WO2011014891A2

WO2011014891A2 - Detonator firing circuit

Info

Publication number: WO2011014891A2
Application number: PCT/ZA2010/000038
Authority: WO
Inventors: Andre Koekemoer; Albertus Abraham Labuschagne
Original assignee: Detnet South Africa (Pty) Ltd
Priority date: 2009-07-30
Filing date: 2010-07-28
Publication date: 2011-02-03
Also published as: AR077785A1; ZA201102955B; AR077784A1; WO2011014891A3; WO2011014892A3; WO2011014892A2; ZA201102956B

Abstract

A circuit (10) for firing an initiating element (24) of a detonator, the circuit including an integrated circuit controller (12), an energy storage device (18), a first switch (28), and current regulating structure (26), wherein the current regulating structure and the first switch are connected in series with the initiating element, the current regulating structure includes a switching arrangement (32), the current limiting structure (36) which is connected in parallel to the arrangement, and the controller is operable, with the voltage of the energy storage device below a defined level, to close the first switch to allow a test current to pass through the structure and the element and, with the voltage of the device at an operative level, which is greater than the defined level, to close the first switch to cause closure of the arrangement, to allow a firing current, produced by the device, to pass through the arrangement and the element.

Description

DETONATOR FIRING CIRCUIT BACKGROUND OF THE INVENTION [0001] This invention relates to a circuit for controlling the firing of a detonator.

[0002] An electronic detonator includes an initiating element which is ignited by the application of a current in excess of a predetermined value. Prior to firing however it is necessary, in a test phase, to test the integrity of the initiating element to ensure that it is fully functional.

[0003] During the test phase the current through the initiating element is limited to a level which is well below the predetermined value, for example by using a resistor, to ensure that ignition of the element does not take place. When the element, is to be ignited however the resistor again acts to limit the current and the energy which is available to ignite the element is thus restricted.

[0004] The igniting of the element can be reliably achieved. However the time which elapses between generating a firing signal and the instant at which ignition takes place can vary from element to element. In many applications the variation is of no consequence. In accurately controlled blasting operations though sub-millisecond accuracy in firing times is called for. A practical difficulty which arises in this respect is that the initiating element is small and has a low thermal mass. Consequently its capacity to generate heat is restricted and an explosive, to which the element is exposed, is ignited after a time interval once the firing signal is generated. The duration of the time interval is however neither predictable nor repeatable due to the aforementioned factors. This aspect can be addressed to some extent by dissipating substantially more energy in the initiating element but this is not readily achieved when an integrated circuit device is used to control the firing process in an electronic detonator. The maximum voltage at which this type of integrated circuit can function is limited and consequently the maximum voltage which is available to generate energy for igniting the initiating element is also restricted. A need therefore exists for a detonator firing circuit which, although capable of functioning under the control of an integrated circuit controller, can direct substantially more energy to an initiating element to ensure more reliable and rapid initiation thereof.

SUMMARY OF INVENTION

[0005] The invention provides a circuit for firing an initiating element of a detonator, the circuit including:

a) an integrated circuit controller,

b) an energy storage device,

c) a first switch, and

d) current regulating structure, and wherein:

i) the current regulating structure and the first switch are connected in series with the initiating element, ii) the current regulating structure includes a switching arrangement and current limiting structure which is connected in parallel to the switching arrangement, and

iii) the controller is operable:

(a) with the voltage of the energy storage device below a defined level, to close the first switch and thereby allow a test current to pass through the current limiting structure and the initiating element, and (b) with the voltage of the energy storage device at an operative level, which is greater than the defined level, to close the first switch and thereby cause closure of the switching arrangement, so as to allow a firing current, produced by the energy storage device, to pass through the closed switching arrangement and the initiating element.

[0006] The switching arrangement may include a voltage-dependent switch which is closed when a voltage across the voltage dependent switch exceeds a predetermined value.

[0007] In general terms the voltage-dependent switch may include a voltage comparator which compares a first voltage which is dependent on the voltage across the voltage-dependent switch to a reference voltage and a second switch which is closed by the comparator when the first voltage is greater than the reference voltage. [0008] Preferably the switching arrangement includes a silicon controlled rectifier

(SCR), a resistive element connected across the gate and cathode of the silicon controlled rectifier, and a zener diode which is connected to the gate and which breaks down when the voltage of the energy storage device is at the operative level and the first switch is closed. [0009] The energy storage device is preferably a capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention is further described by way of example with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of a detonator firing circuit according to the invention; and

Figure 2 illustrates an alternative voltage-dependent switch for use in the firing circuit. DESCRIPTION OF PREFERRED EMBODIMENT

[0011] Figure 1 shows a detonator firing circuit 10 according to the invention. The circuit includes an integrated circuit controller 12 connected to input terminals 14 and 16 respectively of a detonating harness (not shown). A voltage reduction circuit, (not shown) is optionally connected between the input terminals and the controller. [0012] A capacitor 18 is connected across output terminals 20 and 22 of the controller in parallel to a series connection of an initiating element 24 of a detonator, a voltage dependent, current regulating structure 26 and a first electronic switch 28 which is responsive to an output signal from the controller applied via a line 30.

[0013] The structure 26 includes a switching arrangement 32 which is in parallel with current limiting structure 34 which, in this case, comprises a resistor 36.

[0014] The switching arrangement 32 includes a silicon controlled rectifier (SCR) 38, a zener diode 40 which is connected to the gate of the SCR and which acts as a programmable gate bias block / inhibitor and a resistor 42, between the cathode and the gate of the SCR, which protects the gate junction against stray voltages. [0015] In operation of the firing circuit, during a test phase, the voltage on the capacitor 18 is kept below a programmed defined level. The switch 28 is closed by means of a signal from the controller 12. A test current, which is well below the firing current of the initiating element, is then passed through the firing element and the resistor 36. The SCR is kept open. [0016] When the detonator is to be fired the voltage across the capacitor 18 is increased to an operative level which is above the programmed defined voltage referred to. The switch 28 is then closed by means of a signal from the controller. Substantially the full voltage across the capacitor is then impressed across the series connection of the zener diode and the resistor 42. The breakdown voltage of the zener diode is exceeded and current flows through the initiating element, the zener diode, the junction of the gate and cathode of the SCR and the switch 28. The voltage at the gate of the SCR increases and the SCR is turned on hard.

[0017] The resistance of the SCR is substantially less than the resistance of the resistor 42 and of the resistor 36. The resistance of the first switch 28, which is closed, is effectively zero. Consequently practically all of the energy which is discharged by the capacitor is transferred to, and dissipated by, the firing element. Rapid ignition of the element takes place within a time interval which is substantially predictable and repeatable. [0018] The nature of the current regulating structure 26 is such that, during the test phase, the integrity of the firing element is determined by passing a test current through a passive element i.e. the current limiting resistor 36 while, during an operative mode, the full firing current is passed with a substantial degree of efficiency through an active element, i.e. the closed SCR which, for practical purposes, has zero resistance. Dissipation of energy in the resistor 36, during the firing mode, is thus eliminated.

[0019] The switching arrangement 32 shown in Figure 1 depicts a specific preferred circuit. In more general terms though the switching arrangement comprises a voltage-dependent switch which is closed when a voltage across the switch exceeds a predetermined value. Figure 2 illustrates an alternative switching arrangement 32A 'which can be substituted for the switching arrangement 32, and which is connected via terminals A and B to corresponding connection points A1 and B1 in Figure 1.

[0020] The switching arrangement 32A includes a comparator 60 which compares a reference voltage V_R which is developed across a zener diode 62, to a voltage Vc, produced by a voltage-dividing resistive network 64 between the terminals A and B.

[0021] When the voltage across the terminals A and B is lower than the designed switching voltage, V_c is lower than V_R. Both transistors 66A and 66B are off.

[0022] When the voltage across terminals A and B is higher than the designed switching voltage, V_c is greater than V_R and the comparator turns the transistors on. The transistor 66A passes the firing current through the initiating element 24 and the transistor 66B lowers the reference voltage V_Rto enhance the switching action with a positive feedback process.

[0023] A capacitor 68 and diode 70 in the voltage-dependent switch maintain the supply current to the comparator and the base control currents to the transistors 66A and 66B.

Claims

1. A circuit for firing an initiating element of a detonator, the circuit including: a) an integrated circuit controller,

b) an energy storage device,

c) a first switch, and

d) current regulating structure, and wherein:

iii) the controller is operable:

(a) with the voltage of the energy storage device below a defined level, to close the first switch and thereby allow a test current to pass through the current limiting structure and the initiating element, and

(b) with the voltage of the energy storage device at an operative level, which is greater than the defined level, to close the first switch and thereby cause closure of the switching arrangement, so as to allow a firing current, produced by the energy storage device, to pass through the closed switching arrangement and the initiating element.

L. A firing circuit according to claim 1 wherein the switching arrangement includes a voltage-dependent switch which is closed when a voltage across the voltage dependent switch exceeds a predetermined value.

3. A firing circuit according to claim 1 wherein the switching arrangement includes a silicon controlled rectifier, a resistive element connected across the gate and cathode of the silicon controlled rectifier, and a zener diode which is connected to the gate and which breaks down when the voltage of the energy storage device is at the operative level and the first switch is closed.

4. A firing circuit according to claim 2 wherein the voltage-dependent switch includes a voltage comparator which compares a first voltage which is dependent on the voltage across the voltage-dependent switch to a reference voltage and a second switch which is closed by the comparator when the first voltage exceeds the reference voltage.