JPH0773178B2 - Isolation circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulation separation circuit using a photocoupler for electrically insulating separation between input and output when transmitting a digital signal or the like in one direction.
The present invention relates to an insulation separation circuit in which a constant voltage diode for setting a primary side operating voltage to a photocoupler is connected in series to a primary side input terminal of the photocoupler.

[0002]

2. Description of the Related Art Conventionally, as shown in FIG. 4, an insulation separation circuit has a protection resistor R for limiting a primary side input forward current to a photocoupler PC with respect to a primary side applied voltage to the photocoupler PC, and a photo resistor. A constant voltage diode for setting the primary side operating voltage to the coupler, that is, a Zener diode ZD for setting a threshold voltage is connected in series to the primary side input terminal of the photocoupler. In the conventional circuit described above, in consideration of the large variation in the CTR (current transfer ratio) of the photocoupler, the minimum expected value of the CTR of the photocoupler used, that is, the photo The Zener diode was selected by determining the value of the protection resistance based on the maximum operating current of the coupler.

[0003]

However, in the circuit designed as described above, a photocoupler having a large CTR, that is,
When a photocoupler with a small operating current value is used, even if an operating voltage, that is, a voltage lower than the Zener voltage, is applied to the primary side, a slight leakage current of the Zener diode causes a signal on the secondary side. It may be transmitted.
Further, the CTR is also affected by temperature characteristics such that its value changes depending on temperature, and eventually, if the value of the primary side operating voltage at which a signal is to be transmitted from the primary side to the secondary side is constant due to the change or dispersion of the CTR. Instead, it was a design problem. In particular, in the case where a large number of insulation isolation circuits are configured, the operating voltage of each circuit is not uniform, and even if the same input voltage is applied to a plurality of circuits, depending on the insulation isolation circuit, a signal is transmitted to the secondary side. There was a problem that some things could not be transmitted and the signal transmission time varied. An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional circuit, and to maintain the operating voltage constant irrespective of changes and variations in the CTR, and to ensure reliable operation at a desired voltage. To provide a circuit.

[0004]

To achieve this object, a characteristic structure of an insulation separation circuit according to the present invention is that a constant voltage diode for setting a primary side operating voltage to the photocoupler is provided at the primary side input terminal of the photocoupler. Insulation separation circuit connected in series, in front of the constant voltage diode,
The point is that a constant current circuit that reduces the leakage current of the constant voltage diode is connected in parallel.

[0005]

The constant current circuit serves as a bypass path for reducing the primary side input leakage current passing through the constant voltage diode when the primary side applied voltage to the photocoupler is lower than the Zener voltage set as the operating voltage. When the applied voltage becomes equal to or higher than the Zener voltage, the photo coupler is surely energized. Therefore, when the voltage is lower than the set Zener voltage regardless of the variation of the CTR and the temperature characteristic, the operation of the photo coupler is surely blocked. When the voltage is higher than the set voltage, a sufficient current will flow to actuate the device promptly.

[0006]

According to the present invention, it is possible to provide an insulation separation circuit capable of controlling the operating voltage of a photocoupler to a substantially constant value regardless of variations in CTR and temperature characteristics.

[0007]

【Example】

Examples will be described below. As shown in FIG. 1, the insulation separation circuit includes DC signal input terminals a1 to 10 of 0-100V.
A protective resistor R1 of KΩ and a constant-voltage diode D2 having a Zener voltage of 50 V are connected in series to connect to the primary side input terminal b1 of the photocoupler PC, and the ground terminal a2 and the primary side ground terminal b2 of the photocoupler PC are connected. To secure a current path to the light emitting diode LED,
A constant current diode D1 of 2 mA as a constant current circuit is connected in parallel between the latter stage of 1 and its ground terminal a2. A 2 mA constant current diode D3 is further connected between the protection resistor R1 and the constant voltage diode D2 to limit the current value flowing from the primary side input terminal b1 into the photocoupler PC. Meanwhile, the light emitting diode LE
The collector of the secondary-side phototransistor PT which operates in response to the light emission of D is connected to a 5V DC power source through a protection resistor R2, the emitter terminal c2 is grounded, and the collector terminal c1 is taken out as an output terminal. That is, when no voltage is applied between the DC signal input terminals a1 and a2, the phototransistor PT is turned off, and a voltage (H) of 5V is output from the collector terminal c1 thereof, and the DC signal input terminals a1 and a2 are connected. When a voltage of 50 V or more, which is a Zener voltage, is applied to the photo transistor PT, the photo transistor PT is turned on, and a voltage (L) of 0.6 V from its collector terminal c1.
Is output.

The operation of the above circuit will be described based on the voltage-current characteristic curve shown in FIG. Now, the light emitting diode LE
Considering a case where the input current to D is set to 2 mA and the voltage applied to the DC signal input terminals a1 and a2 gradually rises from 0 V to 100 V, the resistance R1 and the constant voltage are kept constant until the Zener voltage exceeds 50 V. A current i ₁ of maximum 2 mA flows in a loop of the current diode D1 and the ground, and a leak current to the light emitting diode LED via the Zener diode D2 is blocked. When the voltage applied to the DC signal input terminals a1 and a2 rises and the voltage across the constant current diode D1 exceeds 50V, a current starts to flow to the light emitting diode LED. When the applied voltage further rises and a current of 2 mA flows through the light emitting diode LED, the constant current diode D
3 limits the subsequent current i _{2 to 2} mA. Therefore, the operation of the photocoupler PC is blocked until the voltage exceeds the Zener voltage, and when the voltage exceeds the Zener voltage, the photocoupler PC is instantaneously passed through the light emitting diode LED to operate.
Furthermore, even if an overvoltage is applied, overcurrent protection is performed by the constant current diode D3, so that heat damage to the light emitting diode LED is avoided.

Another embodiment will be described below. In the above embodiment, the case where the voltage applied to the signal input terminals a1 and a2 is a DC signal having a constant polarity has been described, but as shown in FIG.
The constituent elements can be made nonpolar by arranging them in parallel or in series so that the polarities are inverted. In the previous embodiment, the case where the voltage applied to the signal input terminals a1 and a2 is a DC voltage of 0-100V has been described, but the present invention is not limited to this and the value is arbitrary, and the photocoupler PC The threshold voltage for starting the operation can be set appropriately by using a constant voltage diode having an arbitrary Zener voltage.

It should be noted that reference numerals are given in the claims for convenience of comparison with the drawings, but the present invention is not limited to the structures of the accompanying drawings by the entry.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an insulation separation circuit.

FIG. 2 is a characteristic diagram of a voltage-current characteristic curve of an insulation separation circuit.

FIG. 3 is a circuit diagram of an insulation separation circuit showing another embodiment.

FIG. 4 is a circuit diagram of an insulation separation circuit showing a conventional example.

[Explanation of symbols]

 D1 constant current circuit D2 constant voltage diode PC photo coupler

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Koji Watanabe 2-28-2 Shinmachi, Setagaya-ku, Tokyo Sakurashinmachi San Heights 202 (72) Inventor Mitsuo Igarashi 1790-14, Ageo-shi, Saitama Prefecture (72) Inventor Kida Toshihiko 4-1-2, Hirano-cho, Chuo-ku, Osaka-shi, Osaka Prefecture Osaka Gas Co., Ltd. (72) Inventor Hideo Inui 4-1-2, Hirano-cho, Chuo-ku, Osaka City, Osaka (72) Inventor Ichisei Suzuki 13-131 Noda, Nakagawa-ku, Nagoya-shi, Aichi Prefecture (72) Inventor Hideo Ose 14th, Niigata, Oi-gaku, Saya-cho, Kaifu-gun, Aichi Prefecture (72) Masahiko Takayama Plains, Osaka-shi, Osaka Prefecture Kamikita, 1-Chome, 22-17, Kokukei System Co., Ltd. (56) References JP 61-2411 (JP, A)

Claims (1)

[Claims] 1. A primary side operating voltage of a photocoupler (PC)
The constant voltage diode (D2) that sets the
Insulation connected in series to the primary side input terminal of PC (PC)
A constant voltage diode (D2) before the constant voltage diode (D2).
Constant current circuit (D) that reduces the leakage current of the ion (D2)
An insulation separation circuit in which 1) is connected in parallel .