BACKGROUND OF THE INVENTION

[0001] This invention relates generally to an electronic detonator and more particularly is concerned with a method of adjusting firing energy delivered to an initiating element in an electronic detonator.

[0002] An initiating element in an electronic detonator should be sufficiently insensitive to stray currents and radio fields so as to minimise the possibility of spurious ignition during handling and blast preparation.

[0003] In order to fire an electric detonator reliably and to overcome circuit resistance use can be made of a high-voltage capacitor-discharge circuit to supply to the detonator at least ten times the nominal amount of firing energy required for initiation. The energy typically comes from a stand-alone internal energy source such as a storage capacitor as lead-in wires may be damaged by earlier or adjacent explosions.

[0004] The energy source may, in addition to firing the initiating element, be required to power an electronic delay circuit which is incorporated in the detonator. This can result in limited energy being available to fire the initiating element.

[0005] Although it is possible to use a more sensitive initiating element a practical need to test, trouble-shoot and identify or program an electronic detonator in a safe way prior to a blast, dictates the maximum sensitivity of the initiating element. [0006] The invention is concerned with a detonator which at least partly addresses these factors.

SUMMARY OF INVENTION

[0007] The invention provides an electronic detonator which includes an energy source, an initiating element, a firing circuit for causing discharge of energy from the energy source to the initiating element and at least one circuit element with a resistance value connected in series with the firing circuit and the initiating element.

[0008] The circuit element may be of any suitable kind and preferably is a resistor.

[0009] To prevent an electrical short circuit compromising safety the circuit element is preferably encapsulated or passivated, and may be integrated into a control microcircuit.

[0010] The energy source may be of any appropriate kind and preferably is a capacitor, which may be connected in series with the firing circuit and the initiating element.

[0011] The invention also extends to a method of adjusting the no-fire voltage of a detonator while maintaining the difference between the no-fire voltage and a safe testing voltage in a safe range, the method including the step of including a circuit element with a resistance value in series with an initiating element in the detonator and with a firing circuit which is connected to an energy source which is used for firing the initiating element. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] 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 typical capacitor-discharge firing circuit in an electronic detonator; Figure 2 is a graph indicating the proportion of initiating elements, in a plurality of respective detonators, fired as a function of capacitor voltage; and Figure 3 is a circuit diagram of part of an electronic detonator in accordance with the present invention.

DESCRIPTION QF PREFERRED EMBODIMENT

[0013] Figure 1 of the accompanying drawings illustrates circuit elements included in a typical capacitor-discharge firing circuit 10 in a detonator. The circuit includes an initiating element 12, of any appropriate kind, which has a total internal resistance R(i) designated by a reference 14, a capacitor 16 which has a capacitance C and which, in use, is charged to a voltage V, and a firing circuit 20 which includes a switch 22. The firing circuit has a total internal resistance R(T) designated by a reference 24.

[0014] The energy E(i) which is discharged into the element 12, upon closure of the switch 22, is given by the expression:

E(i) = 0.5*C*V2(R(i)/(R(i) + R(f))

[0015] E(i) is clearly a function of the capacitor voltage V which can be varied to control the amount of energy delivered to the initiating element. [0016] Figure 2 is a graph illustrating the distribution of a percentage of initiating elements, in respective electronic detonators, fired as a function of capacitor voltage. Marked on the graph are the following values for the detonators: A - the safe testing voltage i.e. a voltage which can safely be used for testing a detonator and which will not cause firing of the detonator; B - the no-fire voltage i.e. a voltage which, if exceeded, could cause a detonator to fire; D - the all-fire voltage i.e. a voltage which, if exceeded will, with a high degree of certainty, cause a detonator to fire; and X - the safety margin of B, the no-fire voltage, above the safe testing voltage A.

[0017] With the standard circuit elements shown in Figure 1 it is difficult to adjust the no-fire voltage B while guaranteeing the safe testing margin X. Fabrication of an initiation device with an appropriate sensitivity is a time consuming and expensive task.

[0018] In order to adjust the no-fire voltage B while maintaining the difference X between the no-fire voltage and the safe testing voltage A in a safe range an additional circuit element designated 28 with a resistance R(adj), is connected in series with the capacitor 16 and the initiating element 12, as shown in Figure 3.

[0019] Upon closure of the switch 22 the energy E(d) delivered to the initiating element 12 is given by the expression:

E(d) = 0.5*C*V2(R(i)/(R(i) + R(f) + R(adj).

[0020] It is therefore possible, by varying the value of the resistance R(adj), to adjust the energy E(d) which is delivered to the initiating element while maintaining the safety margin X (see Figure 2) at an optimum value with respect to the normal operating voltage region of the detonator.

[0021] The resistance value R(adj) of the circuit element 28 is chosen so that the sum of the resistances of the elements 24 and 28 (R(f) + R(adj)) is substantially greater than the resistance R(i) of the initiating element 12. This reduces the effect of production spreads in the resistance value R(i) of the initiating element 12. A single resistance value R(adj) of the circuit element 28 is chosen to enable the mass production of the electronic detonators with a significant degree of certainty that the safety margin X will be maintained.

[0022] Figure 3 illustrates the element 28 enclosed in dotted outline 32. This indicates that the resistor 28 may be passivated, integrated into a control microcircuit, or encapsulated in a suitable resin to prevent accidental shorting of the resistor at any stage of manufacture or use of the detonator.

![0023] The invention makes it possible to adjust the no-fire voltage B, while maintaining the safety margin X. Each initiating device 12 can then have a voltage which is marginally above the no-fire voltage B applied to it and so guarantee that the initiating device can be tested in confidence at the safe test voltage A.