DE2544482A1 - Switching surge suppression system - is for voltage and current limited supplies with intermediate controlled current build up circuit and uses two transistors - Google Patents

Abstract

The system is for control of safe power supplies used in explosion risk environments. The object is to limit the capacitative current flow due to long leads on the output side when switching on. Astable multivibrator (2) has a controlling input from a divider (R5, R6) across the circuit output. The vibrator output controls a transistor (V1) shunting a capacitor (C) in series with a resistor (R3) across the supply with their common point connected to one input of an operational amplifier (3). The amplifier output controls the base of a second transistor (V2) in series with one line. With the first transistor (V1) conducting the amplifier output is practically zero and the series transistor (V2) is non-conducting. If the control transistor turns off then the capacitor (C) begins to charge giving an input to the amplifier such that the resultant output turns on the series transistor (V2) at a rate comparable to the charging rate of the capacitor thus producing a controlled voltage build up.

Description

"Start-up circuit for Zener barriers with active current

limitation "The invention relates to a start-up circuit for Zener barriers with active current limiting. It is used for intrinsically safe Circuits for explosion-proof rooms.

With "Zener Barrier" a circuit of any active and Passive components that form an intrinsically safe electrical circuit make and serve to limit an output voltage or an output current.

In the earlier patent application P 24 11 633 is already proposed been to make current limiting shai lungs.

This is done, for example, by the fact that a controllable semiconductor component is arranged in the power supply line so that a constant current source is switched through. When the current limit is reached, a programmable to Uriijunkti on-Transis Lor the iiaible iterbaut ement switched off in such a way, that the energy occurring at the output is limited.

The inrush currents on the output side are disadvantageous here with capacitive load. This means that the capacitive inrush current can occur with long cables become so high that it is far above the maximum permissible current limit value. Through this commissioning may become impossible.

It is therefore the object of the invention to provide a start-up circuit for Zener barriers to find that delivers a slow, defined voltage rise at the output, so that the charging current surge of the capacitance, which is connected at the output, is below the permissible response limit remains.

According to the invention this is achieved in that a transistor is provided is, which controls an operational amplifier via an RC element and that at the output of an operational amplifier the control electrode of one with its emitter-collector path series transistor lying between input A and output B of the start-up circuit whose voltage rise at the output corresponds to the voltage rise at the RC element.

An astable multivibrator is provided, which can be used when the Voltage at the output blocks the transistor and when the output is de-energized, the transistor periodically switches through and blocks.

In an advantageous embodiment, the series transistor and resistor are Part of the current limiting circuit.

It is advantageous to reliably prevent the failure of the Zener barrier when switching. The automatic restart of the is also advantageous Arrangement after failure of the voltage at the output. This automatic reset takes place after each triggering and makes the previously usual manual operation superfluous. Further advantages can be seen from the description.

An embodiment of the start-up circuit according to the invention is explained in more detail below with reference to the drawing.

There is a 'known' at the input terminals A and 0 for positive voltage Voltage limiting circuit 1. An astable multivibrator 2 is arranged behind it. The multivibrator is connected via the series resistor R1 and the base resistor R2 the transistor V7 on. The working resistance of the transistor V1 or series resistor of the capacitor C is denoted by R3.

A charging capacitor C is at ground potential from the collector of the transistor placed. An operational amplifier 3 is located behind the transistor V1 controls the series transistor V2 via a resistor R4. The transistor V2 is with his Emitter-collector path in the current path between input A and output B.

A known current limiting circuit is connected to the emitter of transistor V2 and ground 4 connected. This limiting circuit exists twice and corresponds the formwork according to patent application P 24 ii 633. The output of this circuit leads to terminal B. Between output terminal B and Nasse are the resistors R5 and R6 placed, from whose center tap a feedback leads to the multivibrator 2.

The start-up circuit according to the invention for Zener barriers works on the following way.

The voltage limiting circuit 1 at input A is said to be an undesirable one Prevent the input voltage from rising.

The astable multivibrator 2 is over those input in a defined Hold position. This is done through the feedback of the output voltage, tapped between the resistors R5 and n6. It will do so in an advantageous manner a reset is achieved after each trip.

When the transistor V1 is switched on, there is a low collector-emitter voltage ej. I) the capacitor C is discharged. On the inverting / of the operational amplifier 3 there is no voltage. This means that the output of the operational amplifier opens Supply voltage potential and the entire start-up circuit is de-energized. This has a Blocking of series transistor V2 results in and the output voltage level becomes the same Zero.

When the transistor V1 is blocked, the capacitor C is across the resistor R3 charged and the operational amplifier 3 driven with a positive voltage. The output of the operational amplifier 3 becomes more negative compared to the operating voltage and enables a base current in series transistor V2. As the charge increases of the capacitor C also changes the base current of V2 and switches it slowly by.

In this way, when the entire start-up circuit is switched on achieved gradual rise to the input line voltage.

The disconnection of a current that is too short is carried out in the subsequent current limiting switch 4 reached. This circuit is available twice for safety reasons.

An advantageous current limiting circuit is already shown in, for example way / the patent application P 24 11 633 has been proposed.

A defined value is set via the voltage divider resistors R5 / R6 given to the astable tultivibrator 2.

In an advantageous embodiment of the start-up circuit according to the invention the function of the components V2 and R4 by the current limiting circuit 4 with accepted. This can be achieved because the components V2, R4 of the start-up circuit according to the invention are also present in the current limiting circuit 4.

L e r s e i t e

Claims (3)

Claims 1. Start-up circuit for Zenertiarr even with active Current limitation, characterized in that a transistor (V1) is provided, which controls an operational amplifier (3) via an RC element (R3, C) and that on the output of the operational amplifier (3) the control electrode of one with its emitter-collector-Jtreck series transistor lying between input (A) and output (B) of the start-up circuit (v2) is connected, the voltage rise of which at the output corresponds to the voltage rise on the RC element (R3, C). 2. Start-up circuit according to claim 1, characterized in that a astable multivibrator (2) is provided, which when there is voltage at the output (B) blocks the transistor (V1) and when the output is dead, the transistor (V1) periodically switches through and blocks. 3. Start-up circuit according to one of Claims 1 and 2, characterized in that that series transistor (V2) and resistor (R4) part of the current limiting circuit (4) are.