DE2835608C2 - Circuit arrangement for generating a pulse-width-modulated signal derived from a measuring current or a measuring voltage - Google Patents

Description

The invention relates to a circuit arrangement for generating one of a measuring current or a measuring voltage derived pulse width modulated signal in the preamble of the claim? mentioned Art.

Such a circuit arrangement is known from Siemens-Zeitschrift 47 (1973) No. 4 pp. 280-283. It is there as part of a so-called time division principle (pulse width-pulse height modulation) working electronic electricity meter and consists of a current measuring circuit and from a pulse width modulator connected to the current measuring circuit. The current measuring circuit is as electronically compensated current transformer with a primary winding, an indicator winding and another Winding executed. The indicator winding is connected to the input of an amplifier, its Output feeds a current into the further winding. This current creates a flow in the current transformer, which is opposite to the flow generated by the measuring current. This negative feedback is does not reduce the non-linearity of the current transformer. The pulse width modulator controlled by the current measuring circuit generates a square wave, its pulse width-pulse pause ratio is modulated as a function of the measuring current

The invention is based on the object of ensuring the linearity of the circuit arrangement mentioned at the beginning to enhance.

The invention is characterized in claim 1. Further developments of the invention are set out in the subclaims specified.

The measures specified in claim 1 ensure that the pulse width modulator is included in the negative feedback circuit of the current transformer This causes non-linearities of the pulse width modulator corrected automatically improves the stability of the entire circuit arrangement and drifting is reduced. Furthermore, there is the advantage that failure of components not only of the current transformer, but also the pulse width modulator can be recognized with simple means. As soon as a major change or failure of a component of the circuit arrangement is the compensation the flow in the current transformer is prevented, signals appear on the indicator winding of the current transformer whose values significantly exceed the values present under normal operating conditions, so that the faulty condition can be easily determined.

An exemplary embodiment of the arrangement according to the invention is shown in FIG. 1.

The current flowing in the indicator winding 2 of a current transformer 1 feeds the inverting input of an amplifier 3 with the feedback network Ri, Ci, from whose output voltage the current i _{m is} derived via the resistor Rv, which is derived from the integrator amplifier 4 with the feedback capacitance Cm, the Schmitt trigger 5, the controlled electronic switches 6 and? as well as the reference voltage sources 8 and 9 feeds the pulse width modulator formed The switches 6 and 7 are switched so that while one (for example 6) the reference voltage Ur + (8) via R _r to the input of 4, the other switch (7) the reference voltage opposite polarity U _r - (9) switches to the inverting input of the amplifier 10 via R _r. At ; _m = 0, in the switching position shown, a positive current ir \ = Ur + IRr flows to the summation point of amplifier 4, at the output of which a negative linear voltage change occurs. If this reaches a lower switching level ί ^ d ^ s Schmitttriggers 5. it flips over and switches all switches to the opposite one

so switch position. Now a negative current in = Ur-IRr ensures a positive increase until an upper switching level Uh of 5 is reached and the cycle starts all over again. If U _{r +} - - U, - and i _r \ = - in = i _r = UrIRr and if faults in the components are disregarded, then the duration ii of the phase with positive i _{r is} equal to

Duration ft with negative i _r . 1st / m = 0 so:
, _ C _M (U " - U _h )

" ΓΠΓ ~

and

C _m (U _h - U ")

For the pulse duty factor λ one obtains:

During the time fi, a current / * = - /> flows to the input of the amplifier 10 and during h i'k = + ir- is its mean value

The amplifier 10 filters out the harmonics of the high-frequency switching frequency U = and drives

Via the resistor R _a, a current 4 through a third winding 11 of the current transformer 1 is generated in the indicator winding 2 which is determined by the input current / and the feed current u . The amplifier 3 is derived therefrom, a voltage from the upper R _v the input current / _m of the modulator circuit consequence has thus described circuit forms a negative feedback loop that is stable when caused by the input current / in the current transformer 1 magnetomotive force generated by the of U at correct winding sense of 11 is compensated. /, corresponds to the uncompensated remainder remaining due to the finite loop gain.

A product is formed in a manner known per se, in that the switches 13 and 14, synchronously with 6 and 7, switch through the secondary voltage derived from the measurement voltage U via a voltage converter 12 via the resistors Ru , the two currents i _p and i _p 'proportional to the product. develop. Both currents - like i _r and 4 - are opposite and have the same magnitude.

The voltage or the current at the winding 2 is a sensitive measure for correct functioning of the negative feedback loop. If errors occur in the circuit that interfere with the control loop, these increase Values strongly. A predetermined permissible level can be exceeded by means known per se detected and indicated by electrical, optical or acoustic signals. Deviations the two reference sources 8 and 9 from each other are also easy to use with known means To bring a report. These measures practically eliminate errors that go undetected.

In addition to the use of the arrangement according to the invention for normal power and energy measurement it is suitable for the acquisition of the current in high voltage lines by in the course of the high voltage line the modulator arrangement according to the invention is switched and that generated by the modulator current proportional duty cycle with optoelectronic means in a known manner on Erdpoter.tial where it is used for energy, power or current recording for measurement or protection purposes can be further processed.

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1 sheet of drawings

Claims (4)

Patent claims: 1. Circuit arrangement for generating a pulse width modulated signal derived from a measurement current or a measurement voltage, with a current transformer which has a primary winding for the measurement current or a current of an indicator winding derived from the measurement voltage, which is connected to a further winding via a negative feedback circuit that a current fed into the further winding by the negative feedback circuit generates a flux which is opposite to the flux generated by the measuring current or the measuring voltage, and with a pulse width modulator connected to the current transformer, the output signals of which actuate electronic switches, characterized in that the electronic switches (6, 7) would require the one switching phase fa) of the pulse width modulator (4 to 9, Rv, Cm, Rr) a positive reference direct current (k = + i _r ) and during the other switching phase (t _t ) a negative reference direct current (k = - i _r ) in the further wic feed in the cooling (11). 2. Circuit arrangement according to claim 1, characterized in that the indicator winding (2) is connected to a circuit that generates an electrical, acoustic or optical signal, if the voltage or the current at the indicator windlig (2) exceeds a preselectable level. 3. Circuit arrangement according to claim 1 or 2, characterized in that d '} the reference direct current fa) is fed into the further winding (t 1) via an amplifier (10) and a filter (Ke, Cc) 4. Circuit arrangement according to one of the preceding claims, characterized in that the electronic switches (6, 7) are changeover switches and at the same time components of the pulse width modulator (4 to 9, R _v , C _m Rr) .