement ensures that in any case the relay pulls up 'and the relay contact is closed only when the connection terminals are correctly poled, 56 ,'that. cur- rent can immediately flow via the didde to the load circuit. As soon as the relay has responded, the diode is shunted by the bridging contact, and thus any power loses is then considerablv reduced. This overall circuit arrangement also has the advantage that the diode absorbs any starting current surge. Thus devices having a high capacitive input impedance can ,be connected. 'The diode can be provided in kndwn" manner with a series current-limiting resistance.

Following switch-on, the relay contact carries the supplied operating current, Bnd only a low holding current flows via tlie' re- lay winding, so that there is. a corisiderably smaller residual power ',,Jo'ss' than" that "ex- perienced' with a series diode. This residual power loss can be avoided 'if a bistable polarised relay ,is employed, and , is connected in series with a capacitor. In this case the capacitor is charged by the start- ing current surge, and simultaneously the relay switches to one stable position in which the parallel contact shunts the series'diode.

If the current is disconnected at the 'mains terminals, the capacitor discharges, and with this discharge current the polarised relay switches to its other stable position, so that the parallel contact opens.

The invention will now be described with reference to the drawings, in which: Figure 1 illustrates one exemplary em- bodiment of a circuit arrangement for protection against mis-poling in accordance with the invention, comprising a monostable, unpoled relay; Figure 2 illustrates one alternative ex emplary ,embodiment of the circuit arrangement for protection against mis-poling, comprising a'bistxable polarised relay; and Figures 3 and 4, illustrate further modified circuit arrangemetts.

Figure 1 illustrates a load R1 which is connected via te'rtninals 1 and 2 to a d.c.,, voltage supply. sdurce. By way of protec -tion from tmis-poling there is firstly provided a series'connected diode V which only allows current to flow to the load in the case of correct polarity of the terminals 1 and 2, to, provide a potential U,,+ at the output' 'o'f the diode V iiii this embodiment.

However, when the poling of the connected d.c. voltage supply source is correct, a;elay K1 is simultaneously excited, and closcs..it's.makcntact.kl to shunt the diode V. Thus,- following the starting current surge tlirough the, diode V, the contact kl carries the, continuous, operating current, so that there is only a much smaller power loss that occurs as -a result of the holding current flowing via the winding of the relay K1.

Figure 2 illustrates an alternative embodiment of the proposed protection circuit, which is a modification of the circuit shown in Figure 1, 'in which a load R2 is connected via: the terminals 1 and 2 to d.c. voltage supply source, with a diode V which only allows current to flow through the load when the polling is correct, and provides a poten tial UL+ at the diode output, in this em bodiment In. place of the monostable. relay K1 used in Figure 1, there is a bistable, polarisèd relay K2, which is connected in a shunt arm in series with a capacitor C. This capacitor C is charged by the starting cur rent surge V if correct polarity is applied, whereupon the bistable relay K2 switches over and closes the contact k2.As in Figure 1. the diode V is then shunted so that con tinuousoperating current flows via the make-contact k2. Thus, once again there is no ' power loss in the diode V, but in this case the winding of the relay K2 consumes no'significant power when the relay K2 has been operated as the position which has once been assumed is maintained for the entire. duration in which the load is current.When the operating d.c. voltage supply source is disconnected, the capacitor C discharges via the relay K2, and switches the latter into its other stable position, in which. the makecontact k2 is opened, thus the load is protected from mis-poling by the diode. V. even if the operating voltage is reconnected dnaccurately when the device is switched on again, A; further embodiment using a non-polar.

ised, relay is illustrated in Figure 3. Here, the diode V is arranged in series with a limiting resistor RB, which safeguards any fuses, rectifiers or the switch-on contacts of a battery line, by limiting any starting current surge. A capacitor'C3 is connectedtin parallel with the load in order to receive any transient supp]y swings. A makecontact k3 of an unpolarised relay K3 operates in'the same way as described for the relay K1 in Figure 1, but shunts both the diode V and limiting resistor RB. However, it would also be possible to use a bistable, polarised relay having a series capacitor, as described with reference to Figure 2.

Figure 4 illustrates a further development of Figures 1 and 3, in which a relay K4 has its winding connected to the full load supply voltage UL of a load R4 but can be energised by an auxiliary voltage U that is produced for the- relay K4 in the load R4, which may be, for example, an inverter already provided in the load circuit. As theauxiliary voltage of such an inverter does not become available until an initial delay has expired for example, approximately 100 ms following the initial connection of a mains voltage, the relay K4 does not operate its contact until the connected device is fully operative.The surge capacitor C4 is charged by them, so that -both the diode V and the contact k4 are only required to supply the normal stabilised load, as the initial surge has passed, and the contact k4 is only lightly loaded. A bistable, poled relay having a series capacitor could also be used, as was described with reference to Figure 2.

WHAT WE CLAIM IS:- 1. A d.c. load protection circuit arrangement for protecting electrical load circuits in the event of a connection being made to a supply source in the wrong polarity, a series diode being provided that is conductive when the correct polarity is applied, and said diode being connected in parallel with a make-contact of a relay that is connected in a shunt arm on the side of said diode remote from the supply source terminal connections.

2. A circuit arrangement as claimed in Claim 1, in which said relay is a bistable' polarised relay connected in series with a capacitor in said shunt arm.

3. A circuit arrangement as claimed in Claim 1 or Claim 2 in which a limiting resistor is connected in series with the diode and is bridged together with said diode when said make-contact is closed.

4. A circuit arrangement as claimed in any preceding Claim, in which said relay is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. emplary ,embodiment of the circuit arrangement for protection against mis-poling, comprising a'bistxable polarised relay; and Figures 3 and 4, illustrate further modified circuit arrangemetts. Figure 1 illustrates a load R1 which is connected via te'rtninals 1 and 2 to a d.c.,, voltage supply. sdurce. By way of protec -tion from tmis-poling there is firstly provided a series'connected diode V which only allows current to flow to the load in the case of correct polarity of the terminals 1 and 2, to, provide a potential U,,+ at the output' 'o'f the diode V iiii this embodiment. However, when the poling of the connected d.c. voltage supply source is correct, a;elay K1 is simultaneously excited, and closcs..it's.makcntact.kl to shunt the diode V. Thus,- following the starting current surge tlirough the, diode V, the contact kl carries the, continuous, operating current, so that there is only a much smaller power loss that occurs as -a result of the holding current flowing via the winding of the relay K1. Figure 2 illustrates an alternative embodiment of the proposed protection circuit, which is a modification of the circuit shown in Figure 1, 'in which a load R2 is connected via: the terminals 1 and 2 to d.c. voltage supply source, with a diode V which only allows current to flow through the load when the polling is correct, and provides a poten tial UL+ at the diode output, in this em bodiment In. place of the monostable. relay K1 used in Figure 1, there is a bistable, polarisèd relay K2, which is connected in a shunt arm in series with a capacitor C. This capacitor C is charged by the starting cur rent surge V if correct polarity is applied, whereupon the bistable relay K2 switches over and closes the contact k2.As in Figure 1. the diode V is then shunted so that con tinuousoperating current flows via the make-contact k2. Thus, once again there is no ' power loss in the diode V, but in this case the winding of the relay K2 consumes no'significant power when the relay K2 has been operated as the position which has once been assumed is maintained for the entire. duration in which the load is current.When the operating d.c. voltage supply source is disconnected, the capacitor C discharges via the relay K2, and switches the latter into its other stable position, in which. the makecontact k2 is opened, thus the load is protected from mis-poling by the diode. V. even if the operating voltage is reconnected dnaccurately when the device is switched on again, A; further embodiment using a non-polar. ised, relay is illustrated in Figure 3. Here, the diode V is arranged in series with a limiting resistor RB, which safeguards any fuses, rectifiers or the switch-on contacts of a battery line, by limiting any starting current surge. A capacitor'C3 is connectedtin parallel with the load in order to receive any transient supp]y swings. A makecontact k3 of an unpolarised relay K3 operates in'the same way as described for the relay K1 in Figure 1, but shunts both the diode V and limiting resistor RB. However, it would also be possible to use a bistable, polarised relay having a series capacitor, as described with reference to Figure 2. Figure 4 illustrates a further development of Figures 1 and 3, in which a relay K4 has its winding connected to the full load supply voltage UL of a load R4 but can be energised by an auxiliary voltage U that is produced for the- relay K4 in the load R4, which may be, for example, an inverter already provided in the load circuit. As theauxiliary voltage of such an inverter does not become available until an initial delay has expired for example, approximately 100 ms following the initial connection of a mains voltage, the relay K4 does not operate its contact until the connected device is fully operative.The surge capacitor C4 is charged by them, so that -both the diode V and the contact k4 are only required to supply the normal stabilised load, as the initial surge has passed, and the contact k4 is only lightly loaded. A bistable, poled relay having a series capacitor could also be used, as was described with reference to Figure 2. WHAT WE CLAIM IS:-

1. A d.c. load protection circuit arrangement for protecting electrical load circuits in the event of a connection being made to a supply source in the wrong polarity, a series diode being provided that is conductive when the correct polarity is applied, and said diode being connected in parallel with a make-contact of a relay that is connected in a shunt arm on the side of said diode remote from the supply source terminal connections.

2. A circuit arrangement as claimed in Claim 1, in which said relay is a bistable' polarised relay connected in series with a capacitor in said shunt arm.

3. A circuit arrangement as claimed in Claim 1 or Claim 2 in which a limiting resistor is connected in series with the diode and is bridged together with said diode when said make-contact is closed.

4. A circuit arrangement as claimed in any preceding Claim, in which said relay is

connected in a shunt arm connected in parallel with part of said load circuit.

5. A d.c. load protection circuit arrangement substantially as described with reference to any one of Figures 1 to 4.