The invention relates to power units having two output voltages of opposite polarity and common zero potential which, for connection to an input a.c. voltage, has a rectifier and a linear regulator for each output voltage.

Such a power unit is known from the article "Integrated Voltage Regulator" in Elektronik Entwicklung 6/1979, pp.24 to 46.

Installations are known (Fig 1) which have a central mains supply connected via a transformer to a transmitter with an incorporated power unit and a plurality of remote electronic regulators each having an incorporated power unit. To reduce the number of lines which have to be installed, one connection of the secondary winding of the transformer is chosen as the zero potential. The secondary winding supplies a voltage serving as an input a.c. voltage U- to the transmitter and the regulators. The transmitter voltage at an output A of the transmitter is also related to this zero potential. The transmitter voltage is applied to inputs E, to E₄ of the regulators 3a-d, respectively, and is used for the control thereof.

It is a problem to provide a power unit of the aforementioned type having a balanced load of the common zero potential line.

According to the invention there is provided a power unit with two output voltages of opposite polarity and common zero potential line which power unit, for connection to an input a.c. voltage, has a rectifier and a linear regulator for each output voltage, characterized in that a current balancing circuit is arranged between the rectifiers and the regulators and that the line carrying the zero potential is connected to one of the two lines of the input a.c. voltage.

Power units according to the invention have a balanced current on the zero potential line. As a result, it is possible to use a common neutral conductor which reduces expenditure in connection with the line. It is even possible to connect regulators with a switching behaviour and can be controlled with a d.c. voltage from the remote transmitter, without feedbacks occurring.

The invention, will now be described, by way of example, with reference to the accompanying drawings, in which:-

• Fig 1 a block circuit diagram of a known installation with a central transmitter and regulators remote therefrom;
• Fig 2 a circuit diagram of a power unit according to the invention with a current balancing circuit; and
• Fig 3 a circuit diagram of a further power unit according to the invention.

Referring to Fig 1, a known installation has a central mains supply connected via a transformer 1, to a transmitter 2 with incorporated power unit and a plurality of remote electronic regulators 3a-3d each having an incorporated power unit. To reduce the number of lines which have to be installed, one connection of the secondary winding of the transformer 1 is chosen as the zero potential. The secondary winding supplies a voltage serving as an input a.c. voltage U- to the transmitter 2 and the regulators 3a-d. The transmitter voltage at an output A of the transmitter 2 is also related to this zero potential. The transmitter voltage is applied to inputs E_l to E₄ of the regulators 3a-d, respectively, and is used for the control thereof.

Fig 2 shows a power unit, to whose input terminals is applied the input a.c. voltage U-. The lower input terminal in Fig 2 has zero potential and is designated by O. The input a.c. voltage U-is rectified by two rectifiers D1 and D2. As the rectifiers have opposite polarity, two voltages occur at the two following capacitors C1 and C2 with different polarities related to the common zero potential. These two voltages are applied by means of low-valued resistors R1 or R2 of a current measuring circuit 4 to in each case one linear regulator IC1 or IC2. In the represented embodiment, the regulators comprise known integrated voltage regulators. Regulators IC1 and IC2 are wired to capacitors C3 to C6 in order to prevent undesired oscillations. The two output voltages +U_A and -U_A, related to the zero potential 0, are then available at the indicated terminals.

To current measuring circuit 4 is connected a comparator circuit 5 and to the latter a current control circuit 6. Together all the three circuits 4, 5 and 6 form a current balancing circuit.

The current measuring circuit 4 comprises a bridge circuit formed from resistors R3 to R6 and which measures the voltage drop corresponding to the current J, or J₂ through resistor R1 or R2. These voltage values reach the comparator circuit 5, which comprises the operational amplifier IC3 and the resistors R7 and R8 at the inverting or non-inverting input, resistor R9 from the non-inverting input to the zero potential 0 and resistor R10 from the output to the inverting input of the operational amplifier IC3. The comparison result at the output of the operational amplifier IC3 is a positive or negative voltage value, depending on whether current J1 or current J2 preponderates. This voltage value reaches the base of transistor T1 or T2 across a diode D3 or D4. The collector of transistor T1 is connected to the input of regulator IC1 and its emitter is connected across a resistor R13 to the zero potential. A resistor R11 is connected from the base of transistor T1 to zero potential. Transistor T2 is connected in the same way with resistors R12 and R14 between the input of regulator IC2 and zero potential.

The voltages corresponding to currents J, and J₂ are determined by the current measuring circuit 4 compared with one another in comparator circuit 5 acting as a porportional controller and a resulting voltage value is supplied to transistor T1 or T2 acting as the control element. Transistor T1 or T2 then carries such a current that, apart from the very small control deviation, currents J, and J₂ are the same.

Fig 3 shows a power unit in which one output voltage +U_A is much more highly loaded than the other output voltage -U_A. Due to the lighter loading, a circuit comprising a resistor R12 and a Zener diode D6 is used as the regulator for this output voltage -U_A. The input and output connection isthe same as the circuit according to Fig 2 and consequently the same references are used for the same parts.

Downstream of capacitor C1 a series connection of a resistor R13 and a Zener diode D5 is connected to zero potential. Zener diode D5 has the same Zener voltage as Zener diode D6 and resistors R12 and R13 are identical. This power unit also has a current measuring circuit 4',to which is connected a comparator circuit 5'. The following current control circuit 6' has only one transistor T3 in series with a resistor R11 connected from the input of regulator 7 to zero potential O. The addition of the apostrophe means that in principle the circuit with the same reference numeral is used. Transistor 13 is adequate, provided that the output voltage -U_A, is more lightly loaded than output voltage +U_A, because then a balancing current only has to flow in the lightly loaded branch.

Current J₆ is the same as current J₇, it being assumed that current J₂ is lower than current J₆. Current J, is subject to greater fluctuations and greater loading and consequently so is current J₈. Current J₈ is determined by the current measuring circuit 4' and from this a control signal for transistor T3 is obtained in such a way that current J₅ is the same as current J₈.

As J₆=J₇ and now J_s=J_s and J₃=J_S+J₆ and J₄=J₇+J₈, it follows that J₃=J₄, so that balanced loading of the neutral conductor is obtained.