JPS599470Y2 - Isolated current transmission circuit - Google Patents

Description

[Detailed description of the invention] This invention is an input/output isolated type that can directly isolate and transmit current signals using a current transformer and supply them with high accuracy to loads whose resistance values fluctuate. Related to current output circuit.

The general structure of an insulated type current output circuit for current is to alternately supply current to the primary winding of a current transformer in opposite directions at a fixed period, and to completely eliminate the current induced in the secondary winding. Many types use wave rectification, smoothing, and supply to the load.

FIG. 1 shows an example of such an input/output isolated type current output circuit.

1 and 2 are input terminals to which the input current I1 is applied, 1 is connected to the midpoint tap P1 of the primary winding n1 of the current transformer T1, and 2 is connected to the transistors Q1 and Q2 as current switch means, respectively.
It is connected to both ends of the primary winding n0 via an emitter-collector circuit.

An excitation signal is supplied from the square wave oscillator 5 through the transformer T2 between the base and emitter of the transistors Q1 and Q2, and Q1 and Q2 are alternately opened and closed every half cycle of the square wave, and the input current h is The current flows alternately in opposite directions through the primary winding n0.

A capacitor C1 connected between input terminals 1 and 2 is for absorbing noise caused by switching.

The AC rectangular wave current induced in the secondary winding n2 is full-wave rectified by diodes D1 and D2, and smoothed by a capacitor C2 connected between the common cathode of D1 and D2 and the midpoint tap P2 of n2. , output current I to load R via output terminals 3 and 4.

supply. 6 is a voltage receiving type receiver connected in parallel to this load R, and the voltage drop (■) that occurs across the load R.

Receives R as an input signal.

In the case of an input/output isolated type current output circuit with such a structure,
If the current transformer T1 has ideal characteristics (zero leakage and infinite self-inductance), even if the load R changes, L
and ■.

However, it is difficult to achieve an ideal one, and one with good characteristics is extremely large and expensive.

Consider again the case shown in FIG.

In this case, if the load resistance R is constant, there is no problem if the voltage drop across R is received by a receiving instrument with high input impedance, but as shown by the broken line, multiple loads R. , R2...Rn is connected in series, and the voltage drop of each load is detected by the receiving instrument A. ,A2...An
If the configuration is to lead to the total load resistance depending on the number of receiving instruments? Because of the difference, the voltage between output terminals 3 and 4 changes, resulting in an output current obtained on the secondary side of the current transformer.

changes.

In other words, the output current ■ due to load fluctuations. ×The voltage drop due to the load resistance R fluctuates, and the voltage of the secondary winding on the output side of the current transformer T1 fluctuates.

This voltage change in the secondary winding changes the leakage magnetic flux of the current transformer T1, and as a result, the current transmission ratio changes, making it impossible to obtain an output current proportional to the input signal.

That is, since the output current changes depending on the fluctuation of the load resistance, there is a drawback that the input/output accuracy decreases.

The present invention provides a circuit configuration that eliminates such drawbacks, and the voltage as seen from the secondary winding side of the current transformer to the output terminal side remains constant regardless of the size of the load. It is characterized by the addition of a circuit such that

FIG. 2 shows an embodiment of the present invention, and the difference from FIG. 1 is that the output points of the diodes D, D2 and the output terminals 3,
A circuit indicated by a dotted line block B is inserted between 4 and 4.

Block B is a base-grounded circuit consisting of a transistor Q3 and a constant voltage element Zener diode D3. The other end of the output terminal 4 and 2 through
It is connected to the center tap P2 of the next winding n2.

Here, if the Zener voltage of the Zener diode D3 is 2 (the Zener voltage Vz is selected to be larger than the voltage (= oxRm) seen from the load side between the output terminals 3 and 4), then the voltage across the capacitor C2 is is transistor Q
It is the sum of the base-emitter voltage VB of 3 and the Zener voltage 2.

Now, if the load resistance increases or decreases, VBE will increase or decrease slightly.

However, VBE<V. (=constant), so capacitor C
Since the voltage across T2 is kept almost constant, the current transformer T
There is almost no voltage change in the secondary winding of No. 1.

Therefore, the output current ■ is exactly proportional to the human power current ■1.

is obtained.

Although Q3 is illustrated as one transistor, it is also a diode D.
1. Output current for the rectified current of D2■.

In order to increase the input/output efficiency of the transistor, it is actually desirable to use composite transistors such as combinary Darlington.

In the embodiment shown in FIG. 2, the human input signal is given as a current, but the input is given as a voltage signal. It is also possible to supply a current proportional to the voltage to the primary winding of the current transformer.

FIGS. 3 and 4 show examples of circuit configurations on the primary winding side of the current transformer in such a configuration.

In Fig. 3, the input voltage e led to input terminals 1 and 2
, is a current proportional to e1 led to the current converting transistor Q5 whose emitter is connected to the resistor RE and is proportional to e1 in the collector circuit.

to occur. The collector of Q5 is a diode Ds, D
6 to one end of each of the pair of primary windings n1 and n12.

nil, n1. The other ends of each of the transformer T3 are connected to both ends of the secondary winding of the transformer T3 to which the primary winding of the rectangular wave power source 6 is connected.

The midpoint of this secondary winding is connected to one of the input terminals 2.

With this structure, the collector current of the transistor Q5 is .

is supplied via the primary winding nil of the current transformer T1, the diode D5 or the primary winding n1, the diode D6 for each half cycle of the AC power supply 6, and the current h is alternately supplied to the primary winding. flows in the opposite direction.

In the structure shown in FIG. 4, the collector of the transistor Q5 is connected to the midpoint P1 of the primary winding n1 of the current transformer T1, and
Both ends of 1 are connected to transformer T3 via diodes D5 and D6.
The operation is almost the same as that shown in FIG. 3.

As described above, according to the present invention, by adding a simple base grounding circuit that combines a transistor and a constant voltage element, the secondary voltage of the current transformer can be adjusted to the size of the load.
It is possible to maintain a constant value regardless of the

Therefore, without using an expensive current transformer, high-precision transmission is possible in a human output isolation type current output circuit for current with load fluctuations, and the industrial benefits thereof are great.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an example of a conventional input/output isolated type current output circuit, Fig. 2 is a circuit diagram showing an embodiment of the present invention, and Figs. 3 and 4 are current output circuit diagrams. This is another example of the circuit structure on the primary side of the transformer. 1, 2: Input terminal, 3.4: Output terminal, ■1: Input current, ■o: Output current, eI: Input voltage, 5: Square wave oscillator, T1: Current transformer, D. , D2, D5, D6
: Rectifier diode, 6: AC power supply, R1, R2...
...Rn: load resistance, Q. ．． Q2, Q3, Q5: }
Ransistor, D3: Zener diode, A1, A2・
...An: Receiving instrument.

Claims (1)

[Scope of utility model registration request] a current transformer; a chopper circuit that alternately switches the current flowing through the primary winding of the current transformer in the opposite direction at a constant cycle;
rectifying means for rectifying the alternating current rectangular wave current output from the secondary winding of the current transformer; smoothing means for smoothing the output from the rectifying means; and a transistor having an emitter connected to one end of the smoothing means. and a constant voltage element having one end connected to the base of the transistor and the other end connected to the other end of the smoothing means, and an output current being extracted from the other end of the constant voltage element and the collector of the transistor. A current output circuit with isolated input and output.