JPH0488820A - Protective circuit for switches - Google Patents

Abstract

PURPOSE:To attain effective protection with no change of a design by installing a plurality of transistors turned ON successively when switches are turned ON and supplying the load circuit side with currents when the transistor at a final stage is turned ON. CONSTITUTION:When a switch SW is turned ON, currents are made to flow through a load substrate 2 through a resistor R2, and potential at a point (1) rises. Currents are made to flow through the base of a transistor Q3 through a resistor R4, and the transistor Q3 is turned ON. When the transistor Q3 is turned ON, the potential of the base of a transistor Q2 is increased through a resistor R3, thus turning the transistor Q2 ON. A transistor Q1 at a final stage is turned ON immediately after the transistor Q2 is turned ON, and the load substrate 2 is supplied with currents for drive from a DC power 1. The generation of large inrush currents in a line L1 when the transistor Q1 is turned ON is prevented.

Description

[Detailed Description of the Invention] The present invention provides protection for switches that effectively protect switches such as switches and relay circuits interposed between a power source and a load circuit from inrush current. Regarding circuits.

In electrical equipment in which a control board (4) and a control board (1) are incorporated, a switch or relay circuit is generally interposed between the power supply side and the load board (control board). FIG. 2 shows a conventional example of an electrical device in which a switch SW is interposed between a DC power source 1 and a load board 2. When the switch SW is turned on, the DC power source 1
An output voltage ■α is applied to the load board 2 from.

By the way, a capacitor Ct having a large capacity (several tens of μF to several hundreds of μF) is usually connected to the power supply line 20 of the load board 2. Therefore, in the circuit configuration shown in the figure, when the switch SW is turned on, a very large inrush current of several tens of amperes to several tens of OA momentarily flows through the line interposed with the switch SW, and heat generation occurs at the contacts of the switch SW. This may cause problems such as deterioration or welding. This kind of defect is
A similar problem occurs when a relay circuit is interposed between the power source 1 and the load board 2.

Therefore, in this type of circuit, an inrush current prevention circuit is generally provided on the power supply 1 side or the load board 2 side to protect the switch SW and the relay circuit from inrush current.

By the way, when installing an inrush current prevention circuit on the power supply 1 side or the load board 2 side as described above, a new switching circuit or relay circuit is installed between the power supply 1 and the load board 2. When it is necessary to add an electrical device, major design changes are required on the power supply 1 side or the load board 2 side in order to protect them.Therefore, it becomes difficult to add various functions to this electrical equipment. However, it has the disadvantage that it is difficult to expand the range of applications and improve usability.

The present invention has been made in order to eliminate such drawbacks of the conventional technology, and when it is necessary to add switching circuits and relay circuits, it is possible to effectively protect them without major design changes. The purpose of the present invention is to provide a protection circuit for switches that can easily expand the uses of electrical equipment and improve its usability.

The present invention is a switch protection circuit that protects switches interposed between a power supply and a load circuit, and which protects switches interposed between two lines connecting the power supply side and the load circuit. It is equipped with multiple transistors that turn on in sequence when switches are turned on, and when the last transistor turns on, it supplies current to the load circuit side, and when the capacitor installed in the load circuit is sufficiently charged. The device is characterized by providing means for turning on the transistor at the last stage.

According to the above configuration, after the capacitor on the load circuit side is sufficiently charged, the transistor at the last stage is turned on and current is supplied to the load circuit side. Even if a large capacitor is provided, current is supplied with the capacitance substantially reduced. Therefore, a large inrush current is not generated in the line in which the switches are interposed, and the switches can be effectively protected.

Embodiment 1 (1) An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a circuit diagram showing an electrical device equipped with a protection circuit for switches according to the present invention.

The power supply 1 and the load board 2 are connected to two lines L1. Connected via L2. One end of the line L1 is connected to the output terminal V of the power supply 1.
Connected to cc. On the other hand, one end of the line L2 is connected to the power supply 1
It is connected to the ground terminal GND of. A capacitor C2 is interposed in the middle of the power supply line 20 of the load board 2.

A switch SW for turning on and off the load board 2 is interposed in the middle of the line. And the lines L1 and R2
An inrush current prevention circuit (protection circuit for switches) 3 is interposed between the switch SW and the inrush current to effectively protect the switch SW from inrush current.

Details of this inrush current prevention circuit 3 are as follows.

That is, the transistor Q is connected to the output side of the switch SW of the line Ll.
1 is interposed, and the transistor Q1 is interposed with the transistor Q1.
2 and Q3 are connected. Transistors Q2 and Q are provided in the middle of a line L3 connecting lines L+ and R2. Further, a resistor R1 is interposed between the transistors Q2 and Q of the line L3, and a resistor R1 is interposed between the transistors Q2 and Q of the line L3.
A resistor R is interposed between the intermediate point of the resistor R3 and the line. Further, a resistor R2 is connected in parallel with the transistor Q1 in the middle of the line.

A line L4 is also connected between the line L4 and R2.

Resistors R4 and R1 are interposed in the middle of the line L4.

Further, the midpoint between the resistors R4 and R9 of the line L4 and the transistor Q3 are connected via a line L5, and a capacitor C1 is interposed between the line L5 and the line L2. The capacitance of the capacitor C1 is selected to an appropriate value as described later.

In the inrush current prevention circuit 3 having the above configuration, the switch S
When W is turned on, a large inrush current is not generated in the line L+ when it is turned on, and the contacts of the switch SW are not deteriorated or welded. The reason for this will be explained below along with the operation of the rush current prevention circuit 3.

When the switch SW is off, the potential at the output side of the transistor Q on the line is O, so in this state,
Each transistor Q,,Q2. Q:1 is in the off state. When the switch sw is turned on from this state, a current flows to the load board 2 through the resistor R2, and the potential at point 2 rises.

Then, current flows through the resistor R4 to the base of the transistor Q3, the emitter and collector of the transistor Q3 become conductive, and the transistor Q3 turns on. When transistor Q3 is turned on, the potential of the base of transistor Q2 is lowered through resistor R2, thereby turning on transistor Q2. When the transistor Q2 turns on, the last stage transistor Q1 turns on, and from then on, the DC power supply 1 is connected to the load board 2 through the transistor Q.
Driving current is supplied to.

Here, the timing at which the transistor Q1 turns on is determined by the capacitor C2 interposed in the power supply line 20 of the load board 2.
This is done after the battery is fully charged.

That is, by interposing the capacitor C1 between the line L2 and the line L2, when the switch SW is turned on, the resistance Ra
Since some of the charge flowing into the base of transistor Q3 through the switch SW is accumulated in capacitor CL, the timing at which transistor Q is turned on can be delayed by that amount, and ultimately the timing at which transistor Q at the last stage is turned on is determined by switch SW. It is possible to delay the turn-on operation by a little bit.

In this way, sufficient charge is accumulated in the capacitor C2 from when the switch SW is turned on until the transistor Q1 is turned on, so even if the capacitance of the capacitor C2 is large, the load is When the substrate 2 supplies current, its capacitance is substantially reduced so that no large inrush current occurs in the line L1 when the transistor Q1 is turned on. Therefore, if the capacitance of capacitor C1 is selected to an appropriate value, switch SW
It is possible to reliably prevent problems such as deterioration or welding of the contacts.

In the illustrated example, a case has been described in which the switch SW is interposed in the middle of the line L1, but when a relay circuit is connected in the middle of the line, the contact terminals of the relay circuit can be effectively protected in the same way.

4゜Also, in the illustrated example, one switch SW is provided and one inrush current prevention circuit 3 is provided to protect this switch SW, but in electrical equipment equipped with multiple switches SW or relay circuits, In order to protect the individual switches SW and relay circuits, the above-mentioned inrush current prevention circuits may be placed around them.

According to the present invention described above, since the protection circuit of the switches is directly connected to the switch or relay circuit, it is possible to effectively protect them from rush current.
When it is necessary to add switches, there is no need to make major design changes to the electrical equipment in which these switches are installed. Therefore, there are advantages in that the uses of the electrical equipment can be expanded and the usability can be improved without causing an increase in cost.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a drive circuit provided with a protection circuit for switches according to the present invention. FIG. 2 is a circuit diagram showing a conventional drive circuit. ■...DC power supply, 2...Load board, 3...Inrush current prevention circuit, C,, C2...Capacitor, L,, L2
゜L:l, L4, Ls...railway, Q,,Q,,Q
,...transistor, SW...switch.

Claims (1)

[Claims] (1) A protection circuit for switches that protects switches installed between a power supply and a load circuit, which is installed between two lines connecting the power supply side and the load circuit. When the transistor in the last stage is turned on, current is supplied to the load circuit side, and when the capacitor provided in the load circuit is sufficiently charged, the first stage transistor is turned on. A protection circuit for switches, characterized in that a means for turning on a transistor is provided.