GB2169460A - Initiator fuse firing circuit - Google Patents

Abstract

An initiator fuse, which sets off a primary explosive charge, is fired by an electrical pulse applied to the gate of a 'Hexfet' (Trade Mark) (HF) whose source-drain path is in series with the initiator fuse RF. A capacitor CR connected across the Hexfet-fuse combination is charged from a DC supply; when a pulse fires the 'Hexfet', the capacitor discharges through the series combination to fire the fuse. The high gate capacitance provides a high level of RFI immunity on the control line. To further improve interference resistance, the circuit can be in a screened module. The fuse can be connected to drain or source of the 'Hexfet', the latter having the advantage that it enables the fuse to be directly grounded to the screen. <IMAGE>

Description

SPECIFICATION Initiator fuse circuit This invention relates to electrical circuit arrangements for firing the initiator fuses much used in connection with explosives.

Where electrical operations are used for firing explosive charges, one standard practice is for the electrical signal to operate an initiator fuse which fires a small primary explosive charge which in turn fires a secondary charge to cause the ignition of the main explosives.

The use of an initiator fuse reduces the power needed to fire the main charge, but has the disadvantage that premature firing could occur due to RF interference. This, of course, is somewhat of a nuisance.

An object of the present invention is to provide an electrical circuit arrangement which overcomes the disadvantages of the known fuse operating arrangements.

According to the invention, there is provided an electrical circuit arrangement for firing an initiator fuse, which includes a field effect transistor (FET) having a very low "on" resistance and a relatively high input capacitance connected in series with the initiator fuse, the FET normally being in its non-conductive condition, a capacitor which is in a charged condition when the fuse is to be used and which is connected to the FET, and an input connection to the FET via which a firing pulse may be applied to the FET, wherein the application of a said firing pulse to the gate of the FET switches the FET rapidly on and causes the charge on the capacitor to be discharged via the FET and the fuse, thus operating the said fuse.

In a preferred arrangement, as will be seen the form of FET actually used is that known as a Hexfet (Trade Mark).

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figures 1 and 2 show characteristic curves of a Hexfet, such as used in the circuits embodying the invention.

Figure 3 is a circuit embodying the invention in which the Hexfet is connected in the common source configuration.

Figure 4 is another circuit embodying the invention in which the Hexfet is connected in the source follower configuration.

Figure 5 shows an example of the mechanical construction of a module whose circuit embodies the invention.

As indicated above, the currently preferred semiconductor device is a field effect transistor of the so-called Hexfet (Trade Mark) type.

Such devices are characterised by their very low 'on' resistance. They are based on "MOSFET" technology and thus offer all the advantages of MOSFET's, with the added bonus of a very low forward volts drop at very high currents. This is achieved by having a large area silicon gate structure, which leads to a high capacitance at both the input and output of the device.

"Hexfets" as used in the arrangements to be described herein are manufactured by 'International Rectifier' and their type IRF 520 is suitable for use in an initiator fuse firing circuit embodying the invention. It has a rated drainsource voltage of 100 volts and a pulsed drain current of 32 amperes with a forward "ON" resistance of 0.3Q. Its typical input capacitance is 450 picofarads and its typical output capacitance is 200 picofarads. Fig. 1 shows the relationship between the gate-tosource voltage and the drain current of this device. No appreciable drain current flows until a threshold voltage of 3 volts is exceeded on the gate, and the drain current does not rise linearly until 5 volts gate voltage is exceeded.

The specified minimum gate threshold voltage is 2 volts and the maximum gate voltage is +20 volts.

Fig. 2 shows the total gate charge (in nanocoulombs) of the IRF 520 against the gate-tosource voltage for a drain current of 10 amperes and a drain voltage of 20 volts. The first gradient on this curve depicts the charging of the gate source capacitance, the plateau indicates the charging of the gate-to drain capacitance and the point at which the second gradient starts indicates the point where the device is fully switched on. Thus approximately 8 nanocoulombs of charge are required to fully switch 10 amperes drain current and 10 nanocoulombs would be required to switch 20 amperes. Higher current "Hexfets" such as the IRF 640 capable of switching 72 amperes pulsed drain current require approximately 30 nanocoulombs total gate charge to fully switch 20 amperes, the typical input capacitance being 1275 picofarads.It follows from the curves of Figs. 1 and 2 that interfering signals due to radio frequency interference (RFI) must be of sufficient amplitude and duration to exceed both the minimum gate threshold voltage and the total gate charge, to cause unwanted switching of the 'Hexfet' Thus devices with the highest input capacitance and having the highest threshold voltage provide the highest RFI immunity.

The circuit shown in Fig. 3 is a common source firing circuit for use with current rise times down to ten nanoseconds, and this circuit forms the basis of an RFI resistant module. The energy required to fire the initiator fuse RF is derived from a reservoir capacitor CR which is charged from the supply rails via a series resistor Rs. CR is a tantalum capacitor of approximately 2 microfarads with a supply voltage of 24 volts, and it is discharged via the fuse when the 'Hexfet' is switched on.

This 'Hexfet' is an N-channel device. The 'Hexfet' is switched on by a pulse fed from a remote pulse circuit via a coaxial cable CC terminated by a gate resistor RG, which is of approximately 50Q. If it is assumed that the gate threshold voltage is 4 volts, then any interfering signal would have to generate 80 milliamps in the screened cable to cause spuri ous firing. To provide extra security the voltage supply V5 can remain off until shortly before firing was required. The charging time constant CR R5 must be designed to be equal to, or less than, the firing delay time. The circuit must thus be powered up and triggered before initiation can take place.

In use, the circuit is armed by applying voltage to charge the capacitor CR. After this has been done, the firing pulse is applied to the gate of the 'Hexfet'. The capacitor CR provides RF immunity in the drain circuit, while gate capacitors and the gate's threshold voltage provide RF immunity for the gate circuit.

Fig. 5 shows a sectioned schematic of a RFI resistant module based on the common source firing circuit of Fig. 3. A cylindrical housing approximately 20mm diameter and 20mm long accommodates the initiator fuse RF and a circular printed circuit board PCB on which components are terminated. A 'Hexfet' dice HD is used, and is soldered to a copper track of the PCB and aluminium bonded wires are used for the other connections. Although a conventional wire-ended tantalum capacitor CR is shown, a chip tantalum capacitor can also be used. The coaxial drive cable CC and the power supply cable PC are shown soldered directly into the circuit via internai magnetic screening material SM. The power cable is screened and the magnetic screens SM are of high permeability material.The module has a threaded portion TP via which it is screwed into the container of the primary initiator fuse.

A disadvantage of this approach is that the fuse has to be isolated from the module housing as it cannot be earthed, which necessitates the use of an insulated housing.

An alternative approach is shown in Fig. 4 which is of a source follower firing circuit, in which one side of the initiator fuse is connected to earth. This allows a metal housing to be used, eliminating the requirement for a separate magnet screen and simplifying the earth connection to the fuse and cables. Electrically the source follower circuit has a disadvantage, compared to the common source circuit, due to its low voltage gain. As the firing pulse appears across both the gate circuit of the Hexfet and the initiator fuse in series, its amplitude must equal the minimum gate source voltage plus the volts drop across the fuse, which is approximately equal to the supply voltage V5. Thus a firing pulse of about 30 volts amplitude at 1 to 2 amperes would be required.This may be difficult to generate in certain systems, it does, however, lead to an improvement in RFI immunity since the interfering signal must now exceed the gate threshold voltage plus the voltage drop across the initiator fuse.

Thus we have described a simple compact module housing the initiator fuse, Hexfet switch and associated charging circuit, which has a high degree of RFI immunity. It also avoids the need to run a feed cable to the initiator which would need to carry the full fuse current of about 20 amperes with a rise time possibly in the nanosecond region.

Note that the initiator fuse, RG, when it operates triggers a primary explosive, which ignites a secondary explosive, which latter in turn triggers the main explosive charge. The primary initiator as assembled for use contains the fuse, the primary explosive and the secondary explosive.

It should also be noted that the RFI immunity provided by the arrangements described above is relative, but its extent is more than adequate for the majority of applications.

Claims (6)

1. An electrical circuit arrangement for firing an initiator fuse, which includes a field effect transistor (FET) having a very low "on" resistance and a relatively high input capacitance connected in series with the initiator fuse, the FET normally being in its non-conductive condition, a capacitor which is in a charged condition when the fuse is to be used and which is connected to the FET, and an input connection to the FET via which a firing pulse may be applied to the FET, wherein the application of a said firing pulse to the gate of the switches the FET rapidly on and causes the charge on the capacitor to be discharged via the FET and the fuse, thus operating the said fuse.

2. An electrical circuit arrangement for firing an initiator fuse, which includes a field effect transistor (FET) having a very low "on" resistance and a relatively high input capacitance, such as an FET of the type known as 'Hexfet' (Trade Mark), which FET has its source drain path connected in series with the initiator fuse across a direct current supply, the FET normally being in its non-conductive condition, a capacitor also connected across the supply so as to be in a charged condition when the fuse is to be used, and an input connection to the gate of the FET via which a firing pulse may be applied to the gate of the FET, wherein the application of a said firing pulse to the gate of the FET switches the FET rapidly on and causes the charge on the capacitor to be discharged via the source-drain path of the FET and the fuse, thus operating the fuse.

3. An arrangement as claimed in claim 2, and wherein the FET is so connected as to operate in the common source configuration.

4. An arrangement as claimed in claim 2, and wherein the FET is so connected as to operate in the source follower configuration.

5. An arrangement as claimed in claim 1, 2, 3 or 4, wherein the FET and its associated circuit components is mounted on a printed circuit board in a container which is magnetically screened.

6. An electrical arrangement for firing an initator fuse substantially as described with reference to Figs. 3 and 5, or to Figs. 4 and 5, of the accompanying drawings.