Maplin auto electronics projects P3

Maplin auto electronics projects

Electronic ignition except that the rotor is a star shaped wheel and the static magnetic system has a corresponding number of poles, in this case six of each, for a six cylinder engine. Auto advance One reason why this triggering method has come out on top over rival designs is simply due to one staggering implication. Because the system is magnetic; it is, in ef- fect, a very simple a.c. generator on a small scale, and its output is, therefore, proportional to the driven speed. What this means is that at slow rotor speeds the output voltage is low, while for higher speeds the output is also higher by a proportional amount. If the trigger thresh- old of the amplifier's input is voltage dependent, then triggering can be made to occur at the required point anywhere on the leading slope of the output waveform. Figure 2.5 shows how, from different output levels as produced by corresponding rotor speeds, the trigger level is near the peak of the slope if the output is low, and near the beginning if it is high. At a stroke, what we have here is, by way of an added bonus, an automatic ignition advance mechanism, and this with just one mov- ing part — the rotor! The need for ignition advance While the fuel/air mixture in the combustion chamber burns at a constant rate, the engine as a whole however 47 Auto electronics projects 48 Distributor Rotary shaft ferromagnetic \ element W il low reluctance \J U // Permanen t and results in • §£ magnet strong magnetic I Π . j j field for coil / / Pickup coll ••^1^ // (^) ^ 1 Narrow Gap Voltage due to magnetic Maximum narrow field changing as gap voltage rotor moves toward sensor ^/ Voltage due to magnetic Maximum wide field changing as gap voltage rotor moves away from sensor (c) Figure 2.4 Magnetic timing sensor Electronic ignition 49 Wide air gap offers ^^^S I/ II high reluctance / / and results in **^e§> —7/ weak magnetic I Π « II field for coil Η^-Ν // (b) + I Wide Gap Rotor arm key /^^^^^^^^^^^^^^^^^^^^^^^^^^\^^\ Re ' UCt0r I /oV^^^T-| (ts. Li^) /^^V^ \ || Coil ant ^ magnet 1 |wfr ~"^\ Χ^ΤΓ^Λ/ / « || —j—j-i— system under rT Λ 1 dfr*^ \v ^ X/X // J / Ii dust cover (j| ^ ^ ta *'° P oles " I — L Distributor l ι body V J (D) Figure 2.4 Continued Auto electronics projects Figure 2.5 Auto-advance plot using waveform of Figure 2.4(c) is required to operate over a range of crankshaft speeds. For this reason the moment of ignition must occur ear- lier at higher r.p.m. Full combustion of the fuel gas must occur during the period where the piston has full lever- age on the crankshaft, and at high revs the burn actually needs to begin well in advance of this point; at lower speeds, not so much, at idle, hardly at all. The magnetic reluctance type of ignition timing sensor achieves this auto advance action in a much more linear manner than do compromised mechanical or electronic methods, and barring the odd rare mishap such as a screw coming loose, once set it does not need readjustment — for any- one who has personally endured the long drawn out process of ignition retiming, the subtleties of the opera- tion do not need reiteration! 50 Electronic ignition Furthermore, since this requirement has already been taken care of by the sensor, it makes the amplifier much simpler. Otherwise electronic advance might take the form of frequency sensitive switches selecting from a range of time delays, the minimum number of which is two in the crudest example of such a system. More than this requires rather more logic gates, or a microproces- sor. Instead the magnetic reluctor allows the use of a comparatively very few transistors to produce an ampli- fier. The electronic ignition switch Obviously the heart of an electronic system which simu- lates the action of a mechanical switch to operate the coil primary in the traditional way is a transistor, and you might suppose that any power transistor able to carry the maximum on-time current of the primary will suffice. But oh dear me no. Remember that the primary potential is sufficient to produce an arc across the me- chanical switch, and that the ignition coil as a whole, primary included, must be allowed to generate however high a voltage is necessary to bridge the plug gap? We are therefore obliged to use a high voltage power tran- sistor, with a V rating of several hundred volts, and such ' ce ° ' devices are notoriously inefficient, which means to say that the current gain (H fe ) is very small, measured in tens or less rather than hundreds. The usual biasing method is to use a base bias resistor which typically connects directly between the transis- tor's base and the supply rail, and this resistor can be 51 Auto electronics projects formidably beefy to provide the necessary bias current for the transistor to do its job properly, with the attend- ant power consumption and heat dissipation problems. I have actually seen one design where the base bias re- sistor is no more than 9.2 Ω! No, that wasn't a printing error. It's an illustration of how extreme base biasing may have to be to ensure that the switching transistor achieves a saturated on state, es- sential to get the maximum available voltage across the primary of the coil and therefore the maximum primary current. Suppose, in a worst case example, that our tran- sistor has an H fe of 3 at 1 A (yes, just 3 — although fortunately later devices are better than that now), but then in order to conduct 4 A this value reduces to say <2. To ensure adequate biasing we assume a current gain of 1.5, and choose a base bias resistor with a value of 4 Ω, taking into account a base/emitter forward drop of 1 V. This resistor is then sinking 2.6 A and dissipating 28 watts; has to be removed from the rest of the amplifier to avoid cooking it to death, and be provided with its own heatsink! Even in the case of the aforementioned design using the 9.2 Ω component, the resistor is of the high power, metal encapsulated type (see the resistors section of Maplin's catalogue for examples) and is screwed to the outside surface of the amplifier's die-cast case. In comparison the power dissipation of the actual switch- ing transistor is not very much at all, which seems almost perverse. This is because it performs a switching action; it is either on or off. Which leads us to the next crite- rion, namely ensuring that the transistor commutâtes 52 Electronic ignition (switches off) as fast as possible. This is necessary since the coil needs to be switched off quickly in order to de- velop its high tension output (a slowly switched ignition coil fails to make a spark). High speed switching Figure 2.6 shows the essentials of a typical ignition am- plifier as used with a magnetic reluctance type of timing sensor. To summarise so far, TR5 is the inefficient, high voltage power transistor switch for the coil, and R9 is the base bias resistor. In this case the bias current origi- nates from TR4, which is controlled by a Schmitt trigger comprising TR2, TR3, and resistors R3 to R6. The Schmitt trigger is essential to produce the fast edged switching waveform from the slower changing input, provided by TRI. TRI is the basis of the input stage which incorporates the input level threshold as indicated in Figure 2.5. This consists of diode Dl and the base/emitter junction of TRI itself, which together will not begin to conduct until the applied level is >1.2 V. This signal is of course the ramp shaped output from the sensor coil and you can see now that while the amplitude of the ramp is variable, the in- put threshold is constant. Dl also blocks the negative going part of the input waveform, which is superfluous, while Rl is a current limiter to protect Dl and TRI in the event that for example the input is accidentally con- nected to the supply while the power is on. 53 Auto electronics projects Figure 2.6 Essential ignition amplifier for a magnetic reluctor based system Electronic ignition Protection for the engine's mechanical bits can be pro- vided by including CI, which acts as a rev limiter. While it is charged quickly viaDl, this charge leaks away slowly via the base emitter of TRI due to this device's current gain offering a relatively high impedance, and in conse- quence the waveform at TRl's emitter takes on a more triangular shape. As engine speed increases the mean average d.c. voltage drop across R2 also increases until a point is reached where even the lowest level of the waveform exceeds the low threshold of the Schmitt trig- ger; the amplifier ceases to operate and no sparks are generated. CI also affords some RF filtering, but it might be surpris- ing to learn that the input leads are rarely screened. The sensor coil is of such low impedance that this is unnec- essary and in any case since both these wires are run together as a pair, any externally induced current will be equally present in both, cancelling each other out. A real working amplifier Figure 2.7 shows a circuit which is the culmination of six months development including testing in the field on- board a real motor vehicle which, for earlier versions, proved to be destructive (to the circuit, not the vehi- cle). Such is the way of research and development, and these events made definite indications that the unit should be: • electrically robust, • mechanically robust; and, • utterly weatherproof. 55 Auto electronics projects 56 ^ -H2V test ^j 4V LLK-7 S220nF 22R SIOOuF y w/ T20X MlOW T35V ο 1 1— u — LJ—3—<fj /Q\ BU208A I 1 1 d be / 0 0 \ [Ξ NE555 ^ (be) \o/ Figure 2.7 Real amplifier circuit diagram [...]... d off, ICI is r e s e t when t h e ramp is c o m p l e t e d as T R I c o l l e c t o r g o e s high a g a i n , a n d t h e s y s t e m is r e a d y t o g e n e r a t e a n o t h e r spark 57 Auto electronics projects Note t h a t all s t a g e s u s e t h e 0 V rail as t h e s o l e r e f e r e n c e and a r e t h u s immune to supply rail f l u c t u a t i o n s , which will o c c u r often in t... h e r e s i s t a n c e of which is v o l t a g e dep e n d e n t It has a knee v o l t a g e of 3 4 0 V ( t h a t is, 1.414 χ 240 V ) , which is t h e peak value of t h e mains supply Up 59 Auto electronics projects to t h i s point its r e s i s t a n c e is high, but r e d u c e s c o n s i d e r a b l y as s o o n as its k n e e v o l t a g e is e x c e e d e d , and is n o r m a l l y u s e d... c t position on t h e s t r i p b o a r d r e a d y to slide into t h e c a s e , as c a n b e s e e n in P h o t o 2.2 Photo 21 A complete home-made i g n i t i o n amplifier in its case 61 Auto electronics projects Photo 2 2 The s t r i p b o a r d assembly o f the c i r c u i t o f Figure 2 7 w i t h heatsink in p o s i t i o n and remote R4 on s e p a r a t e board Mechanical considerations... h e c k t h e o p e r a t i o n of t h e amplifier b e f o r e fitting into t h e v e h i c l e A simple t e s t r e q u i r e s a 12 V power supply of up to 4 A output ( o r a c a r bat- 63 Auto electronics projects t e r y ) , and a s p a r e ignition coil T h e amplifier on its own draws a p p r o x i m a t e l y 5 0 0 t o 6 0 0 mA B y wrapping s o m e tinned c o p p e r wire around t h e + t e... insulation w e a k n e s s always b r e a k s down during a c c e l e r a t i o n Such a breakdown is usually total, as I found out t h e hard way, leaving me s t r a n d e d So t a k e note! 65 Auto electronics projects 700-, 600 Limited by protection scheme #1 500ω 400- § 30020010010_ 0 © ® +12V t 0V Limited by protection scheme §2 5 milliseconds — Figure 2.9 Oscillograph produced by test r i g : (a)... o , p r o d u c i n g a s p a r k at t h e HT o u t p u t T h e c o u n t e r - e m f from t h e coil p r i m a r y t h a t follows t u r n s CSR1 off again While all this is going on, 67 Auto electronics projects + 12V DC IN Ο DC/DC CONVERTER +500V DC OUT C1 "II" σ INPUT STAGE Figure 2.10 Capacitive discharge ignition block schematic of ) PULSE GENERATOR Ignition coil essential parts t h e c o... f o r m e r , and not u s e a mains t r a n s former in reverse] Mains t r a n s f o r m e r s a r e d e s i g n e d to tap p o w e r from t h e mains at mains f r e q u e n c y , and a r e 69 Auto electronics projects not v e r y good at doing anything e l s e Given t h e shortcircuited output problem, the converter could be a single-ended flyback c o n v e r t e r design The future O n e p o s s... and it is only a q u e s t i o n of time b e f o r e motor c a r s follow suit and become equally distributorless 70 3 Microcontrollers The m i c r o c o n t r o l l e r is the workhorse of the modern electronics industry T h a t s t a t e m e n t may b e strong, but it is not an e x a g g e r a t i o n , for it is b e c o m i n g i n c r e a s i n g l y diffic u l t t o p u r c h a s e a n y s i g . and, • utterly weatherproof. 55 Auto electronics projects 56 ^ -H2V test ^j 4V LLK-7 S220nF 22R SIOOuF y w/ T20X MlOW T35V ο 1 1— u — LJ 3 <fj /Q BU208A I 1 1 d be. example the input is accidentally con- nected to the supply while the power is on. 53 Auto electronics projects Figure 2.6 Essential ignition amplifier for a magnetic reluctor based system. P oles " I — L Distributor l ι body V J (D) Figure 2.4 Continued Auto electronics projects Figure 2.5 Auto- advance plot using waveform of Figure 2.4(c) is required to operate