GB2222036A

GB2222036A - Apparatus for providing stroboscopic illumination

Info

Publication number: GB2222036A
Application number: GB8917146A
Authority: GB
Inventors: Richard Edwin Barfoot
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-07-30
Filing date: 1989-07-27
Publication date: 1990-02-21
Also published as: GB8818227D0; GB8917146D0

Abstract

Apparatus for providing stroboscopic illumination, e.g. for checking the timing of an internal comustion engine has capacitive pick-up means 18 with conductive plates which fit closely about a high tension lead 6 to provide a high capacitance connection therewith, a gas discharge tube 2 for providing stroboscopic illumination and a low loss electrical connection 8 between the pick-up means 18 and a control electrode 3 of the discharge tube, the capacitance of the connection between the pick-up means 18 and the high tension lead 6 being sufficiently high and the capacitive losses of the electrical connection being sufficiently low that a voltage is applied to the control electrode 3 of a magnitude sufficient to trigger the discharge tube 2 without the need for amplification means. Preferably, further conductive plates 5 connected to earth 17 are provided to suppress interference. The conductive plates 4, 5 may have conductive foam inserts 32 (Fig. 4), which will conform with H.T. leads of various diameters, the plates being supported by a hinged clamp. <IMAGE>

Description

APPARATUS FOR PROVIDING STROBOSCOP@@ ILLUMINATION This invention relate to a?pa-atus for providing stroboscopic illumination.

It is particularly suitable for use in checking the timing cf an internal combustion engine.

Timing devices for use with internal combustion engines require an input from the engine ignition system This is conventionally provided by a direct electrical connection, eg to the ignition high tension lead, or by a remote sensing device such as a transformer with a moving ferrite core pick-up arranged to fit around one of the ignition leads. It is, however, necessary to provide control and amplification circuitry to enable this arrangement to function satisfactorily so adding to the cost of the system.

According to the present Invention there its provided apparatus for providing stroboscopic illumination comprising: pick-up means for providing a high capacitance connection with a high tension condutor; a gas discharge tube for providing strob@scopic illumination and connection means for providing a low loss electrical connection between the pick-up means and a control electrode of the discharge tube such that, in use, the capacitance of the connection between the pick-up and the high tension conductor is sufficiently high and the capacitive losses of the connection means sufficiently low that a voltage is applied to the control electrode of a magnitude sufficient to discharge the tube without the connection means needing to include amplification means.

Preferred features of the invention will be spparent from the following description and the subsidiary claims of the specification.

The invention will now be further described, merely by way of example, with reference to the accompanying drawings, in which Figure 1 is a circuit diagran of a preferred @ embodiment of the invent ion; Figure 2 is a perspective view of an instrument case and remote pick up for attaching to the exterio:- of a high tension lead according a preferred arrangement of the invention: and Figures 3 and 4 are perspeztive views of alternative arrangements for the construction of the remote pick-up shown in Figure 2.

An electric current will pass through a gas discharge tube containing an inert gas, eg xenon, when a critical discharge voltage is reached between its main electrodes. In some types of gas discharge tube, an additional control electrode is provided which is conventionally positioned between the two main electrodes. By applying a voltage to this electrode, it is possible to initiate or influence the discharge between the two malta electrodes. When a capacitor is discharge through the two main eiectr;des of the tbe under the control of voltage applied to the control electrode an instantaneous light c.utput is obtained which may be used to provide stroboscopic illumination.

The voltage spplied te the control electrode, when the tube is used as a stroboscopic timing light, is conventionally provided by a trigger coil and associated circuitry. Alternatively, a direct connection may be made to the high tension lead of an Internal combustion engine ignition system. Either of these two arrangements will provide a voltage to the control electrode in excess of, for example, the five thousand volts necessary to cause a conventional xenon tube to discharge.

The circuit shown in Figure 1 comprises an inverter circuit 1 for providing a high voltage across a gas discharge tube 2. The discharge tube 2 is provided with a control electrode 3 which is electrically connected by a lead 8 to first conductive plates 4 of a remote pick-up 18. Additional conductive plates 5 are connected to ground 17 via a lead 9.

As indicated above, the input from the ignition system is conventionally provided by a direct electrical connection to the RT leads or by some form of remote pick-up. A direct electrical connection is disadvantageous as a number of different adaptors areusually required to provide connections to different systems. The connection als@ disturbe the HT connections of the system which may cause further problems to çrtse. A direct connection wiil also carry high voltage and high current signals and may be potentially dangerous for the user.In view of this, remote pick-ups are generally preferred although, as indicated above, these tend to be more complex and expensive due to the need to provide amplification circuitry.

The apparatus described herein aims to provide a remote, capacitive pick-up which provides a signal of sufficient magnitude to trigger discharge of the tube 2 without the need for amplification circuitry. The apparatus is also much safer thar. thcse providing a direct electrical connection to the HT system as the leads 8 and 9 can be touched without receiving a major shock.

The inverter circuit 1 consists of a power transistor 10, a primary transformer winding 11, a secondary transformer winding 12 and, in the arrangement shown half wave rectification diodes i3. h capacitor i: is provided for coupling and s load capacitor 15 is connected across the main electrodes of the discharge tLbe 2.This capacitor i5 is charged to several hundred volts (eg 20C. - 60Ov) by the circuit when a low voltage is supplied across an input i6 and ground 17, The remote pick-up i8 comprises first conductive plates 4 which are arranged t provide a high capacitance connection wit. a high tension conductor 6 (such as an HT lead of a vehicle ignition system) by extending along the lead, eg by a distance of 2cm or more, and by fitting closely about the lead 6. The capacitance of the connection between the plates 4 and the lead 6 is preferably at least 10-15 picoforads. The connection between a conventional capacitive picx-up and the associated lead would typically have a capacitance of only about 25% of this. The high capacitance connection is achieved both by the length of the plates 4 and, more importantly, the close fitting of the plates 4 about the lead 6.

The lead 8 provides a low loss electrical connection between the plates 4 and the control electrode 3 of the gas discharge tube 2.

The remote pick-up 18 preferably comprises interference supression means i the form of second conductive plates 5 which are also arranged to provide a high capacitance connection with the high tension lead 6. As indicated above, the plates 5 are connected to ground 17 by a lead 9. The purpose of these plates 5 is to dissipate any interference signals transmitted from other high tension leads (not shown) which may be in close proximity to the high tension lead 6. The majority of the interference is due to other HT leads close to and parallel with that to which the pick-up 18 is connected.

The plates 5 are used to reduce the level of interference within the lead to which the pick-up i8 is connected. The plates 5 may be slightly longer than the plates 4 and thus provide a connection with the lead 6 with a capacitance of around 15-20 pi-oforads.

Longer plates 4 and 5 could be used to increase further the capacitive connect on but these would be more awkward to use and prone to damage.

Thus within a given s=e ccnstrain, tnere are conflicting requirements between increasing the length of the plates 4 to increase the voltage of the sign: provided to the gar discharge tube 2 and increasing the length of the plates - to Improve dissipation of interference signals.

As shown in Figure i, the leads 8 and 9 include a two lead plug connection 7 connecting the leads from the pick-up .8 to the gas discharge tube 2 and inverter circuit i. The two leads 8 and 9 may comprise a single length cf cable of two core construction sraced by a significant distance, eg lOmm, thus ensuring a mutual capacitance in the region of only 10-15 picoforads/metre. The lead 8 or both the leads 8 and 9 may consist of a specially fine wire, eg 0.5-1.5mm in diameter, to reduce further their mutual capacitance. The dividing insulation between the leads 8 and 9 may also be locally punched out to reduce further the mutual capacitance (see Figure 2).Alternatively, two separate conductors may be used, optionally spaced by clips (not shown), at regular intervals along their length. In either case, a finned construction (not shown) or other means may be provided around the lead 8 to reduce the signal loss to ground caused by local insulation contact with ground or other steady potentials.

The mutual capacitance between the leads 8 and 9 should preferably be 5-15 picoforads/metre or less, this being determined mainly by the diameter of the conductor any the spacing and material between hen This compares with a mutual capacitance of 50 picoforads/metre or more between the conductor and shielding of a conventional co-axial cable.

With the arrangement described herein, a gas discharge tube 2 of relatively high sensitivity is provided. This can be achieved by alteration of the main electrode spacing, the spacing, surface area and position of the trigger electrode 3, and the nature of the gas filling.

The trigger electrode 3 is typically attached to the exterior of the gas discharge tu'e 2. The electrsde 3 is then electrically connected to the conductive plates 4 which are fitted in close proximity with the high tension lead 6. A large surface area and intimate contact with the high tension lead insulation ensures that tne plates ; have a high capacitance connection with the lead 6.

The electrical connections between the @onauctive plates 4 ci the remote pick-up 18 an- the gas discharge the trigger electrode 3 must be of relatively small mutual capacitance in order to ensure low loss so that a signal of sufficiently high magnitude is provided to trigger discharge of the tube 2 without the need for amplification circuitry between the pick-up 8 and the trigger electrode 3.

A conventional gas discharge tube requires a voltage cf around 4KV on the control electrode 3 to cause it to discharge. Although a conventional tube may be used if a sufficiently high capacitance connection is made with the lead 6. and e sufficiently low loss connection is provided between the remote pick-up 18 and the gas discharge tube 2, it is preferred if a more sensitive tube, eg one which fires at a voltage of about 2KV, is used, The voltage within a high tension lead is typically in the range 6-10KV.

The conductive plates 5 ere preferably of a different surface area to the conductive plates 4. By reversing the two lead plug connection 7, an alteration in the signal strength an@ signal tc nose ratio can thus be obtained.

Alternative arrangements for altering the signal strength and signal to noise ratio can be obtained by variation in the capacitance between the two leads 8 and 9.

It may be desirable to adjust the sensitivity of the apparatus due to the variations between different vehicles on which it may be used. As indicated, the connections on the plug connector 7 may be reversed to increase the sensitivity of the pick-up (although interference suppression will then be reduced).

means for providing an adjustable loss may be provided in the cable 8. One possible form of loss adjustment is to provide an adjustable gap within the plug connector 7 across which the voltages jump. Preferably, the apparatus 5 designed to work in the worst possible case, de with an ignition system with a relatively low voltage h system. With vehicles emplQving much higher voltages in their HT systems it may be desirable to reduce the sensitivity of the pick-up in order to reduce the interference picked ur from other leads.

Figure 2 shows an instrument case 20 suitable for housing the inverter circuit 1 and gas discharge tube 2 shown in Figure i. Connection means 22 for connecting the apparatus to a supply voltage and attachment means 2i for holding the conductive plates 4 and 5 in position about the high tension lead 6 are also provided.

Figures 3 and 4 show alternative designs for the detailed construction of the remote pick-up 18. The first type shown in Figure 3 has conductive plates 4 and 5 as described above with two separate pairs of plates 4 and 5. These are mounted within a housing 27 constructed of a resilient uterial, eg a plastics material, having a hinge formed by the thin section 28 and an optional securing clip 29.

The plates 4 and 5 are preferably mad of metal, eg of copper, aluminium or steel.

Alternatively, the housing 27 may be extended to support thIn flexible plates of a conductive material (not shown) which are preferably formed to fit around the largest size lead to be encountered, and which springs to conform to leads of a smaller circumference.

The conductive plates may also be formed by local metal plating or adhesive foil (not shown). The more flexible the conductor plates are, the easier it is to fit them closely around the high tension lead.

The errangement shown in Figure 4 comprises a conductive foam insert 32 which conforms with high tenior. leads cf various ciameters. The effective capatitance may th=n be determined by the degree of tightening ot. an adjustable securing clip 31 which comprises a ratchet arrangement on a ramp. The foam may comprise any material which is not a good insulator.

Foams jaded with carbon or other conouctive materials may also be uses.

A more compact construction may be obtained by omitting plates 5 and providing interference suppression means in the form of a controlled capacitance to ground cr other steady potentiai on the lead 8, eg cf the order of 5-i5 picofarads, although the signal strength will then be less and the interference noise greater.

ThIs apparatus described thus provides a stroboscopic timing device with a particularly sensitive gas discharge tube, having a trigger electrode connected directly to a capacitive coupling with a high tension lead of an internal combustion engine ignition system. Such a device has been found to function well, is safer and more convenient to use than prior art devices and is significantly less complex to manufacture than the prior art devices with a remote electrical connection since the latter require additional electronic circuitry to function satisfactorily.

As indicated above the apparatus is able to provide a signal of sufficient magnltude to cause reliable triggering of a stroboscopic discharge tube without the need for amplification circuitry. This is achieved by providing a high capacitance connection with the HT lead, a XoW loss electrical connection between the pick-up means and the discharge tube and the selection of a discharge tube of appropriate sensitivity. The apparatus may be further enhanced by providing means to suppress interference ir. the HT lead to which the pick-up means is connected.

Claims

CLAYS

1. Apparatus for providing strobos@opic illumination comprising: pick-up means for providing a high capacitance connection with a high tension.

conductor; a gas discharge tube for providing stroboscopic illumination and connection means for providing a low loss electrical connection between the pick-up means and a control electrode of the discharge tube such that, in use, the capacitan-ze of the connection between the pick-up and the high tension conductor is sufficiently high and the capacitive losses of the connection means sufficiently low that a voltage is applied to the control electrode of a magnitude sufficient to discharge the tube without the connection means needing to include amplification means.

2. Apparat@s as claimed in claim 1 in which the pick-up means comprises conductive plates or other conductive means arranged to fit closely about the high tension conductor.

3. Appartus as claimed in claim 2 in which the conductive plates or other conductive r.eans is arranged to extend along the high tension. conductor for at least 2cm.

4. Apparatus as claimed in claim 2 or 3 in which a pair of conductive plates or other conductive means are carried by attachment means for holding the pate= or other conductive means in position about the high tension conductor.

5. Apparatus as claimed in claims 2, 3 or 4 in which the conductive means comprises a conductive foam insert arranged to conform with high tension conductors of various diameters.

6. Apparatus as claimed in any preceding claim in which the pick-up means is arranged to provide a connection with the high tension lead with a capacitance of at least 10-15 picofarads.

7. Apparatus as claimed in any preceding claim in which the gas discharge tube, eg an xenon tube, requires a trigger voltage of 4KV or less, eg 2KV.

S. Apparatus as claimed in any preceding @ lain in which the connection means comprises a ead formed of fine wire, eg 0.5-1.5mm in diameter.

provided with means for reducing signal loss to ground or other steads potentials.

9. Apparatus as claimed in any preceding claim comprising adjustment means for adjusting the signal strength in the connection means and the signal to noise ratio.

10. Apparatus as claimed in claim 9 in which the adjustment means comprises means for adjusting the signal loss In the connection means.

ii. Apparat@s as claimed in any preceding claim comprising interference supression means for suppressing interferen@e in the high tension conductor.

i2. Apparatus as claimed 1 in which the Interference supression means comprises means for providing a controlled capacitance to ground - other steady potential on the connection means.

;3. Apparatus as claimed in claims 1 in which the interference supression means comprises further cor.ductse plates or other conductive means arranged to fit closely about the high tension conductor and connected to ground or other steady potential.

14. Apparatus as claimed in claims 8 and i3 in which the connection means comprises a length of cable of two-core construction, one core providing an electrical connection between the pick-up means and the trigger electrode and the other core providing an electrical connection between the interference suppression means and ground or other steady potential.

15. Apparatus as claimed in claim 14 in which the mutual capacitance between the two cores of the cable is 15 picofarads/metre or less.

16. Apparatus as claimed in claims 10 and 14 in which the adjustment means comprises means for varying the mutual capacitance between the two cores of the cable.

17. Apparatus for providing strobos@opic illumination substantially as hereinbefore destri@d with reference to the accompanying drawings.