FR2776137A1 - Protective circuit for use in the transmission line for analogue signals - Google Patents

Abstract

Signals (RVB) from a computer are fed to a pre-amplifier stage (PT,R1). Its output is fed to the base of a transistor (T2) acting as an amplifier with collector and emitter resistors (LR1,LR2) selected so that the collector voltage level of a series transistor (T1) is equal to the signal voltage. A diode (D) is connected between base and emitter of the series transistor so that normally it is non-conducting

Description

The present invention relates to a protection circuit, to be placed between a

generator

  analog signal and a transmission line for this signal.

  Such a circuit is used for example in a road vehicle, where there is a risk

  significant electric shock on the lines.

  A protection circuit is known from document DE-OS-28 12 157. According to this document, a switching transistor is associated with an auxiliary transistor controlled in conduction by a signal at the terminals of a current measurement resistor, which

  adds current to the base of the switching transistor.

  An object of the invention is to make the circuit even more efficient.

  To this end, the protection circuit comprises on the one hand a first semiconductor amplifier element mounted both as a conventional amplifier with a first load element and as a follower with a second load element, and on the other hand a second semiconductor amplifier element of the same conduction type as the first, of which a current receiving electrode is connected to the first amplifier element on the side of the second charging element, a current supply electrode delivered to the line, and a control is connected by a resistor

  to the first amplifier element on the side of the first load element.

  These aspects of the invention as well as other more detailed aspects will appear more clearly.

  clearly through the following description of an embodiment constituting a

nonlimiting example.

  Figure 1 shows a road vehicle.

  FIG. 2 is a diagram of an embodiment of the circuit.

  The vehicle 1 in FIG. 1 is provided with a central unit generating an analog signal and a line for transmitting this signal which terminates in a peripheral incorporated in the dashboard DB of the vehicle. The central processing unit CU is for example the computer of a navigation device which delivers RGB signals of color components for a display, the line is for example a shielded bipolar line LI (a

  by color), and the device is for example a DISP liquid crystal display.

  The circuit of FIG. 2 is supplied by a voltage VCC for example of 12 volts. It is to be inserted between an RGB input terminal, on which the CU computer provides a signal, and the LI line. The signal from the input terminal is applied to a preamplifier stage known per se, consisting of a transistor PT mounted as a follower with a load resistor R1. The signal from this preamplifier stage is applied to

  the control electrode, here the base, of a first semi-amplifier element

  conductor, here a bipolar transistor T1 of the NPN type, mounted both as a conventional amplifier with a first load element, here the resistor LR1 placed in the collector connection, and as a follower with a second load element, here the resistor LR2 placed in the transmitter connection. On the other hand a second semiconductor amplifier element of the same type of conduction as the first, here a transistor T2 of NPN type, has a current receiving electrode, namely its collector, connected to the first amplifier element Tl on the side of the second element. of charge LR2, a current supply electrode, namely its transmitter, delivering to the line LI, and a control electrode, namely its base, connected by a resistor to the first amplifier element T1 on the side of the first charge element LR1 . If for example the line LI was in short-circuit with the ground, an excessive current in LR1 would make fall the tension at the point 2, thus also the basic tension of T3,

  thus limiting the current supplied to the line.

  The bipolar transistors could possibly be replaced by field effect transistors. In this case the control electrode would be called the gate, the current receiving electrode called the drain, and the current supply electrode called the source. A diode D is connected between the line LI and the base of the second transistor T2, with its cathode connected to the base of T2, therefore in a direction such that it is blocked in normal service. It would have a blocking effect if accidentally the LI line came into contact with the 12 volt supply. A small model is enough because it never passes a lot of

  current, because it is in series with R3.

  The main current path of the second transistor T2 is shunted by a capacitance between its collector and its emitter, to short-circuit the dynamic resistance of this transistor. The circuit formed with T1 and T12 is followed by a low-pass filter consisting of

  inductance L in series, and two capacitors C1 in parallel.

  An exemplary embodiment uses the following values: - Rl = 470 ohms, LR1 and LR2 = 33 ohms, R3 = 4700 ohms - C1 = 1 nF, L = 1 pH

- D = 1N4148

Claims (4)

CLAIMS:   1. Protection circuit, to be placed between an analog signal generator and a line for transmitting this signal, characterized in that it comprises on the one hand a first semiconductor amplifier element mounted both as a conventional amplifier with a first element charging and in follower with a second charging element, and on the other hand a second semiconductor amplifier element of the same type of conduction as the first, of which a current receiving electrode is connected to the first amplifier element on the side of the second charging element, a current supply electrode flows to the line, and a control electrode is connected by   a resistance to the first amplifier element on the side of the first load element.   2. Protection circuit according to claim 1, characterized in that a diode is connected between the line and the control electrode of the second amplifier element   semiconductor, in a direction such that it is blocked in normal service.   3. Protection circuit according to claim 1, characterized in that the main current path of the second semiconductor amplifier element is shunted by a capacitor. 4. Protection circuit according to claim'l, characterized in that it is followed by a   low-pass filter with inductance and capacities.