JPH02223335A - Protective circuit - Google Patents

Abstract

PURPOSE: To protect a circuit element connected to a line by conducting a varistor and turning off transistor, when a surge voltage is generated in the line. CONSTITUTION: When voltage surge is generated in a line, a varistor VC1 generates, the forward bias voltage of a diode D3. The bias of a transistor TR2 is reversed, a transistor TR1 is switched accordingly, and the flow of a current into a load is prevented. Thus, the circuit element connected to the line can be prevented against the breakdown, caused by the abnormally excessive high-voltage surge on the line generated by, e.g. a thunderbolt and the fluctuation in the neighboring electric-power transmission lines.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] This invention protects circuits connected to a line from being destroyed by abnormal transient high voltage surges on the line caused, for example, by lightning strikes or fluctuations in adjacent power transmission lines. Concerning protection circuits that protect devices, particularly, but not limited to, telephone lines connected to exchange lines via solid state line switches providing hook switch functionality and loop current interrupt functionality. The present invention relates to a circuit device suitable for protecting elements.

[Prior Art] Protection circuits for telephones of the type described above are known;
It consists of a voltage limiting circuit and a current limiting circuit.

The voltage limiting circuit comprises a voltage dependent non-linear resistor connected in parallel with the line terminals of the telephone to substantially absorb transient high voltage surges on the line, and includes circuitry in the telephone associated with the line terminals of the telephone circuit. Protect the element. Furthermore, in addition to the voltage limiting circuit, a current limiting circuit is also provided. The current limiting circuit includes a resistor device in series with the conductive path line side of the line switch, and a diode device connected between the line end of the resistor and the line switch control element. The value of the resistor is such that when the current threshold is exceeded, the voltage drop across the resistor exceeds the forward voltage drop of the diode device, thereby limiting the current through the conductive path of the line switch.

[Problem to be Solved by the Invention] A drawback of this conventional protection circuit is that a portion of the current generated by the transient voltage surge flows through the line switch;
Therefore, the power rating of the line switch must be sufficient to accommodate this transient current and the selected power rating.

This affects the cost of transistors used in line switches.

It is therefore an object of this invention to protect electrical equipment connected to a line through a solid state switch from destruction due to transient voltage surges on the line, thereby reducing the power rating of the transistors used in the line switch. The purpose of the present invention is to provide an improved protection circuit that enables

[Means for Solving the Problems] According to the present invention, a circuit element connected to a line via a line terminal means is protected from the influence of a transient abnormal high voltage surge, and the circuit element is selected for the line. In a protection circuit comprising a controllable semiconductor switch whose conductive path is connected in series between said line terminal means and said circuit element for connecting one of said line terminal means
a first resistance means of a predetermined value connected in series between a terminal and a conductive path of the semiconductor switch;
and a first voltage reference means connected in parallel to the conductive path of the semiconductor switch and the connection of the control element thereof; a second voltage reference means connected in series;
voltage-dependent second resistance means having a predetermined threshold value connected between the connection between the resistance means and the conductive path of the semiconductor switch and the other terminal of the line terminal means; the first and second voltage reference means are connected in parallel to the first voltage reference means, and the resistance value of the first resistance means is a normal value. The voltage drop at the junction of the first resistance means and the conductive path of the semiconductor switch and its control element due to the line current is smaller than the reference voltage of the first voltage reference means, and the line current has a normal magnitude. When you exceed the
The sum of said voltage drops increases until the reference voltage of said first voltage reference means is reached, at which point said voltage reference means clamps the voltage across said first resistive means, thereby causing a current to flow therethrough. and when the voltage-dependent second resistance means detects its threshold voltage, the voltage drop across the first resistance means is the sum of the reference voltages of the first and second voltage reference means. increasing the current flowing through said first resistive means until equal to the sum, when the voltage across said first resistive means is thereby clamped and the reference of said second voltage reference means A protection circuit is provided which is selected such that the voltage renders the semiconductor switch means non-conducting.

According to this invention, the first voltage reference means further includes 2 (11
A protection circuit is provided comprising series-connected semiconductor diodes.

Objects and features of the invention will become apparent to those skilled in the art from the following description of the embodiments, taken in conjunction with the accompanying drawings.

[Example] Referring to FIG. 2, the circuit is connected to a switch line (not shown).
It is provided with line terminals Ll and L2 for connection to. A voltage protection varistor VCI is connected between terminals Ll and L2. NPN transistor TRI and PNP
) A line switch in the form of a complementary transistor consisting of transistor TR2 connects the line terminal and the telephone transmission circuit TX.
are connected in series between. The collector φ emitter path of the line switch control transistor TR3 is connected between the base of the transistor TR2 and the terminal L2.

A resistor R2 is connected between the base of the transistor TR2 and the collector of the transistor TR3 to provide a small but sufficient base current to the transistor TR2. The base of transistor TR3 is coupled to the output of a microprocessor (not shown) in the transmit circuit. The signal at this output controls the function of the line switch in a well known manner. Resistor R1 connects terminal Ll and transistor TR
A series connection of two diodes DI and D2 is connected between the terminal Ll and the base of the transistor TR2. Resistance R1
and two diodes D1 and D2 form the current limiting part of the protection circuit. The operation of this known current protection circuit is as described above.

Referring to Figure 1, this circuit, except for the protection circuit,
Since it is the same as the circuit shown in the figure, the explanation will be omitted. The protection circuit shown in FIG. 1 includes a resistor R1 connected in series between the terminal Ll and the collector-emitter path of the transistor TRI, and diodes DI and D2 connected in series between the terminal L1 and the base of the transistor TR2. , a varistor vC1 connected between the connection portion of the resistor R1 and the emitter of the transistor TR2, and a terminal L2, and another diode D3 connected between the emitter and the base of the transistor TR2. The resistance value of resistor R1 is selected such that the voltage drop across it at normal operating line currents (≦150 mA) is less than the forward bias voltage of diode DI. Typically, the resistance of resistor R1 is approximately 3 ohms.

In operation, normal line current flowing through resistor R1 and the saturated line switch causes transistor TR
The forward bias voltage across the base-emitter junction of R2 is 0.6 volts and the voltage drop across resistor R1 is 0.
．． Less than 6 volts.

Therefore, the voltage drop across diode DI, D2 is the sum of the forward bias voltage across the base-emitter junction of transistor TR2 and the voltage drop across resistor R1, which is the forward bias across diode DI, D2 when normal line current flows. less than the sum of the voltages (approximately 1.2 volts).

When a high surge voltage occurs on the line, the line current starts to increase, thereby increasing the voltage drop across resistor R1,
Of course, the voltage across the base-emitter junction of transistor TR2 remains unchanged.

Diodes D1.D2 become forward biased when the sum of the voltage drop across resistor R1 and the voltage across the base-emitter junction exceeds the sum of the forward bias voltages of diodes DI and D2. These forward biased diodes DI, D2 clamp the voltage across the resistor R1 and the voltage across the base-emitter junction of the transistor TR2, thus preventing the base drive current of the transistor TR2 from increasing, thereby Limit the current flowing through TRI.

At the same time, the voltage across varistor VC1 increases and when it reaches the operating voltage of varistor vC1, typically 180-250 volts, the current flowing through resistor R1 crosses the voltage drop across resistor R1 and the base-emitter junction of transistor TR2. The sum of the voltages is the voltage across the diodes DI, D2. It increases until it exceeds the sum of the forward bias voltages of D3. From line terminal Ll, diodes Dl, D2 . A current path is set through D3 and varistor VCt to line terminal L2. The forward bias voltage of diode D3 reverse biases transistor TR2, turning it off, thereby turning off transistor TRI and substantially blocking current flow through the line switch during the remainder of the voltage surge. do.

Preferably, a polarity protection circuit (not shown) is inserted between the line terminal and the line switch to ensure that the polarity of the voltage surge is accurately presented to the protection circuit's diodes.

Referring to FIG. 3, the ACC deer switching circuit, a pair each with a protection circuit according to the present invention, for providing full wave protection in case of abnormal power surges on the line (not shown) connected to the input. It has an AC input (~) coupled to a load RL via a solid state switching circuit. In this circuit, varistor VC1 is common to both protection circuits. Switch contacts S1 and S2, when energized, energize transistors TR3 and TR23, which in turn energize line switch transistors TRI, TR2 and TR.21. Turn on TR22.

In operation, during the first half cycle, the current path for normal line current is through resistor R1, the collector-emitter path of transistor TRI, and load RL.

It returns to the line via diode D24. The elements of the protection circuit are the same as those in FIG. If a voltage surge occurs, varistor VCI conducts and creates a forward bias voltage on diode D3, which inverts the bias of transistor TR2 and switches it off, thus switching off transistor TRI and allowing current to flow to the load. to prevent

During the second half-cycle, the current path for normal line current is through resistor R21, the collector-emitter path of transistor TR21, load RL, and diode D4 back to the line. When the voltage across varistor VCI exceeds its threshold, it begins to conduct and creates a forward bias voltage on diode D23, reversing the bias on transistor TR22 and switching it off, thus turning off transistor TR2.
Switch 1 off. Diode D5, DO, D
25. D2B shunts current from load RL.

Although this invention has been specifically described above, it should be understood that equivalent substitutions can be easily made without departing from the technical scope of this invention.

This invention is particularly effective when applied to telephone devices.

[Brief explanation of the drawing]

FIG. 1 shows a portion of a telephone line circuit including a protection circuit according to one embodiment of the invention. FIG. 2 shows a portion of a telephone line circuit with conventional protection circuitry. FIG. 3 shows an ACC deer switching device circuit incorporating the protection circuit of the present invention. R1, R2...Resistor, VCl...Varistor, TX...
...Telephone transmitter circuit. Applicant's agent Patent attorney Takehiko Suzue

Claims (11)

[Claims] (1) The line terminal means and the A protection circuit comprising a controllable semiconductor switch whose conductive path is connected in series with a circuit element, comprising: a controllable semiconductor switch connected in series between one terminal of said line terminal means and the conductive path of said semiconductor switch; The first of the predetermined values
a first voltage reference means connected in parallel to the first resistance means and the connection of the conductive path of the semiconductor switch and its control element; a second voltage reference means connected in series with the first voltage reference means; and a connection between the first resistance means and the conductive path of the semiconductor switch and the other terminal of the line terminal means; a second circuit comprising voltage-dependent second resistance means connected thereto and having a predetermined threshold, the series connection of said first and second voltage reference means being connected to said first The resistance value of the first resistor means is connected in parallel with the voltage reference means such that the voltage drop at the junction of the first resistor means and the conductive path of the semiconductor switch and its control element due to a line current of normal magnitude is is smaller than the reference voltage of the first voltage reference means, and increases until the sum of the voltage drops reaches the reference voltage of the first voltage reference means when the magnitude of the line current exceeds a normal magnitude. , upon which said voltage reference means clamps the voltage across said first resistor means, thereby limiting the current flowing therethrough, and said voltage dependent second resistor means detects its threshold voltage. then increasing the current flowing through the first resistive means until the voltage drop across the first resistive means is equal to the sum of the reference voltages of the first and second voltage reference means; the protection circuit being selected such that the voltage across the first resistor means is thereby clamped and the semiconductor switch means is made non-conducting by the reference voltage of the second voltage reference means when . (2) The protection circuit according to claim 1, wherein the first voltage reference means includes two semiconductor diodes connected in series. (3) The protection circuit according to claim 1 or 2, wherein the second voltage reference means includes a semiconductor diode. (4) The protection circuit according to any one of claims 1 to 3, wherein the voltage-dependent second resistance means includes a varistor. 5. The controllable semiconductor switch comprises one or more bipolar transistors, the collector-emitter path of which forms the conductive path and the base of which is coupled to the control means. Any 1 of paragraph 4
Protection circuit described in section. (6) A protection circuit according to claim 5, wherein the base is coupled to a conductive path of another transistor, and the base of this other transistor is coupled to control means. (7) The semiconductor switch is of complementary type NPN and PN.
Claim 5: comprising a P-transistor, the collector-emitter path of the NPN transistor forming the conductive path, and the first voltage reference means being connected in parallel with the best-emitter junction of the PNP transistor. The protection circuit described in item 6 or item 6. (8) The protection circuit according to claim 7, wherein the semiconductor switch is constituted by a high voltage bipolar device. (9) The line terminal means is connected to the DC input means with a constant polarity by the polarity protection means connected to the line.
A protection circuit according to any one of claims 1 to 8, which is connected to an output means. (10) The controllable semiconductor switch is a telephone line switch, the control element of which is coupled to control means cooperating with the telephone circuit for controlling and switching the loop current circuit of the telephone. The protection circuit according to any one of items 9 to 9. (11) A pair of line terminal means and a pair of load terminals are provided, and one terminal of the line terminal means is the protection circuit according to any one of claims 1 to 9. is coupled to one terminal of the load terminal via the conductive path of the first resistor and the semiconductor switch, and the other terminal of the line terminal means is according to any one of claims 1 to 9. A connecting an AC power line to a load coupled to the other terminal of the load terminals via a conductive path of the first resistor and the semiconductor switch of another protection circuit as described.
C power switch device.