JPH08241138A - Constant-current power circuit - Google Patents

Abstract

PURPOSE: To provide the constant-current power circuit which can suppress heat generation as much as possible when a load is short-circuited. CONSTITUTION: This circuit has a transistor Q1 for constant current supply which is applied with a supplied voltage at its base through a Zener diode ZD1 and at its emitter through a resistance and supplies a constant output current from its collector to the load Z, the emitter and collector of a transistor Q2 for protection for protecting the transistor Q1 for constant-current supply in a load short-circuit state are connected to both the terminals of the Zener diode ZD1 , and a resistance R5 for biasing is interposed between the base and emitter of the transistor Q2 for protection; and a Zener diode ZD2 for protection and a resistance R4 are connected in series between the collector of the transistor Q1 for constant-current supply and the base of the transistor Q2 for protection and the Zener voltage of the Zener diode ZD2 for protection is so set that the protection transistor Q2 turns on when the load is short-circuited.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to protection of a constant current supply transistor when a load of a constant current power supply circuit is short-circuited.

[0002]

2. Description of the Related Art Conventionally, as shown in FIG. 1, a supply voltage VIN is applied to a base of a constant current supply transistor 1 via a resistor R3, a diode D1 and a zener diode ZD1, and a resistor R3 and a resistor R1. Is applied to the emitter of the constant current supply transistor 1, the base of the constant current supply transistor 1 is grounded via the resistor R2, and a constant output current Io is supplied from the collector of the constant current supply transistor 1 to the load Z. In addition, a series circuit including a resistor R4 and a zener diode ZD2 is interposed between the emitter and collector of the constant current supply transistor 1 to protect the constant current supply transistor 1 when the load is short-circuited. A constant current power supply circuit is known (see Japanese Utility Model Laid-Open No. 60-123015).

In this conventional constant current power supply circuit, when the load Z is short-circuited, a short circuit current larger than the constant output current Io is indicated by the arrow A, the resistor R3, the diode D1, the zener diode ZD1, the diode D3 and the resistor R4. , While flowing to the load Z through the Zener diode ZD2,
Resistance R3, resistance R1, resistance R4, zener diode Z
Since the current flows to the load Z via D2, heat generation of the constant current supply transistor 1 is suppressed, and the constant current supply transistor 1 is protected.

[0004]

However, in this conventional constant current power supply circuit, the resistor R3, the diode D1,
Zener diode ZD1, diode D3, resistance R
4. The heat generated by the current flowing through the Zener diode ZD2 is large. Moreover, since the resistor R3 is interposed,
Constant output current Io flowing through the load Z during constant current operation
A steady loss corresponding to the resistance value R3 occurs.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a constant current power supply circuit capable of suppressing heat generation due to a short-circuit current as much as possible when a load is short-circuited.

[0006]

In order to solve the above problems, the constant current power supply circuit according to claim 1 of the present invention applies a supply voltage to a base via a Zener diode and an emitter via a resistor. In a constant current power supply circuit having a constant current supply transistor that is applied to the load and supplies a constant output current from the collector to the load, protection for protecting the constant current supply transistor when the load is short-circuited across the Zener diode. For connecting the emitter and collector of the protection transistor, interposing a bias resistor between the base and emitter of the protection transistor, and connecting the collector of the constant current supply transistor and the base of the protection transistor. A protective Zener diode and a resistor are connected in series between the protective Zener diode and the load. The Zener voltage as the protection transistor is turned on is set to fault occurs. Preferably, a capacitor is connected in parallel with the bias resistor.

[0007]

According to the present invention, when the load is short-circuited, the zener diode for supplying constant current breaks down and the protection transistor is turned on. As a result, the constant current supply transistor is turned off and the output current is cut off.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a constant current power supply circuit according to the present invention will be described below with reference to the drawings.

[0009]

First Embodiment FIG. 2 shows a first embodiment of a constant current power supply circuit according to the present invention.

In FIG. 2, reference sign VIN 'is a supply voltage application terminal, and reference sign Q1 is a PNP type constant current supply transistor. The supply voltage application terminal VIN 'is connected to the emitter of the transistor Q1 via the resistor R1 and is connected to the base of the transistor Q1 via the Zener diode ZD1 so that the supply voltage VIN is connected to the transistor Q1 via the resistor R1. The transistor Q is applied to the emitter and also through the Zener diode ZD1.
1 is applied to the base. The collector of the transistor Q1 is grounded via the load Z, and the load Z is normally supplied with a constant output current Io. The base of the transistor Q1 is grounded via a resistor R2 for setting the bias voltage of the Zener diode ZD1.

The emitter and collector of a PNP type protection transistor Q2 for protecting the constant current supply transistor Q1 when the load is short-circuited are connected to both ends of the zener diode ZD1. A bias resistor R5 is interposed between the base and emitter of the protection transistor Q2. A resistor R is provided between the collector of the constant current supply transistor Q1 and the base of the protection transistor Q2.
4 and the protective Zener diode ZD2 are connected in series.

The zener voltage of the protective zener diode ZD2 is larger than the difference voltage (VIN-Vo) between the normal supply voltage VIN and the voltage Vo across the load Z, and the zener voltage of the short-circuited supply voltage VIN and the load Z is large. The Zener voltage V is set to be smaller than the voltage difference (VIN-Vo ') with the voltage Vo' between both ends at the time of short circuit, so that the protection transistor Q2 is turned on and the constant current supply transistor Q1 is turned off at the time of load short circuit.
F is set.

According to the first embodiment, during the normal operation, the protection transistor Q2 is off, and the constant current supply transistor Q1 is turned on as in the conventional case. Therefore, the emitter-collector of the constant current supply transistor Q1. A constant output current Io flows through the load Z throughout. Here, when the load Z is short-circuited, the emitter-collector voltage of the constant current supply transistor Q1 rises as compared with the normal operation, and the voltage applied across the Zener diode ZD2 becomes higher than the Zener voltage. Therefore, the Zener diode ZD2 Breakdown, which turns on the protection transistor Q2 and turns off the constant current supply transistor Q1. For this reason,
The constant output current Io is cut off. Even if the short-circuit state continues, the base current I1 as a short-circuit current flowing through the emitter-base of the protection transistor Q2, the resistor R4, and the zener diode ZD2 when the protection transistor Q2 is turned on is a constant output current Io. Since it is much smaller than the above, heat generation due to the short-circuit current can be minimized when the load is short-circuited.

[0014]

Second Embodiment FIG. 3 shows a second embodiment of the constant current power supply circuit according to the present invention.

In the first embodiment, when the supply voltage VIN is suddenly applied and sharply raised, in other words, when the rise time of the supply voltage VIN is very short, the constant current supply transistor Q1 is protected before being turned on. Although the transistor Q2 is turned on and the load Z is not short-circuited, the constant current power supply circuit may malfunction in the short-circuit protection mode.

In the second embodiment, in order to prevent this malfunction, as shown in FIG. 3, a capacitor C1 is connected in parallel with a bias resistor R5, and a protection transistor Q2 is provided.
The operation start of (1) is delayed compared with the operation start of the constant current supply transistor Q1.

[0017]

According to the constant current power supply circuit of the first aspect of the present invention, heat generation due to the short-circuit current can be suppressed as much as possible when the load is short-circuited. In addition, a circuit for protecting the constant current supply transistor can be manufactured at low cost.

According to the constant current power supply circuit of the second aspect of the present invention, it is possible to avoid the malfunction that the constant current power supply circuit enters the short-circuit protection mode even though the load is not short-circuited.

[0019]

[Explanation of symbols]

 Q1 ... Transistor for supplying constant current Q2 ... Transistor for protection ZD1 ... Zener diode ZD2 ... Zener diode for protection R4 ... Resistor R5 ... Resistor for bias Z ... Load

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional constant current power supply circuit.

FIG. 2 is a diagram showing a first embodiment of a constant current power supply circuit according to the present invention.

FIG. 3 is a diagram showing a second embodiment of the constant current power supply circuit according to the present invention.

Claims (2)

[Claims] 1. A constant current power supply circuit having a constant current supply transistor which applies a supply voltage to a base via a Zener diode and a emitter via a resistor, and supplies a constant output current from a collector to a load. The emitter and collector of a protection transistor for protecting the constant current supply transistor are connected to both ends of the Zener diode at the time of load short circuit, and a bias resistor is provided between the base and the emitter of the protection transistor. And a protection Zener diode and a resistor are connected in series between the collector of the constant current supply transistor and the base of the protection transistor, and the protection Zener diode protects the load when the load is short-circuited. Current transistor whose zener voltage is set so that the Circuit. 2. A capacitor is connected in parallel with the bias resistor between the base and the emitter of the protection transistor in order to prevent the protection transistor from malfunctioning when the supply voltage rises. The constant current power supply circuit according to claim 1.