Low-voltage-insensitive electro-pyrotechnic device

Abstract

A low-voltage-insensitive electro-pyrotechnic device comprising electrical input terminals, a bridge element connected to the electrical input terminals, and a transient voltage suppressor connected between the bridge element and one of the electrical input terminals. The device may utilize one bidirectional TVS on one side of the bridge element or two unidirectional TVSs on opposite sides of the bridge element, and may also include one or more resistors and/or an RF filter between the input terminals and bridge element.

Description

FIELD OF THE INVENTION

The present invention relates primarily to the field of electro-pyrotechnic devices, and more particularly, to a low-voltage-insensitive electro-pyrotechnic device.

BACKGROUND OF THE INVENTION

Conventional electric detonators comprise hotwire initiators inserted into a metal shell with load charges. The hotwire initiator is directly electrically exposed to external electrical input or stimulus from the legwires to the outside world, making it susceptible to stray electrical voltage or current entering the legwires, leading to possible inadvertent firing of the detonator. Such electric detonators are likewise vulnerable to ESD strikes (either wire-to-wire or wire-to-shell), which can also lead to inadvertent firing. Since hotwire resistance is typically in the range of 1 to 5 Ohm, significant current will flow through the hotwire at voltages of 1 to 5V without ESD protection. This makes electric detonators susceptible to low voltage sources such as watch batteries, etc.

SUMMARY OF THE INVENTION

A low-voltage-insensitive electro-pyrotechnic device according to an embodiment of the present invention comprises electrical input terminals, a bridge element connected to the electrical input terminals, and a transient voltage suppressor (“TVS”) connected between the bridge element and one of the electrical input terminals. The device may utilize one bidirectional TVS on one side of the bridge element or two unidirectional TVSs on opposite sides of the bridge element, and may also include one or more resistors and/or an RF filter between the input terminals and bridge element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a detonator according to a preferred embodiment of a low-voltage-insensitive electro-pyrotechnic device according to the present invention, and having a single bidirectional TVS.

FIG. 2 is a diagram of a suitable layout for the circuit board included in the embodiment of FIG. 1.

FIG. 3 is an electrical schematic for the embodiment of FIG. 1.

FIG. 4 is a schematic for an alternative embodiment having a single bidirectional TVS, but two parallel resistors rather than one.

FIG. 5 is a schematic for another alternative embodiment in which there are two unidirectional TVSs rather than a single bidirectional TVS.

FIG. 6 is a schematic showing an embodiment that further incorporates an RF filter.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1-3 depict a low-voltage-insensitive electro-pyrotechnic device according to a preferred embodiment of the present invention in which there is a single bidirectional TVS and a parallel resistor. As seen in FIGS. 1 and 3, a hotwire initiator 20 is secured in the detonator 10 with an endplug 15, with a TVS 22 placed before the hotwire initiator 20 and a parallel resistor 24 placed before the TVS 22 and between the two input terminals 26 from the legwires 28. FIG. 2 shows an example of a circuit board layout (with the positions where the TVS 22 and resistor 24 sit on the board depicted in dashed lines) using a 5.0 mm×13.3 mm board 13 that can be accommodated inside the metal shell 30 of a common mining and blasting detonator as shown in FIG. 1, with contact pads 21 provided for connection to the hotwire initiator 20.

The TVS 22 blocks voltage applied across the legwires 28 until the clamping voltage is reached, at which point the TVS 22 starts conducting current to the hotwire initiator 20. The TVS 22 can be discrete axial or radial, preferably a surface mount component. The TVS 22 is preferably chosen to conduct above 100V, more preferably 180V and most preferably above 200V but below 450V, typical voltage of electric blasting machines. Examples of such TVSs are smaj170ca, smaj190ca, p4sma200ca and p4sma220ca. With the TVS 22 consisting of a bidirectional p4sma200ca, the assembly should block any DC input voltage up to 200V and will only start conducting at voltages above that (regardless of polarity since it is bidirectional).

The parallel resistor 24 placed before the TVS 22 and between the two input terminals 26 from the legwires 28, mitigates ESD charge buildup and also facilitates an electronic assembly continuity check performed at the downholes (to ensure there is electrical continuity between the legwires to the electric detonator below in the downholes). The value of this resistor can be between 1 kOhm to 100 kOhm, most preferably 25 kOhm to 75 kOhm. Alternatively, a combination of resistors in series or parallel can be used to achieve the desired final resistance (e.g., 1 kOhm to 100 kOhm). For example, as shown in FIG. 4, two 150 kOhm resistors 25 can be employed in parallel to produce a circuit resistance of 75 kOhm.

As shown in FIG. 5, an alternate embodiment could instead employ two unidirectional TVSs 23. Such an embodiment could also be used with a single resistor or a combination of resistors in series or parallel (not shown).

Optionally, as shown in FIG. 6, an RF filter 32 (comprising a pi(LC) filter, RC, RL networks, ferrite beads or chips, or a capacitor) can also be incorporated in the front of the assembly, to enhance blocking of higher frequency AC voltage such as from extraneous RF or EMI signals.

Also, instead of a TVS, another means for upwardly shifting the all-fire voltage as measured at the input terminals could be used, such as a combination of (e.g., 100) forward voltage diodes (each producing a drop of 1 volt) connected in series between one of the input terminals and the hotwire initiator.

In use, the detonator 10 is connected to an electric blasting machine (not shown) yielding an output of at least 250V from a 450 uF capacitive discharge, such as an REO Model CDJ 450J blasting machine with output of 500V 450 uF capacitance network. Upon arming and firing the blasting machine, the surge in voltage and current to the hotwire initiator 20 results in deployment of the detonator 10 within 20-30 us or .so.

Although the present invention has been described in detail in the context of a preferred embodiment of a detonator used in mining and blasting, one skilled in the art will appreciate that numerous variations, modifications, and other applications are also within the scope of the present invention. For example, although an embodiment of the invention for use in mining and blasting has been described, the invention could be used in other applications such as military, defense and aerospace applications where a low-voltage-insensitive electro-pyrotechnic device is needed. By using a TVS as high as 300V to 500V or even 1000V by having serially connected TVSs, it is possible to have all-fire voltage shifted to similarly high-voltage ranges. Further, in automotive applications such as airbags, such a low-voltage-insensitive electro-pyrotechnic device (for example, using a 45V TVS) can be used as an initiator that is impervious to typical automotive voltages of 12V to 42V. Thus, the foregoing detailed description is not intended to limit the invention in any way, which is limited only by the following claims and their legal equivalents.

Claims

1. A low-voltage-insensitive electro-pyrotechnic device, comprising: a) first and second electrical input terminals;b) a bridge element connected to said electrical input terminals; andc) a first transient voltage suppressor connected in series between said bridge element and said first electrical input terminal.

2. The device of claim 1, wherein the device is a detonator.

3. The device of claim 1, wherein the device is a detonator for use in mining and blasting.

4. The device of claim 1, wherein said bridge element is a hotwire initiator.

5. The device of claim 1, wherein said transient voltage suppressor is bidirectional.

6. The device of claim 1, further comprising a second transient voltage suppressor connected between said bridge element and said second electrical input terminal, wherein said first and second transient voltage suppressors are unidirectional.

7. The device of claim 1, further comprising a resistor connected between said electrical input terminals and said bridge element.

8. The device of claim 1, further comprising two or more resistors connected between said electrical input terminals and said bridge element.

9. The device of claim 1, further comprising one or more resistors connected between said electrical input terminals and said bridge element, and further comprising a second transient voltage suppressor connected between said bridge element and said second electrical input terminal, wherein said first and second transient voltage suppressors are unidirectional.

10. The device of claim 1, wherein the device is a detonator for use in mining and blasting and further comprises at least one resistor connected between said electrical input terminals and said bridge element.

11. The device of claim 10, wherein said transient voltage suppressor has a clamping voltage of at least 100V.

12. The device of claim 10, wherein said transient voltage suppressor has a clamping voltage greater than 100V and less than 450V.

13. The device of claim 10, wherein said transient voltage suppressor is a p4sma200ca.

14. The device of claim 10, wherein said resistor is connected in parallel between said input terminals and said bridge element and has a resistance greater than 1 kOhm and less than 100 kOhm.

15. The device of claim 14, wherein said resistor has a resistance greater than 25 kOhm and less than 75 kOhm.

16. The device of claim 14, further comprising an RF filter.

17. The device of claim 1, further comprising an RF filter.

18. A low-voltage-insensitive electro-pyrotechnic device, comprising: a) first and second electrical input terminals;b) a bridge element connected to said electrical input terminals; andc) an upwardly voltage shifting means connected in series between said bridge element and said first electrical input terminal.

19. The device of claim 18, wherein said upwardly voltage shifting means comprises at least one transient voltage suppressor.

20. The device of claim 18, further comprising an RF filter.