GB2066968A - Circuit testing apparatus - Google Patents

Abstract

Circuit testing apparatus for testing a component associated with an ignition circuit (15) of an internal combustion engine is disclosed. The apparatus including a trigger simulation means (10, 11) for association with an input of the component being tested and an output display means (16) for association with an output of the component such that the performance of the component may be determined. An electronic tachometer (12) suitable for use with the above testing apparatus is also described. <IMAGE>

Description

SPECIFICATION Circuit testing apparatus The present invention relates to circuit testing apparatus and in particular to apparatus used for testing ignition circuits of the kind employed in internal combustion engines.

The ignition testing apparatus of the present invention has particular application in testing and evaluation of various components associated with ignition circuits. Ignition testing apparatus as known prior to this invention are generally unsatisfactory in operation, either because of unaccuracy or because of inadequacy in that they could not cover a sufficiently wide range of operations.

It is a primary object of the present invention to provide circuit testing apparatus which is suitable for testing various key components associated with ignition systems and which is relatively simple and inexpensive in construction. A further object of the present invention is to provide a compact testing device which is portable. A still further object of the present invention in a preferred form, is to provide a device which may be applied to capacitive discharge ignition systems including ignition systems which comprises a plurality of individually triggered ignition circuits. Yet another object of the present invention is to provide apparatus as aforesaid which may be used by relatively unskilled personnel.

With regard to the last point, the portable nature of the device makes it especially suitable for marine use, and other applications where circuit failure or malfunction remote from a workshop can be inconvenient. In the event of engine ignition problems, for example, at a remote location, the device permits "on the spot" diagnosis, and that may reveal that repairs can be carried out at that location.

It will be convenient to hereinafter describe the invention with particular reference to ignition testing, but that is not to be understood as being the only application of the invention.

Prior ignition testing apparatus generally require the components being tested to be disconnected from the ignition system so as to be tested in isolation and under static conditions rather than actual working conditions.

This method is inconvenient and does not lend itself fully to pinpointing a fault in the ignition system. Such testing apparatus generally involve the use of cathode ray tube displays, whereas, the testing device according to the present invention employs, in one preferred form, a test spark plug to display the output associated with the ignition circuit.

In a basic preferred form, apparatus according to the present invention includes trigger simulation means for association with an input of said component or components and output display means for association with an output of said component or components such that the performance of said component or components may be determined. The preferred apparatus also includes output display means which enable direct viewing of the ignition circuit output. The output display means is preferably a standard 5mm test spark plug.

An apparatus as described above can be powered from an A.C. or D.C. source, whichever is preferred for the particular application.

The apparatus can be used with any ignition system, such as a conventional kettering system, capacitive discharge system or a system having a plurality of SCR circuits. The quality of spark output or lack thereof as detected by the apparatus can be related to ignition component malfunction or inferior performance in a manner well known to persons skilled in engine analysis.

The apparatus may optionally include tachometer display means to test the tachometer output of the ignition circuit or to monitor engine speed during the ignition testing operation.

The testing apparatus may also include an additional facility enabling testing of the ignition circuit trigger pulses associated with the distributor. This testing facility may comprise a visual pulse readout device such as an LED display. The pulse readout device may also be employed for internal testing of the apparatus, which should preferably be carried out prior to applying the apparatus to an ignition testing operation.

The testing apparatus of the present invention may be adapted to testing ignition coils such as those associated with ignition circuits.

In adapting the testing apparatus to coil testing applications the trigger simulation means which in one form comprises trigger pulse forming means coupled to a suitable amplifying means may be connected to current adjustment means preferably in the form of a rheostat, ,and current display means which may by a suitably calibrated moving coil ammeter. The pulse generating means preferably should exhibit a similar waveform to that formed in the ignition circuit being tested. The amplifying means preferably comprises a transistor, triac or SCR or other suitable current controlling device.

An additional testing facility which may be used in isolation or in conjunction with the apparatus as above described incorporates a condenser testing facility. The condenser testing facility includes a condenser leakage testing circuit comprising a pulse forming means operating at or near the peak working voltage of the condenser being tested, and current display means which may in a preferred form be a suitably calibrated moving coil ammeter.

The condenser testing facility further includes a capacitance testing circuit comprising an alternating current power supply means, operating at the normal working voltage of the condenser being tested, and a voltage display means such as a moving coil voltmeter. Preferably the sensitivity of the voltmeter is adjustable.

The tachometer display means suitable for use with the testing apparatus of the present invention preferably comprises an electronic tachometer. The electronic tachometer may be used in conjunction with conventional distributors as well as various electronic distributors whose waveform outputs differ from those of conventional distributors. The electronic tachometer preferably has associated with it an engine speed limit circuit to automatically limit engine speed to the maximum allowable level. The function of the speed limit circuit is to ensure that excessive engine speeds which may damage the engine do not occur during testing of the ignition circuit.

According to one aspect of the present invention the electronic tachometer comprises pulse shaping means for shaping input pulses to a pulse generating means, said pulse generating means being triggered by said shaped input pulses and being adapted to provide output pulses having constant amplitude and duration; and tachometer display means, connected to the output of said pulse generating means, and adapted to provide an output indicative of the pulse rate of said output pulses.

In a preferred form the pulse shaping means includes voltage dividing means which may be of the resistive type and is preferably adjustable, a high pass filter network, a trigger bias network and peak value pulse detection and clipping means. The pulse generating means may comprise a suitably dimensioned monostable circuit. The output display means provides a steady state read out of the pulsed monostable output, and is preferably a moving coil ammeter or voltmeter but other suitable means could be adopted, for example an integrating circuit means coupled to a digital display means.

The electronic tachometer according to the present invention may in a preferred form be used to control an engine speed limit circuit to automatically limit engine speed to a safe operating level. The speed limit circuit preferably comprises an integrating means connected to the output of said pulse generating means for providing an output voltage proportional to the frequency of said output pulses, a comparator means for comparing this output voltage with a set voltage, said set voltage corresponding the maximum allowable engine speed, the comparator being adapted to provide an output when the output voltage of said integrating means exceeds said set voltage, said comparator output being adapted to activate an engine switch off means. The comparator means preferably comprises a differential amplifier means.The engine switch off means may include a suitable relay driven by an amplifying means. The relay is adapted to switch off ignition power to the engine in the event that the engine is over-revved.

In another possible form of the invention the apparatus includes trigger simulation means in the form of a trigger pulse forming means and amplifying means. The trigger pulse forming means, in this form of the invention, is adapted to provide a plurality of outputs each having different waveforms. fFhis form af apparatus is particularly suited for testing ignition circuits of different makes of internal combustion engines. Each output dof the plurality of outputs being suitable for testing a particular type of engine.

This form of the apparatus may be powered in a similar manner to the preferred form of apparatus previously described. Similarly, this form of the apparatus may include or be adapted to provide a tachometer output display means, a testing facility for testing ignition circuit distributor trigger pulses, a coil testing means, and a condenser testing facility all of the type previously described.

One of the plurality of outputs of the pulse generator means of this form of the invention may be chosen and connected to the remainder of the apparatus by any convenient means. Preferably a switch is provided to enable a desired output to be connected to the remainder of the apparatus.

Conveniently, any form of the apparatus, whether it has a single output or a plurality of outputs, may include means for producing a high voltage input for the capacitor discharge ignition system. Preferably the high voltage input, in the form of the apparatus of the invention powered by a D.C. source, is produced by utilizing an oscillator of known type and a transformer.

By applying the testing apparatus of any form of the present invention to the ignition circuit components in a serial sequence the failed component may be readily detected.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings wherein: Figure 1 shows in block diagram form the CDI and distributor testing apparatus according to a preferred embodiment of the present invention; Figure 2 shows in block diagram form the ignition coil testing apparatus according to a preferred embodiment of the present invention; Figures 3a and 3b show in block diagram form the condenser leakage and capacitance testing apparatus respectively according to a preferred embodiment of the present invention; Figure 4 shows the schematic circuit of one form of trigger simulator, pulse readout device and tachometer output tester according to the present invention; Figure 5 shows the schematic circuit of another form of trigger simulator, pulse rea dout device and tachometer output testing according to the present invention;; Figure 5a shows a schematic circuit of yet another form of trigger simulator according to a preferred form of the present invention; Figures 6a and 6b show the schematic circuit of one form of condenser leakage and capacitance testing apparatus respectively according to the present invention; Figure 7 shows a schematic circuit diagram of one form of ignition coil testing circuit according to the present invention; Figure 7a shows a schematic circuit diagram of a further form of ignition coil testing circuit according to the present invention.

Figure 8 shows in block diagram form a high voltage power supply which may be included with the apparatus of the invention for providing a high voltage to the capacitor discharge ignition system; Figure 9 shows in block diagram form a pulse generator for use with another preferred embodiment of the apparatus; and, Figure 10 shows a particular circuit of the general form of a high voltage power supply shown in the block diagram of Fig. 8 and also a particular form of the pulse generator shown in Fig. 9.

Figure 11 shows in block diagram form the electronic tachometer and speed limit circuit according to a preferred embodiment of the present invention, Figure 12 shows the schematic diagram of the electronic tachometer according to a preferred embodiment of the present invention, Figure 13 shows the schematic diagram of the engine speed limit circuit according to a preferred embodiment of the present invention.

Fig. 1 shows the testing apparatus of the present invention connected to a typical capacitive discharge ignition (CDI) system 1 5 comprising a distributor, an ignition coil and a CDI unit. The testing apparatus comprises a trigger pulse generator 10 coupled to amplifier 11, a tachometer 12, a pulse display 13, a switch 14 and a test spark plug 16. Testing of the distributor may be carried out by disconnecting the distributor from the CDI unit, where it is connected in normal operation, and connecting it to pulse display 1 3 via switch 1 4. An absence of output pulses at this point may indicate a possible distributor fault. The fault may be confirmed by connecting the pulse generator 10 and amplifier 11 to the CDI unit as shown in Fig. 1.If no other faults exists in the system test spark plug 1 6 will indicate a normal spark output.

A lack of spark output at this point would indicate that a fault lies in the CDI or the ignition coil unit. Since the tachometer 1 2 is connected to the tachometer output of the CDI units, a null output at tachometer 1 2 may indicate that a fault exists in the CDI unit. By further applying the testing apparatus to the ignition coil as described below the location of faults in the system may be determined. Pulse display 1 3 may be connected via switch 14 to an "internal test" mode to indicate that the pulse generator 10 and amplifier 11 are functioning before applying them to any given testing situation.

Fig. 2 shows the testing apparatus of the present invention connected to an ignition coil 24. A trigger pulse generator 20 is shown connected to amplifier 21, whose output is modified by rheostat 22 and applied via current display 23 to the primary circuit of ignition coil 24. The secondary circuit of the ignition coil is connected to a test spark plug 25. By adjusting the current displayed via rheostat 22 to within specifications and evaluating the quality of spark output the condition of the coil may be determined.

Figs. 3a and 3b show the general form of condenser testing apparatus which may be used in conjunction with the apparatus described so far to locate further faults in the ignition system. Fig. 3a shows a leakage testing circuit comprising an alternating current high voltage power supply 30 connected via a rectifier 31 and applied to the condenser 33 undergoing testing, via an ammeter 32.

An excessive current reading on ammeter 32 may indicate that condenser leakage is excessive.

Fig. 3b shows a capacitance testing circuit comprising an alternating current low voltage power supply 34 connected via the condenser 35 undergoing testing to a rectifier 36 and a voltmeter 37. An evaluation of the voltage reading on voltmeter 37 may indicate that the capacitance value of the condenser 35 is not within specification limits.

Fig. 4 shows one preferred form of trigger simulator, pulse display and tachometer output display shown in the block diagram of Fig. 1. The timer 40 may be a National Semi Conductor type LM555 or similar, and may be adjusted to provide the desired pulse rate and duty cycle in the manner well known to those skilled in the art.

The output of timer 40 is applied to a two stage amplifier comprising transistors 41 and 44 connected in an emitter follower configuration. Referring to Fig. 4 it is clear that when the switch 45 is in the "internal test" position, the LED display 42 which is driven by transistor 43, will display the pulse generated by the trigger simulation circuit. This may serve as an indication that the trigger simulation circuit is operating correctly. In the distributor test position the LED display can be used to indicate that the trigger function of the distributor is operating correctly.

The circuit of Fig. 4 may also be adapted to ignition coil testing, by connecting the output of the circuit to the primary winding of the ignition coil, via a suitable current adjustment means such as rheostat and current display means, such as a moving coil ammeter. The secondary winding of the ignition coil may then be connected to a standard 5mm test spark plug, the current measured for a steady discharge into the test spark plug, being the accepted measure of the quality of the coil. It is possible to use fixed current output together with a test plug having an adjustable spark gap width. In such instances the maximum gap width which will produce a steady discharge into the test spark plug is the accepted measure of the quality of the coil.

The lower half of the circuit shown generally at reference numeral 46 in Fig. 4 is the tachometer display which may be used to test the tachometer output of the CDI unit. This part of the circuit may in a further form of the invention, be replaced by the electronic tachometer and speed limit circuit shown in Figs.

11-13. Fig. 5 shows another embodiment of the trigger simulator, pulse display and tachometer output display shown in Fig. 1. The trigger pulse generator in this embodiment comprises a unijunction transistor circuit which generates a spiked pulse signal and is suitable for testing CDI units designed for use with electronic type distributors. The spiked output generated by this circuit simulates the output of an electronic distributor of the type currently in use in some Marine engines, e.g.

"Mercury" type. The output waveform of the electronic distributor type is markedly different from the conventional "breaker point" distributor, hence the need for a modified form of circuit as provided in Fig. 5.

The operation of the pulse generating circuit of Fig. 5 is as follows. Initially condenser 51 is discharged and the pn junction of transistor 52 is reverse biased. When power is switched on the condenser charges via resistance 50 and the voltage across condenser 51 rises.

When the voltage reaches the forward bias voltage of the pn junction, the transistor 52 turns on and gives rise to a large current flow through the junction which also discharges condenser 51. The discharge of condenser 51 causes the pn junction to become reverse biased with the result that the circuit reverts back to the initial state. The sequence of operation described above is then repeated.

The output voltage formed across resistance R thus forms a waveform comprising a series of rapidly rising and falling voltage spikes which simulates the trigger waveform of electronic distributors.

The distributor test facility shown in Fig. 5 is especially useful for testing electronic distributors particularly as they may often be a source of breakdown in the overall ignition system which otherwise might be difficult to detect. The internal testing facility serves a similar function as previously described in relation to Fig. 4. The modified tachometer display circuit shown generally at reference numeral 59 in Fig. 5 is adapted for testing the tachometer output associated with CDI units designed for use with electronic type distributors. According to a further embodiment of the present invention the tachometer circuit of Fig. 5 may also be replaced by the electronic tachometer and speed limit circuit shown in Figs. 11-13.

Fig. 5a shows a further form of trigger simulator according to the present invention.

This form of simulator is particularly suited for testing distributors used in certain "Chrysler" engines. The trigger simulator comprises an operational amplifier feedback circuit adapted to provide a sine wave voltage generator. The operational amplifier 200 may be a National Semi-Conductor type LM709 or similar having external frequency compensation networks 201 and 202. The feedback circuit may comprise a network including resistors 203 to 205 and condensors 207 and 208, which network determines the frequency of the sine wave generator. The frequency of the generator corresponds to the speed of the distributor being simulated and may be adjusted via resistors 204 and 206 within a convenient range. A variable resistor 214 may be provided in the feedback path and adapted to adjust the symmetricity of the waveform output.Feedback resistors 211 and 212 are provided to set the overall gain of the operational amplifier 200. The gain may be further adjusted by means of resistor 213. The output of the operational amplifier 200 is preferably connected via pulse shaping diodes 209 and 210 to an output buffer amplifier stage.

The buffer amplifier may be an operational amplifier 21 5 connected as shown. The buffer operational amplifier may be a National Semi Conductor type LM741 or similar. The output of the buffer amplifier is adapted to be connected to a CDI unit undergoing testing. A LED 218 or other visual indicating means may be connected across the output of the buffer amplifier in series with a test which 216 to provide an internal test facility for the trigger simulator.

Figs. 6a and 6b show particular forms of the condenser leakage and capacitance testing apparatus shown in Figs. 3a and 3b respectively. Referring to Fig. 6a, a transformer 60 steps up the mains voltage supply to 350 volts. This relatively high voltage is rectified by a diode 61 and is applied across the terminals of the condenser under test in series circuit with an ammeter 62. A resistor 64is included to limit maximum current in the circuit. The ammeter 62 gives a direct reading of the leakage current flowing through the condenser under test. Switches 63a and 63b are ganged such that switch 63b is normally open when switch 63a is normally closed and vice versa. This ensures that the high voltage charge stored in the condenser discharges via resistor 64 when the mains power is switched off.

In Fig. 6b the mains voltage is stepped down via transformer 65 to 6.3 volts. This low voltage is connected to the input of a full wave rectifier 66 via the condenser under test. The output of rectifier 66 is connected to a voltmeter comprising ammeter 68, adjusting resistor 69 and load resistor 67. Because the frequency of the mains supply remains sub 'stantially constant, the alternating current passing through the condenser will be proportional to the capacitance value of the condenser. Also, because the current flowing in the ammeter is proportional to the current flowing through the condenser, the ammeter 68 gives a direct reading of the capacitance value of the condenser.

Fig. 7 shows a particular form of the ignition coil testing apparatus shown in the block diagram of Fig. 2. The circuit comprises a trigger pulse generator in the form of a timer 70 which may be adjusted to the desired pulse rate and duty cycle by a suitable network (not Shown). Preferably the timer 70 is the circuit type LM555 described above.

Timer output pulses are amplified by transistors 71 and 72 and are applied to the primary winding of the ignition coil undergoing test ing. The primary winding of the ignition coil forms part of the load circuit of transistor 72 together with rheostat 73 and ammeter 73. A fixed spark gap test plug 75 is connected across the output of the ignition coil. By adjusting rheostat 74 for steady discharge into the test spark plug the current reading on ammeter 73 may be related to the perfor mance of the coil in a manner well known in the art. Preferably the timer 70 is adjusted such that it has a free running period of 1 6 mS and a duty cycle of approximately 75%.

Fig. 7a shows another preferred embodi ment of the ignition coil testing apparatus shown in Fig. 2. The circuit comprises a high voltage pulse generator in the form of an alternating current derived from the mains supply via the secondary winding of a transformer 84 and converted into direct current pulses by rectifying diode 77. The high voltage pulses derived in this way are adapted to charge a condenser 76 via the primary wind ing of the ignition coil. A tapping on the transformer 84 supplies a low voltage which is rectified by diode 79 and provides low voltage pulses to the gate circuit of the triac 80. Because of the relative phasing of the windings of the transformer 84 the low voltage pulses occur on alternate half-cycles with respect to the high voltage pulses charging the condenser 76.Thus the triac 80 is caused to conduct between charging cycles of the condenser 76. Conduction of the triac causes the condenser 76 to discharge through the second anode A2 of triac 80 which is at signal earth potential, into the primary wind ing of the ignition coil.

The trigger pulse generator shown in Fig. 9 is especially useful when testing ignition systems associated with various makes of engines. The timer 90 illustrated is preferably a National Semi-Conductor type LM566 or similar. Several different output waveforms are provided directly by the timer and these are further modified by means of a suitable shaping network 91. A rotary switch 91 is used to select from the plurality of outputs available.

The selected waveform is coupled to the amplifier stage 93. The output of amplifier stage 93 is the output of the trigger pulse generator.

Fig. 10 shows at reference numeral 100 a particular form of the high voltage power supply shown in the block diagram of Fig. 8 and suitable for a capacitive discharge ignition system. The circuit on the right shows a particular form of the trigger pulse generator shown in Fig. 9 and comprises a voltage controlled oscillator 101 having a triangular waveform output 102 and a square waveform output 1 03. The square waveform output is shown connected to pulse shaping networks 104 and 105 to provide additional waveform outputs. All the waveform outputs thus generated are connected to the rotary switch 106 which enable any one waveform output to be selected. The selected waveform passes to the base of output transistor 107 which is shown connected in an emitter follower configuration.Preferably voltage controlled oscillator 101 is a circuit type LM566 and is preferably connected to operate at frequencies below 1 KH,. Timing resistor 108 is adjustable to enable variation of the frequency of the output of the trigger pulse generator.

Fig. 11 shows in block diagram form the electronic tachometer and speed limit circuit according to the present invention. The tachometer circuit comprises a pulse shaping circuit 111 to modify the distributor pulses suitable for triggering pulse generator 11 2. The pulse generator is adapted to provide output pulses of fixed amplitude and duration which will be displayed by a suitable output display 113, such as a moving coil voltmeter. Due to the inertia of the moving coil the pulses will be displayed on the voltmeter as a steady deflection. Other forms of display may be utilised although it may be necessary to provide an integrating facility when using more raped voltage displays.

The engine speed limit circuit comprises integrator 114, comparator 115, amplifier 11 6 and engine switch off means 11 7 connected serially as shown in Fig. 11. The integrator 11 4 provides an output voltage which is proportional to the frequency of the output of pulse generator 11 2. This output voltage is compared to a set reference voltage in the comparator. Whenever, the compared voltage exceeds the set voltage, the comparator 11 4 is adapted via amplifier 11 6 to activate engine switch off means 11 7.

Fig. 1 2 shows one form of the electronic tachometer shown in Fig. 11 and comprises an integrated circuit timer 1 20 which may be a circuit type LM555 or similar, connected for monostable operation and triggered via a network comprising resistors R1, R2, R3 and R4, isolating condenser C1 and clipping diode D1.

The purpose of the clipping diode is to isolate the integrated circuit from high level transients in the trigger circuit.

Timing resistors R6 to R10 are selectable to obtain output pulses of variable duration, thereby providing a variable engine speed range on the output meter 121. By selecting output pulses of longer duration in the monostable circuit, low PRM readings will be effectively expanded in the meter output readings.

Fig. 1 3 shows a preferred form of engine switch off means according to the present invention and includes an integrating circuit comprising resistor R13 and condenser C4.

The output of the integrating circuit is shown applied to one input of a differential pair circuit 1 30. The other input is connected to a variable resistance R16 to provide an adjustable voltage reference which corresponds to the maximum allowable engine speed. The output of the differential pair circuit 1 30 drives the relay 1 32 via driving transistor 131. The relay 132 is adapted to switch off ignition power to the engine.

It is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the present invention.

Claims (21)

1. Circuit testing apparatus for testing a component or components associated with an ignition circuit, said apparatus comprising trigger simulation means for association with an input of said component or components and output display means for association with an output of said component or components such that the performance of said component or components may be determined.

2. Circuit testing apparatus according to Claim 1 wherein said trigger simulation means comprises trigger pulse forming means and amplifying means.

3. Circuit testing apparatus according to any one of the preceding claims wherein said output display means is a test spark plug.

4. Circuit testing apparatus according to any one of the preceding claims for testing a distributor, a CDI unit or an ignition coil associated with an ignition circuit, wherein said trigger simulation means is connectable to the input of said CDI unit, said CDI unit is connected to the ignition coil input and said output display means is connectable to the ignition coil output such that the performance of said distributor, CDI unit or ignition coil may be determined.

5. Circuit testing apparatus according to Claim 4, including a pulse display means connectable to the output of said distributor via a contact means.

6. Circuit testing apparatus according to Claims 4 or 5 including a tachometer means connectable to a tachometer output of said CDI unit.

7. Circuit testing apparatus according to Claim 6 wherein said tachometer means is an electronic tachometer.

8. Circuit testing apparatus according to Claim 7 wherein said electronic tachometer comprises pulse shaping means for shaping input pulses to a pulse generating means, said pulse generating means being triggered by said shaped input pulses and being adapted to provide output pulses having constant amplitude and duration; and tachometer display means, connected to the output of said pulse generating means, and adapted to provide an output indicative of the pulse rate of said output pulses.

9. Circuit testing apparatus according to Claim 8 wherein said tachometer display means comprises a voltmeter.

1 0. Circuit testing apparatus according to any one of Claims 6 to 9 wherein the tachometer means has associated with it a speed limit means.

11. Circuit testing apparatus according to Claim 10 wherein said speed limit means comprises an integrating means connected to the output of said pulse generating means for providing an output voltage proportional to the frequency of said output pulses, a comparator means for comparing this output voltage with a set voltage, said set voltage corresponding to the maximum allowable engine speed, the comparator being adapted to provide an output when the output voltage of said integrating means exceeds said set voltage, said comparator output being adapted to activate an engine switch off means.

1 2. Circuit testing apparatus according to Claim 11 wherein said engine switch off means comprises a relay.

1 3. Circuit testing apparatus according to any one of Claims 1 to 3 for testing an ignition coil associated with an ignition circuit wherein said trigger simulation means is son- nectable to the input of said ignition coil via current adjustment means and said output display means is connectable to the output of said ignition coil such that the performance of said ignition coil may be determined.

14. Circuit testing apparatus according to any one of the preceding claims further including a condenser testing means comprising a condenser leakage testing circuit and a capacitance testing circuit.

1 5. An electronic tachometer suitable for use with circuit testing apparatus according to any one of claims 1 to 6, said electronic tachometer comprising pulse shaping means for shaping input pulses to a pulse generating means, said pulse generating means being triggered by said shaped input pulses and being adapted to provide output pulses having constant amplitude and duration; and tachometer display means, connected to the output of said pulse generating means, and adapted to provide an output indicative of the pulse rate of said output pulses.

1 6. An electronic tachometer according to Claim 1 5 wherein said output display means comprises a voltmeter.

1 7. An electronic tachometer according to Claim 1 5 or 1 6 further including a speed limit means.

1 8. An electronic tachometer according to Claim 1 7 wherein said speed limit means comprises an integrating means connected to the output of said pulse generating means for providing an output voltage proportional to the frequency of said output pulses, a comparator means for comparing this output voltage with a set voltage, said set voltage corresponding to the maximum allowable engine speed, the comparator being adapted to provide an output when the output voltage of said integrating means exceeds said set voltage, said comparator output being adapted to activate an engine switch off means.

1 9. An electronic tachometer according to Claim 1 8 wherein said engine switch off means comprises a relay.

20. Circuit testing apparatus according to Claim 1 substantially as herein described with reference to Figs. 1 - 1 0 of the drawings.

21. An electronic tachometer according to Claim 1 5 substantially as herein described with reference to Figs. 11-13 of the drawings.