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ffigh energy ignition for cars with reluctor distributors This high energy ignition system is a variant of the circuits published in the May and June 1988 issues of SILICON CHIP. It is especially intended for reluctor distributors and features improved protection for the coil and main switching transistor. By JOHN CLARKE & LEO SIMPSON Our high energy ignition sytems described in the May and June 1988 issues of SILICON CHIP have been very popular with car enthusiasts all over Australia and many people have written to say how pleased they are with with them. Based on the Motorola MC3334P ignition IC and MJ10012 high voltage Darlington switching transistor, the circuit has been easy to build and very reliable. But while a thousand of more ignition kits have been built by satisfied readers, we have also had a steady stream of requests from DISTRIBUTOR HOUSING MAGNETIC PICKUP ASSEMBLY ADVANCE PLATE SLEEVE ARMATURE FIXED BASE PLATE 18 SILICON CHIP readers who want to use the high energy ignition module with reluctor distributors. There have been two main reaso:n for this. First, some people have had cars like the Mitsubishi Sigma in which the factory fitted ignition module has failed. Rather than pay several hundred dollars for a new ignition module (which does seem outrageous), readers have wanted to substitute the SILICON CHIP high energy module. Second, some readers who own Ford Falcons or Holdens with six or eight cylinder motors have wanted VACUUM ADVANCE PULL ROD to upgrade their car ignition system by using a reluctor distributor from a later model. Where the car has had a distributor made by Lucas there has been a more pressing reason - you can no longer buy Lucas distributors so if your old one is worn out, you are faced with a substantial cost for a new dizzy made by Bosch. So why not go for a reluctor distributor from a later model, from a wrecker's yard? Our response when asked these questions by readers has been to refer them to the data article on the MC3334P ignition IC published in the May 1988 issue of SILICON CHIP. This article accompanied the high energy ignition system in that issue and featured the chip in a Motorola applications circuit with a reluctor distributor (made by Delco; ie, General Motors). In ·an cases we have stated that we had not tried the circuit but readers have often gone ahead nonetheless. In some cases, with Fords and Holdens, they have been successful but in others, notably the Mitsubishi Sigma, the circuit has OVAL VACUUM ADVANCE ASSEMBLY CARBURETOR CONNECTION Fig.1: a reluctor distributor uses a toothed wheel on the distributor shaft and this runs close to a coil/pole piece assembly. As each moving tooth comes close to the coil, it generates a voltage which then triggers the electronic ignition module. This is what the ignition module looks like when all the components have been installed on the printed board and then fitted into the diecast case. The diecast case serves as a heatsink for the switching transistor. Note the loop in one lead of each of the zener diodes. worked only spasmodically and has not been viable. We wanted to know why? To find out, we went the same route as any reader would have. We obtained a reluctor distributor for a Mitsubishi Sigma from a wrecker's yard - the going rate is about $100. We then set up the distributor so that it could be driven by a motor in a bench jig and measured the waveform and voltage output at different speeds. Having assured ourselves that the distributor was producing the typical reluctor output waveform and that it was within the expected limits, we then connected it to the ignition circuit featuring the MC3334P IC. At first, we thought we had a goer but as we checked further we found that all was not well. At some speeds the reluctor output was not triggering the ignition circuit and so no sparks were being produced. And at low speeds, the coil charging time [dwell) was far too short. Both these factors would have made a car undriveable and hence the circuit was impractical. Our solution was to modify the Motorola circuit so that the reluctor output voltage is not critical. It involved adding four diodes and two resistors. We also discovered that the polarity of the reluctor output voltage affects the ignition timing and so we have incorporated a LED indicator circuit to show when it is correct. These modifications meant that the original circuit board is no longer suitable - a new circuit board is featured with this article. Reluctor distributor Before we go any further, what is a reluctor distributor? It is one of the three types used with solid state ignition systems. The others are the Hall Effect distributor [covered in our June 1988 article) and the flywheel pickup as used on many cars with microprocessor controlled engine management systems. A typical reluctor distributor is shown in the diagram of Fig.1. It consists of a toothed wheel on the distributor shaft, with one tooth for each cylinder of the motor. The toothed wheel runs very close to a soft iron pole piece which is wound with hundreds of turns of wire. The pole piece is attached to one pole of a permanent magnet so that the pole/coil assembly and the toothed wheel form a closed magnetic circuit. As each moving tooth comes close to the pole piece, the coil generates a voltage which swings strongly one way and then the other way, as the tooth passes and then moves away from the pole piece. The voltage waveform is akin to a flattened sawtooth and is shown in the oscilloscope photograph in this article. The beauty of the reluctor is that it is a simple passive device which is completely impervious to oil, dirt and the high temperatures in a distributor. Once the gap between the toothed wheel and the pole piece has been set, and the ignition timing is set, the reluctor distributor should not require any adjustment for the life of the car. That's a big advance over conventional distributors with points. Effectively, the only components which ever require replacement in a vehicle with solid state ignition are the spark plugs. Sometimes though, the ignition module itself fails, which is where we came in; hence this new circuit. MAY 1990 19 COIL CURRENT COIL CURRENT (b) TIME (ms) 10 15 20 25 30 Fig. 2: this diagram shows the primary coil current with and without dwell extension. In (b), the spark duration is fixed at one millisecond and so coil energy is not wasted in useless primary resonance. This allows the coil current to start from a high value for each cycle rather than from zero. High energy ignition As with our previous ignition circuits, mentioned above, the circuit described here gives a much greater spark output than is possible with conventional ignition systems, even though the same ignition coil is used. The reason for this is that the fixed dwell of conventional ignition, as set by the distributor cam and points gap setting, does not apply. Instead, at medium and high engine revolutions, the spark dura- tion is fixed at about 0.8 milliseconds. This means that for each spark, after 0.8 milliseconds has elapsed, the main switching transistor in series with the coil turns on again, so that the coil immediately begins storing energy for the next spark. The diagram of Fig.2 illustrates the benefit of a fixed spark duration. Not only does the coil have a much longer period for the current to build but since the coil does not waste energy in useless r inging of PARTS LIST 1 reluctor distributor, to suit vehicle 1 polarised 2-way connector, to suit distributor 1 PCB, code SC05106901, 102 x 59mm 1 diecast box, 11 0 x 30 x 63mm 4 6mm standoffs 3 solder lugs 1 grommet 1 TO-3 mica washer and insulating bushes 1 T0-3 transistor cover Semiconductors 1 MJ10012 NPN power Darlington (Q 1 ) 1 BC54 7 NPN transistor (02) 4 1N537475V5Wzener diodes (D1 -D4) 20 SILICON CHIP 4 1 N4002 1 A diodes (D5-D8) 1 MC3334P ignition IC (IC1) 1 red LED Capacitors 2 0. 1 µF 1 00V metallised polyester 1 .01 µF metallised polyester 1 4 70pF 1 00V ceramic Resistors (0.25W, 5%) 1 820kD 2 22kD 2 1 Ok!] 2 1kD 1 330!] 1 1000 5W Miscellaneous Automotive wire, screws, nuts, shakeproof washers, solder, heatsink compound, etc. the primary circuit after the spark is extinguished, the coil current stays at a much higher level. For a more detailed explanation of how this comes about, see our article in the May 1988 issue. The net result is that the ignition coil is able to deliver a much hotter spark, even at very high engine revolutions. By the way, our article in the May 1988 issue also gives a thorough explanation of conventional Kettering ignition, and this is essential background information for anyone interested in electronic ignition. Now have a look at the complete circuit diagram of Fig.3. The reluctor coil is connected to pins 5 and 4 of the MC3334P, IC1, via 22k0 resistors. These provide protection from excessive input voltages which may occur with a reluctor which is running with a very small gap. The leads from the reluctor are bypassed with a 470pF capacitor and one side is connected to OV (chassis) via a .OlµF capacitor. Both these capacitors help eliminate any hash picked up by the reluctor leads. Initial bias Pin 3 is the dwell voltage output from IC1 and it is stored in a 0.1µ.F capacitor. Because the Mitsubishi Sigma's reluctor has a smaller than usual output voltage, it is necessary to provide an initial bias voltage to pin 3, to make sure the circuit works over the full engine rev range and particularly at the lower speeds. This bias voltage is provided by diodes D5-D8 which are fed via a lk!J resistor from the + 12V supply. The bias voltage is fed from the diodes to pin 3 via an 820k0 resistor. By using the four diodes, the bias voltage fed to pin 3 is essentially constant at around 2.8 volts, regardless of the battery voltage which can vary widely. In fact, the circuit will work down to battery voltages of 4 volts! The fact that an engine could not be cranked at such a low battery voltage indicates that the battery voltage range is far more than adequate. IC1 stores the bias voltage from the diodes and a reference voltage detected from the reluctor via an in- .----------------------+12V SWITCH VIA IGNITION 330f! , __ _ HTTO OISTRIBUTOR 820k IC1 MC3334P 4x1N5374 (75V 5W) .,. CASE B RELUCTOR IGNITION SYSTEM ELJc C 0 0 B VIEWEO FROM BELOW Fig.3: the key components in the circuit are the Motorola MC3334P high energy ignition IC and the MJ10012 high-power Darlington transistor (Ql). The Darlington transistor switches the heavy currents through the coil. The string of four zener diodes protects the Darlington against excessive coil voltage if a spark plug lead becomes detached. ternal diode. It is this dwell voltage which determines how long the output transistor Ql is conducting, at the various engine speeds. We've already mentioned that the spark duration at medium and high engine speeds is around 0.8 milliseconds. At cranking and low engine speeds, the spark duration is longer and may be up to 3 milliseconds long. This actually has the effect of slightly reducing the heat dissipated in the coil while not having effect on the available spark energy. The output of ICl, pin 7, turns Darlington transistor Ql on and off. Pin 7 is actually the collector of an internal transistor and it is supplied with current via the external 1000 5 watt resistor. When pin 7 is pulled low, all the current through the 1000 resistor is shunted away from the base of Ql which is then turned off. When the internal transistor is turned off, all the current passes into the base of Ql which then turns fully on. Ql is an MJ10012, which is a very rugged high voltage Darlington transistor with a collector current rating of 15 amps peak. It is designed specifically as a coil driver in automotive ignition systems. Never- We obtained this Mitsubishi Sigma distributor from a wrecker's yard for $100. The rotor button has been removed -to show the toothed wheel. This has four teeth, one for each cylinder of the motor. theless, while it is a rugged device, it is possible that it could be damaged if a spark plug lead became disconnected. Zener diode protection If a spark plug lead does become disconnected, the coil secondary voltage can rise to very high levels, perhaps 40,000 volts or more. This can do two things. First, it can damage the coil itself by internal flashover and second, because the primary voltage also becomes high, say 500 volts or more, it could damage the MJ10012. MAY 1990 21 which needs to be explained involves Q2 and the LED. The base of Q2 is connected to pin 7 of ICi so that Q2 turns off every time Qi turns off. Normally, Q2 is on and the LED is alight. When Q2 turns off, for the same 0.8 millisecond period as Qi, the LED is extinguished momentarily and that tells you the firing point of the reluctor. This enables you to do an initial timing of the reluctor, and ensure that the reluctor coil polarity is correct before the ignition coil is connected. Spark plug gaps The high-power Darlington transistor is installed on the outside of the diecast case and fitted with a plastic cover to prevent shorts or "tingles" from inadvertent contact. To protect against this situation, we have included a chain of four 75V zener diodes between the emitter and collector of Ql. With these in circuit, the coil primary voltage is limited so that no damage can result. In fact, while you may expect the voltage to be limited to 300 volts (ie, 4 x 75V), the actual limiting figure is close to 350 volts because the zeners do not turn on really sharply. In our previous ignition circuits featuring the MC3334P and MJ100i2, we specified four 75V iW zener diodes, type iN4761. In most cases these have worked entirely satisfactorily and we have not heard of an MJ100i2 or an ignition coil failure. However, the iN476i zeners in our own prototype of this circuit failed when we deliberately opened up the spark gap on our bench setup. And we have heard of i W 75V zeners failing in a number of units in the field. Therefore, to give a greater margin of safety, and thus extra reliability, we are now specifying 5 watt zener diodes, type iN5374. We strongly recommend that they be used in the previous circuits, too. Trigger point indicator The last feature of the circuit The lower trace on this CRO photograph shows the 35V p-p output from the reluctor pickup. Above this is the coil primary voltage waveform which has a peak to peak voltage of 350V (CRO sensitivity l00V/div; horizontal timebase lms/div). 22 SILICON CHIP In the past it has been common practice by car enthusiasts, when they have fitted electronic ignition, to increase the spark plug gaps. This was done to take advantage of the higher spark voltage and thereby obtain a longer spark "path". We don't recommend this practice. It places much greater voltage stress on the car's high tension components; the coil, distributor, spark plug leads and the spark plugs themsevles. So there is more likelihood of a high tension failure. Construction The circuitry for our high energy ingition system is housed in a small diecast box. It may not look "high energy" but it is. The box measures 110 x 30 x 63mm and provides what little heatsinking the main Darl- Mount the four zener diodes with a loop in one lead to provide stress relief as the devices warm up. The remaining parts should all be installed without stressing their leads and should move freely in the PCB before soldering. I. ~~; ~1.m•• ~ ~ LED. 1f-"'l ·- ~ 07 ~ £2) 06 IGNITION SWITCH . ~ ...... 03 ~ . . ~ I' - - - -- - -- -- -- - - -- Fig.4: here's how to install the parts on the PCB and run the internal wiring. All wiring from the board should be run using 4mm auto cable which has a generous current rating. The case of the Darlington power transistor must be electrically isolated from the metal case. Problems? ... and you don't have our NEW 1990/91 148 page electronic parts and accessories catalogue ... Its our latest TRADE catalogue for the consumer ARISTA ... Your one-stop problem solver. ----TRANSISTOR 0 0 CASE ©-INSULATING BUSH <at>::::i)-SOLDER LUG <at>-WASHER <at>....--SPRING WASHER <at>--NUT Fig.5: the Darlington power transistor is electrically isolated from the case using insulating bushes and a mica washer. Smear heatsink compound on the mating surfaces before bolting the assembly together, then use your multimeter to check that the transistor is correctly isolated. ington transistor needs. Under normal operation, the transistor and the case become warm but not hot; or no hotter than the surrounding metalwork underneath the bonnet. All the circuit components, with the exception of the MJ10012 transistor, are mounted on a printed circuit board measuring 102 x 59mm (code SC 05106901). The wiring diagram is shown in Fig.4. Note that the diecast box is the only type that we recommend. This is because it is splashproof, rugged and provides heatsinking for transistor Q1. We don't recommend folded metal cases because they are not splashproof. Begin construction by mounting the PC pins onto the PCB, then install the rest of the components, according to the wiring diagram. Mount the 5W resistor so that it is raised about 1mm from the PCB surface to allow cooling. The four zener diodes should be mounted with a loop in one of the leads to provide stress relief. For the remaining components it is important to insert them into the PCB without stressing their leads. The component leads should move freely in the PCB holes before they are soldered. Once assembly of the PCB is complete, work can begin on the diecast box. Drill holes for the corner mounting positions of the PCB, a cord entry in the side of the box large enough for the grommet, and finally holes for the earth terminal, transistor mountings and the base and emitter leads. The transistor is mounted on one side of the case with the emitter lead located near the relevant connection on the PCB. The transistor is mounted using a ... Stylus .. . ...Plugs, Jacks and Sockets ... .. .Batteries .. . ... Cable .. . .. .Tools and Technical Aids .. . ... Plug and Power Packs .. . ... Car/Auto Accessories .. . ...Boxed Hi Fi Speakers .. . ... Raw Replacement Speakers ... ... Speaker Accessories ... ... Telephones and Intercom .. . . .. Public Address Accessories .. . ... Security and Alarm Accessories .. . .. .TV/Video/Antenna Accesories .. . ... Videocam Accessories ... ... Audio Accessories ... ... Headphones ... ... Computer Accessories .. . ... Microphone Accessories .. . ... Mixers, Amplifiers, Equalizers ... Just about anything you want ... Get your catalogue complete with "Recommended Retail Prices" free from your local ARISTA dealer or send $2.50 P & H and your return address to: ARIST~ ELECTRONICS PTY LTD PO BOX 191, LIDCOMBE, NSW, 2141 MAY 1990 23 Our test setup used an old sewing machine motor to drive the distributor shaft via a flexible coupling. An electronic speed controller varied the speed of the motor so that performance could be checked over a wide rev range. mica washer and insulating bushes to electrically isolate it from the diecast case . The method of assembly is shown in Fig.5. You can mark the holes for mounting the transistor using the T0-3 mica washer as a template. After drilling, remove any burrs using a larger diameter drill. With the heatsink area (ie, where the transistor mounts onto the case) free of any metal swarf or grit, smear a thin layer of heatsink compound onto the transistor mounting base and the mating area on the case, before placing the mica washer in position. When the transistor is screwed down, check that it is completely isolated from the case by using a multimeter (switched to a high "Ohms" range) or a continuity checker. The PCB is mounted on four 6mm standoffs within the case. We recommend using shakeproof washers on all screws to ensure that they don't become loose. The wiring to the power transistor and to the various external connections should be via 4mm auto cable, soldered to the PC pins. Use 1-metre or longer lengths of wire to provide the chassis, points, coil and battery connections to the circuit. Installation Choose a convenient and well ventilated spot in the engine bay, ,ol O 0 T'" 0 O'> <.O 0 T'" I.() 0 (.) (/) Ch ,~ Fig.7: this is the full size artwork for the printed circuit board. 24 SILICON CHIP 10:L12V FUSE BALLAST RESISTOR N/0 HT 12V RELAY .,. +12V TO IGNITION CIRCUIT TO COLLECTOR OF 01 Fig.6: if making a direct connection to the ignition switch is too difficult (in cars with the ballast resistance in the harness), you can use this relay hook-up to make a more convenient connection to + 12V. away from the heat of the exhaust manifold and clear of any possible splashing from water. If you can, choose a position reasonably close to the coil so that long wires can be avoided. For our prototype, we were able to mount it simply with two large self-tapping screws in one side of the case and into a bulkhead near the wheel well. It was just a matter of having suitable holes drilled in the case and bulkhead. The two screws are then used to secure the unit. A plastic case fitted over the power transistor is a good idea - J because it prevents any possibility of shorts from stray tools. It can also avoid the possibility of a "tingle" to any unsuspecting mechanic working on the car while the engine is running - and that could include you! After mounting, the electrical connections, can be made. The final connection for the transistor ignition is to the + 12V supply which comes via the ignition switch. In some cars this is accessible at the + 12V side of the coil ballast resistor. However, some Ideally, the high energy ignition module should be installed in the coolest available spot underneath the bonnet. Use 12mm x No.to selftapping screws to secure the module to the firewall. vehicles have the ballast resistor as part of the wiring lead to the coil and this means that the + 12V connection must be made at the fuse panel. Once the ignition system is installed, the reluctor polarity and static timing can be tested. Turn the engine by hand until a reluctor tooth is close to the pole piece. Then loosen off the distributor clamp and rotate the distibutor housing back and forth so that the tooth moves past the pole piece. Each time the tooth moves away from the pole piece, the LED should blink. If not, swap the connections to the reluctor coil and try again. You can now fit the lid to the case, connect the coil and replace the distributor cap. The vehicle should now be started and the ignition timing checked using a timing light in the normal way. ~ VBATT 4-24 Vdc RBATT 300 CflLTER 0.1 I -=- 6 Vee RDRIVE 100 Ignition Coil Primary 8.0 mH Power C1' 470 RL 10 k Output and OVP OUT Current Limit B c2• J:0.01 Dwell Reference •Optional Pans for Extended Transient Protection Buffer Sense Rs 0.075 -=- .. A 350 V zener clamp is required when using the standard MJ10012 . This clamp .is not required if a selected version with V(BR)CEO(sus) " 550 V is used. -=- MC3334 Sense 2 1 Power Ground Ground~------~ Fig.8: this is Motorola's suggested circuit for the Delco distributor. It requires several modifications so that it is also compatible with the distributor fitted to Mitsubishi Sigmas. MAY1990 25