JPH0228462Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a current measuring device that measures alternating current and converts it to direct current using a current transformer such as a clamp-type current sensor, and particularly relates to an operational amplifier (hereinafter referred to as "operational amplifier") having a current-voltage conversion function. This invention relates to a current measuring device that converts an alternating current signal into direct current using a single power supply.

As shown in Fig _. 1, the clamp type current sensor has the same function as a current transformer. is an open circuit, and the structure is such that leakage magnetic flux is generated in principle. A load resistance 4 is attached to the secondary winding 3 wound around the iron core 2.
is connected to convert the measured current into voltage.
Therefore, this current measuring device has the disadvantage that measurement accuracy is reduced due to the influence of leakage magnetic flux.

Therefore, as a means for measuring current without being affected by leakage magnetic flux, etc., there is a method using a current-voltage conversion circuit 5 made up of an OP amplifier 5a, as shown in FIG. That is, in this device, due to the feedback effect of the OP amplifier 5a, the load resistance seen from the current transformer secondary winding 3 appears to be almost OΩ.
The secondary side voltage en also becomes OV. Therefore, almost no magnetic flux consumption occurs on the secondary side, allowing highly accurate current measurement.

Furthermore, in processing this type of signal, alternating current signals are rarely used as they are;
In many cases, a rectifier circuit is connected to the output side of the OP amplifier and the signal is treated as a DC signal. For this reason, generally, as shown in FIG.
A current signal is measured by connecting a rectifier circuit 6 which is an ideal diode circuit consisting of an OP amplifier 6a and diodes 6b and 6c. 7 is a smoothing circuit consisting of a resistor R ₂ and a capacitor C ₁ . In the apparatus shown in FIG. 3, the voltage-current converter circuit 5 and the rectifier circuit 6 are each ideal, so that current can be measured with high precision.

However, the device shown in FIG. 3 requires two positive and negative power sources to drive the OP amplifiers 5a and 6a, and cannot be operated with a single power source such as a dry battery. If it were to be operated with a single power supply, a DC-DC converter or the like would be required, which would reduce reliability.

The present invention has been developed in view of the above circumstances, and provides a current measuring device that can measure current with high precision using an OP amplifier and that can drive the OP amplifier with a single power supply. It is.

Hereinafter, an embodiment of the current measuring device according to the present invention will be described with reference to FIG. In the figure, 10 is a current transformer as a current sensor that detects current, and 11 is a current-voltage conversion circuit 11 having an OP amplifier 11a and the like that converts the current detected by the current transformer 10 into a voltage. This circuit 11 is connected to two of the current transformers 10.
The terminals of the next winding 10a are connected to the positive and negative input parts of the OP amplifier 11a, and the positive input part of the OP amplifier 11a becomes the ground side of the single power supply 12 through a parallel circuit of a capacitor C ₁ and a resistor Ra ₁ . Connected to negative polarity Vss. Further, the positive input portion of the OP amplifier 11a is connected to the positive polarity V _DD of the single power supply 12 via a resistor Ra ₂ (where Ra ₁ =R ₂ a). 13 is a comparator circuit using an OP amplifier 12a that compares the output voltage ea of the current-voltage conversion circuit 11 and the 1/2 bias voltage eb of the single power supply 12,
When ea>eb, the subsequent first analog switch S ₁
It has a function of outputting a switch control signal T that turns on the second analog switch _S2 connected via the inverter circuit 13b when ea<eb. These OP amplifiers 11a and 13a both have two power supply parts, and V _DD and Vss of the single power supply 12 are supplied to these power supply parts. and,
A configuration in which the voltage ep converted to DC by the ON operation of analog switches S ₁ and S ₂ is taken out, smoothed by a smoothing circuit consisting of a resistor R ₃ and a capacitor C ₃ , and a voltage signal eo proportional to the measured current is output. It is.

Next, the operation of the current measuring device as described above will be explained. The current Io detected by the current transformer 10 is input to the negative input part of the OP amplifier 11a constituting the current-voltage conversion circuit 11 as a voltage en fed back through the resistor _R1 , while the positive input part is input to the single power supply 12. The resistance of Ra ₁ ,
1/2 bias voltage eb divided by Ra ₂ is input,
This converts the current Io and extracts it as a voltage ea. And this ea is the voltage eb as shown in Figure 5a.
It outputs as a sinusoidal voltage signal centered on , and supplies this to an OP amplifier 13a having a comparator function. As a result, the switch control signal T output from the OP amplifier 13a is as shown in FIG. 5b.
When ea>eb, it becomes H (VDD) level, and when ea>eb, it becomes L (Vss) level, controlling analog switches S ₁ and S ₂ . Analog switch S ₁ ,
_S2 is turned on when its logic input signal becomes H level. By the way, a signal voltage ea' whose direct current is cut off by a capacitor _C2 is led to the switch _S1 . Therefore, analog switch
As is clear from Figure 5, S ₁ turns on.
Since ea>eb, as shown in FIG. 6, only the positive half cycle of the alternating current signal is synchronously output as a rectified signal. This signal is R ₃ , C ₃
If the signal eo is smoothed using a smoothing circuit consisting of the following and outputted, it can be regarded as a measurement current. In other words, the output signal eo of this device is a half-wave rectified value of the signal ea. Note that since the analog switches S ₁ and S ₂ generally use FETs without offset, no voltage loss occurs during this half-wave rectification. Such rectification means is generally called synchronous rectification.

As described above, after converting the secondary current of the current transformer that serves as a sensor into a voltage by connecting it to a current-voltage conversion circuit consisting of an OP amplifier to which a medium bias voltage eb of a single power supply voltage is applied, this current - By comparing the output ea of the voltage conversion circuit with the bias voltage eb using a comparator circuit, only when ea>eb, the output of the current-voltage conversion circuit is DC-cut with a capacitor, and the signal is synchronously rectified and smoothed. It is possible to accurately convert an alternating current signal to direct current using only a single power supply. In other words, even if an OP amplifier is used, it can be realized without using two power supplies or without using a DC-DC converter.

[Brief explanation of the drawing]

Figures 1 to 3 are for explaining conventional devices. Figure 1 is a diagram showing a current transformer such as a clamp type current sensor, and Figure 2 is a diagram showing a current transformer such as a clamp type current sensor, and Figure 2 is a diagram showing a current transformer such as a clamp type current sensor. 3 is a block diagram of a device for measuring current with higher precision than in FIG. 2. FIG. 4 is a block diagram showing an embodiment of the current measuring device according to the present invention. 5 and 6 are operational waveform diagrams of the device shown in FIG. 4. 10...Current transformer, 11...Current-voltage conversion circuit, 11a...OP amplifier, 12...Single power supply,
13... Comparator circuit, 13a... OP amplifier, 13b... Inverter circuit, Ra ₁ , Ra ₂ ...
Resistor, C ₂ ... Capacitor, S ₁ , S ₂ ... Analog switch, R ₃ , C ₃ ... Smoothing circuit.

Claims (1)

[Scope of utility model registration request] A current transformer detects an alternating current signal, and a power supply voltage from a single power supply is supplied to the power supply terminal of the OP amplifier, and the power supply is connected to the power supply from both ends of the single power supply through resistors having approximately the same resistance value. Approximately 1/ of voltage
2 bias voltage eb is applied to the positive input of the OP amplifier, and the current-voltage comprising the OP amplifier converts the alternating current signal from the current transformer supplied to the positive and negative inputs of the OP amplifier into a voltage signal ea. The conversion circuit compares the output voltage ea of this current-voltage conversion circuit with the 1/2 bias voltage eb and determines the output voltage.
A comparator circuit that outputs a switch-on control signal when ea is large, and a signal obtained by turning on the comparator circuit in response to the switch-on control signal and cutting off the DC output voltage ea of the current-voltage conversion circuit with a capacitor. A switch circuit that outputs a synchronous rectification signal and a smoothing circuit that smoothes the synchronous rectification signal output from the switch circuit, and uses the single power source to obtain a DC signal proportional to the AC current signal. A current measuring device characterized by: