JPH0446058B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection circuit for devices controlled by electrical signals, of the type in which the input side is protected against positive and negative overvoltages by the protection circuit.

In electrical control systems used, for example, for gasoline injection systems in motor vehicles, integrated units with narrow operating voltage tolerances are generally used. The operating voltage is normally 5V±10%. The signal at the input side of the unit must also be between 0V and 5V in this case if full functionality is to be guaranteed. To avoid damage to the unit, the input signal should generally not fall below a value of -0.3V and should not exceed +7V.

During operation of a motor vehicle, high positive or negative voltage peaks repeatedly occur on the signal lines of the control device, for example due to the interruption of ignition, inductive loads and other low-resistance loads.

The object of the invention is to limit the voltage to a permissible value at the input of the unit without distorting the signal itself.

In order to solve this problem, a structure having the features shown in claims 1 and 2 is provided.

Analog signal protection circuits, in which the desired limit voltage is adjusted by a voltage divider connected between the positive pole of the working voltage and the negative or ground wire, are optimal because the analog signal retains its value without distortion. be. For the protection of digital signals in which the absolute value of the signal voltage is not important or essential, according to the invention a forward polarized diode is provided in each case instead of the first and third resistors. In both cases, it is particularly advantageous to integrate the entire circuit together with two transistors of opposite conductivity type in an integrated circuit in which the other units of the control device are integrated.

DESCRIPTION OF EMBODIMENTS Two embodiments of the present invention will be described in detail below using the figures.

Digital signal protection circuit in FIG. 1 The terminal marked with a circle, to which the input signal voltage Uein is supplied, is at the same time the input side of a device (not shown) to be protected by this protection circuit.

In FIG. 1, diode D1, resistor R2, and diode D2 are directly connected, and this series resistor is further connected between Ugrenz and ground. In this case Ugrenz is the positive operating voltage U _B of the protection circuit. Current always flows through the series connections D1, R2, and D2. Therefore D1 and D2
The voltage drop at is the diode forward voltage. That is, each voltage is 0.7V. Therefore, a voltage of 0.7V is applied to the base of T1, and a voltage of Ugrenz - 0.7V is applied to the base of T2.

For digital signals (eg switches for start, no load, etc.) only logic states 0="Low" and 1="High" must be detected.
Generally, a TTL- circuit detects a voltage smaller than 0.7V as "Low" and a voltage larger than 2.4V as "High". Therefore, the input signal is exactly
Limited to greater than OV and less than 5V.

The circuit according to Fig. 1 has an input voltage Uein of 0V and +
Limit to lower values between 0.3V and upper values between Ugrenz − 0.3V and Ugrenz. The change in limit value is due to temperature,
It depends on the current flowing through diodes D1 and D2 and the current flowing into the bases of transistors T1 and T2. Ugrenz is the operating voltage U _B of the large resistance integrated element.

Function: Approximately 0.7V to the base of transistor T1 via a voltage divider consisting of diodes D1, D2 and resistor R
voltage is generated with respect to ground, and the transistor
A voltage of -0.7V is generated at the base of T2 with reference to approximately Ugrenz.

In normal conditions, ie in the absence of overvoltages, both transistors T1 and T2 are switched off. Then, when a negative overvoltage is applied to terminal E and Uein drops to about 0V, transistor T1
Current flows to the base of. This current flows into the transistor by making transistor T1 conductive.
A collector emitter current flows from Ugrenz to Uein. This collector emitter current has resistance
In conjunction with RV, it does not reduce the voltage at Uein even further. This is due to the protection circuit, so 0V
This means that the lower input signal Uein does not appear at terminal E. Negative overvoltage at terminal E is therefore eliminated by this protection circuit.

When the voltage at Uein rises to Ugrenz, the base current flows through T2, so that this transistor limits the voltage upwards.

The difference between the protection device for analog signals and the protection device for digital signals according to Figure 2 is that for digital signals, the limit region is not critical near 0V and Ugrenz, but for analog signals, the limit region is completely pure from 0V to Ugrenz. The point is that it remains the same. Therefore, the analog signal re-protection function is below 0V and
It is first performed above Ugrenz. This limits the input signal to −0.3V…0V and
The value will be Ugrenz+0.3V.

This is achieved as follows: Using the circuit according to FIG. 1 when using diodes with low forward voltage (for example Schottky diodes) for diodes D1 and D2.

Resistors (as shown in FIG. 2) are used to adjust the base-bias voltage for transistors T1 and T2.

Function Analog signal circuits operate in the same way as digital signal circuits, and differ from digital signal circuits in the following ways:
The voltage at the base of transistor T1 is across resistor R3
is brought close to 0V, and the voltage at the base of transistor T2 is raised to close to Ugrenz by resistor R1.

Advantages of the Invention A particular advantage of the protection circuit according to the invention is that it is suitable both for the protection of devices controlled by analog signals and for the protection of devices controlled by digital signals.

Another advantage is that only one connection is required for the line to be protected during the integration of the circuit.

[Brief explanation of the drawing]

FIG. 1 shows a schematic circuit diagram of a digital signal protection circuit according to the invention, and FIG. 2 shows a circuit diagram of an analog signal protection circuit according to the invention. Uein...input voltage, Ugrenz...limit voltage.

Claims (1)

[Claims] 1. In a protection circuit for a device controlled by an electrical signal, the input side of which is protected from positive and negative overvoltages by the protection circuit: two transistors (T1, T2) of opposite conductivity types; are provided, the transistors are connected in series with each other to the operating voltage of the device, the mutually connected emitters of the two transistors being connected indirectly to the signal voltage via a preresistor (RV), It is also connected directly to the input side (E) of the device, and further a voltage divider consisting of a first resistor (R1), a second resistor (R2) and a third resistor (R3) is connected to the operating voltage, and the current The base of the first transistor (T1) connected in the direction of flow is connected to the connection point between the second resistor (R2) and the third resistor (R3), and the base of the second transistor (T2) A protection circuit for a device controlled by an electrical signal, characterized in that it is connected to a connection point between a resistor (R1) and a second resistor (R2). 2. In a protection circuit for a device controlled by an electrical signal in which the input side is protected from positive and negative overvoltages by the protection circuit, two transistors (T1, T2) of mutually opposite conductivity types are provided, The transistors are connected in series with each other to the operating voltage of the device, the mutually connected emitters of the two transistors being connected indirectly via a preresistor (RV) to the signal voltage and also to the input side of the device ( a first diode (D1) connected directly to E), a second diode (D2) connected in the forward direction, and a resistor (R2) connected between said two diodes. The voltage generator is connected to the operating voltage, the base of the first transistor (T1) connected in the direction of current flow is connected to the connection point of the resistor (R2) and the second diode (D2), A protection circuit for a device controlled by an electrical signal, characterized in that the base of the transistor (T2) is connected to the connection point between the second diode (D1) and the resistor (R2). 3. A protection circuit for a device controlled by an electrical signal according to claim 2, wherein the diode has a significantly low forward rise voltage.