JPH0314925Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a circuit that detects a short circuit in a load while protecting a transistor circuit.

[Conventional technology]

FIG. 2 shows an example of a conventional circuit using a grounded emitter type transistor circuit as the final stage of a drive circuit (driver) for a load (for example, a solenoid load). In the figure, Tr is an NPN transistor, the emitter is grounded, the collector is applied with the DC power supply voltage +E volts via the load L, and the drive signal input from the terminal T1 is applied to the base of the amplifier A1.
and is supplied via input resistor r1. Resistor r2 is a bias resistor for obtaining a base-emitter voltage (VBE). In order to monitor the collector voltage, the collector is connected to a detection output terminal T2 via a resistor r3, and this terminal T2 is connected to a detection circuit (not shown). Such a circuit has a high amplification factor and good stability when operating normally. However, the weak point of this circuit is that the drive voltage is usually a large voltage with sufficient margin.
If the load L is short-circuited, the transistor
A power supply voltage E is directly applied between the emitter and collector of the Tr, and as a result, the transistor Tr is destroyed and becomes open, which is detected as an open failure of the transistor Tr, but it is not possible to detect a short-circuited state of the load.

[Problem to be solved]

An object of the present invention is to provide a load short-circuit detection and protection circuit capable of detecting a short-circuit in the load L while preventing destruction of the transistor Tr even when the load L is short-circuited as described above.

[Means for solving problems]

The load short circuit detection and protection circuit according to the present invention applies a DC power supply voltage to the collector of the first NPN transistor through the load, inputs a drive signal to the base, and grounds the emitter through a resistor. The base and emitter are connected to the emitter grounded type.
are connected to the collector and base of the NPN transistor respectively, and the drive signal voltage and the first
The NPN transistor voltage is supplied to a comparator circuit for determining logical coincidence of these two inputs, and the output of this comparator circuit is configured to detect a short circuit in the load.

[Effect]

In the load short-circuit detection and protection circuit with the above configuration, when the load is short-circuited, an excessive current tries to flow through the first transistor, but the voltage drop of the resistor connected to the emitter increases and the second transistor uses this voltage as a bias. Transistor 2 is turned on. As a result, the base current of the first transistor is reduced, the flow of excessive current into the collector is suppressed, and destruction of the first transistor is prevented. At this time, since the collector voltage of the first transistor is stabilized at the power supply voltage value, a short circuit in the load can be detected by logical comparison with the input drive voltage.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of a load short circuit detection and protection circuit according to the present invention.

The NPN transistor Tr1 corresponds to the transistor Tr in FIG. 2, and its collector is connected to a positive DC power supply +E via a load L.

The emitter of this transistor Tr1 is resistor RE
and ground through the NPN transistor Tr
Connect to the base of 2. The collector of the transistor Tr2 is connected to the base of the transistor Tr1, and the emitter is grounded.

A2, R2, and R3 of the drive signal input circuit are the second
This corresponds to the amplifier A1 and the resistors r1 and r2, respectively, in the circuit shown in the figure.

Also, the collector of transistor Tr1 is connected to resistor R
1 and one input of an exclusive OR circuit (hereinafter referred to as EXOR circuit) via inverter A3, and the other input of this EXOR circuit is connected to amplifier A2.
Connect to the output of

The operation of the circuit shown in FIG. 1 will be explained below.

(1) During normal operation (when the load L is not short-circuited) The input signal is a binary signal, and when the logic is "1", the voltage level is high (hereinafter referred to as "H"), and when the logic is "0", the voltage level is high (hereinafter referred to as "H"). It is assumed that a low voltage level (hereinafter referred to as "L") is taken.

When transistor Tr1 is turned on and current flows through load L, emitter current flows through resistor RE, creating a voltage across resistor RE that does not provide enough bias voltage to turn on transistor Tr2. The bias voltage for turning on Tr2 is, for example, between 0.6 and 0.7 volts for a silicon transistor.

In the above case, the input voltage level and the collector voltage level of the transistor Tr1 have exactly the opposite relationship. Therefore, the output voltage level of amplifier A2 and the voltage obtained by inverting the collector voltage of transistor Tr1 by inverter A3 are always at the same logic level, so EXOR using these as inputs
The output of the circuit is always "L", indicating a normal state.

(2) In the event of an abnormality (when the load L is short-circuited) When the load L is short-circuited, the collector of the transistor Tr1 becomes equal to the power supply voltage +E, and an excessive current attempts to flow. In this case, the emitter current also increases,
The voltage generated across the resistor RE is the transistor Tr2
(e.g. 0.7 volts or more). When the transistor Tr2 turns on, the base voltage of the transistor Tr1 drops and the base current drops.As a result, the excessive current flowing through the transistor Tr1 is suppressed, and this current is caused by the voltage across the resistor RE.
Since an equilibrium state is maintained by applying a bias voltage (for example, 0.7 volts) that turns on Tr2, transistor Tr1 is not destroyed. The collector voltage of the transistor Tr1 is maintained constant at the power supply voltage + E volts, and this voltage is applied to the inverter A3.
An "L" input is supplied to one input of the EXOR circuit through the EXOR circuit. As a result, when there is a driving input
The output of the EXOR circuit becomes "H" or "L" depending on the level of the drive signal, indicating an abnormal state.

〔effect〕

To provide an economical detection and protection circuit with a simple configuration that detects the occurrence of a load short-circuit while preventing a transistor from being destroyed even if the load of a drive circuit is short-circuited.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is an explanatory diagram of a conventional example. Tr1...first NPN transistor, Tr2...
Second NPN transistor, L...Load, E...
DC power supply voltage, T1...Drive signal input terminal, T
2...Detection signal output terminal, A3, EXOR...
Comparison circuit.

Claims (1)

[Scope of utility model registration request] A DC power supply voltage is applied to the collector of the first NPN transistor via a load, a drive signal is input to the base, and the emitter is grounded via a resistor.
Further, the base and emitter of the first NPN transistor are respectively connected to the collector and base of a second NPN transistor whose emitter is grounded, and the voltage of the drive signal and the emitter of the first NPN transistor are connected to each other.
The collector voltage of the NPN transistor is
A load short circuit detection circuit characterized in that it is supplied to a comparison circuit for determining logical coincidence of inputs, and the output of this comparison circuit is used as a detection signal.