Maplin auto electronics projects P3

Maplin auto electronics projects

except that the rotor is a star shaped wheel and the static

magnetic s y s t e m has a c o r r e s p o n d i n g number of poles,

in this c a s e six of e a c h , for a six cylinder engine

Auto advance

One reason why this triggering method has c o m e out on top over rival designs is simply due to one staggering implication B e c a u s e the s y s t e m is magnetic; it is, in ef-fect, a very simple a.c g e n e r a t o r on a small s c a l e , and its output is, therefore, proportional to the driven speed What this means is that at slow r o t o r s p e e d s the output voltage is low, while for higher s p e e d s 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 o c c u r at the required point anywhere on the leading slope of the output waveform Figure 2.5 shows how, from different output levels as

p r o d u c e d by c o r r e s p o n d i n g r o t o r s p e e d s , the trigger level is near the peak of the s l o p e 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 a d v a n c e mechanism, and this with just one mov-ing part — the rotor!

The need for ignition advance

While the fuel/air mixture in the c o m b u s t i o n c h a m b e r burns at a c o n s t a n t rate, the engine as a whole however

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Distributor Rotary shaft ferromagnetic

Wide air gap offers ^^^S I/ I I

Figure 2.5 Auto-advance plot using waveform of Figure 2 4 ( c )

is required to o p e r a t e over a range of crankshaft s p e e d s For this r e a s o n the moment of ignition must o c c u r ear-lier at higher r.p.m Full c o m b u s t i o n of the fuel gas must

o c c u r during the period where the piston has full age on the crankshaft, and at high revs the burn actually needs to begin well in a d v a n c e of this point; at lower

lever-s p e e d lever-s , not lever-so much, at idle, hardly at all T h e magnetic

r e l u c t a n c e type of ignition timing s e n s o r a c h i e v e s this auto a d v a n c e action in a much more linear manner than

do c o m p r o m i s e d m e c h a n i c a l or e l e c t r o n i c m e t h o d s , and barring t h e odd rare mishap s u c h as a s c r e w coming loose, o n c e set it does not need readjustment — for any-

o n e who has p e r s o n a l l y e n d u r e d t h e long drawn out

p r o c e s s of ignition retiming, the s u b t l e t i e s of the tion do not need reiteration!

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Furthermore, s i n c e this requirement has already been

taken c a r e of by the s e n s o r , it makes the amplifier much

simpler Otherwise e l e c t r o n i c a d v a n c e might take the

form of frequency s e n s i t i v e switches s e l e c t i n g from a

range of time delays, the minimum number of which is

two in the c r u d e s t example of such a s y s t e m More than

this requires rather more logic gates, or a m i c r o p r o c e s

-sor Instead the magnetic r e l u c t o r allows the use of a

c o m p a r a t i v e l y very few t r a n s i s t o r s to p r o d u c e an

ampli-fier

The electronic ignition switch

Obviously the heart of an e l e c t r o n i c s y s t e m which

simu-lates the action of a m e c h a n i c a l switch to o p e r a t e the

coil primary in the traditional way is a t r a n s i s t o r , and

you might s u p p o s e that any power t r a n s i s t o r a b l e to

c a r r y the maximum on-time c u r r e n t of the primary will

suffice But oh dear me no R e m e m b e r that the primary

potential is sufficient to p r o d u c e an arc a c r o s s the

me-chanical switch, and that the ignition coil as a whole,

primary included, must b e allowed to g e n e r a t e however

high a voltage is n e c e s s a r y 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 ° '

d e v i c e s are notoriously inefficient, which m e a n s to say

that the current gain (Hf e) is very small, measured in tens

or less r a t h e r than hundreds

T h e usual biasing method is to use a b a s e bias r e s i s t o r

which typically c o n n e c t s directly between the

transis-t o r ' s b a s e and transis-the supply rail, and transis-this r e s i s transis-t o r can be

formidably beefy to provide the n e c e s s a r y bias current for the t r a n s i s t o r to do its j o b properly, with the attend-ant power c o n s u m p t i o n and heat dissipation p r o b l e m s

I have actually s e e n one design where the b a s e bias

re-s i re-s t o r ire-s no more than 9.2 Ω!

No, that wasn't a printing error It's an illustration of how

e x t r e m e b a s e biasing may have to be to e n s u r e that the

switching t r a n s i s t o r a c h i e v e s a saturated on s t a t e ,

es-sential to get the maximum available voltage a c r o s s the primary of the coil and t h e r e f o r e the maximum primary current Suppose, in a worst c a s e example, that our tran-

fortunately later d e v i c e s are b e t t e r than that now), but then in order to c o n d u c t 4 A this value r e d u c e s to say

<2 T o ensure a d e q u a t e biasing we a s s u m e a current gain

of 1.5, and c h o o s e a b a s e bias r e s i s t o r with a value of

4 Ω, taking into a c c o u n t a b a s e / e m i t t e r forward drop of

1 V This r e s i s t o r is then sinking 2.6 A and dissipating 28 watts; has to be removed from the rest of t h e amplifier

to avoid cooking it to death, and be provided with its own heatsink!

Even in the c a s e of the aforementioned design using the 9.2 Ω c o m p o n e n t , the r e s i s t o r is of the high power, metal

e n c a p s u l a t e d type ( s e e the r e s i s t o r s s e c t i o n of Maplin's

c a t a l o g u e for e x a m p l e s ) and is s c r e w e d to the outside surface of the amplifier's die-cast c a s e

In comparison the power dissipation of the actual ing transistor is not very much at all, which s e e m s almost

switch-p e r v e r s e This is b e c a u s e it switch-performs a switching action;

it is either on or off Which leads us to the next rion, namely ensuring that the t r a n s i s t o r c o m m u t â t e s

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( s w i t c h e s off) as fast as p o s s i b l e This is n e c e s s a r y s i n c e the coil n e e d s to be s w i t c h e d off quickly in order to de-velop its high tension output (a slowly s w i t c h e d ignition coil fails to make a s p a r k )

High speed switching

Figure 2.6 shows the e s s e n t i a l s of a typical ignition plifier as used with a magnetic r e l u c t a n c e type of timing

am-s e n am-s o r T o am-s u m m a r i am-s e am-s o far, TR5 iam-s the inefficient, high voltage power t r a n s i s t o r switch for the coil, and R9 is the b a s e bias r e s i s t o r In this c a s e the bias current origi-nates from T R 4 , which is c o n t r o l l e d by a Schmitt trigger comprising TR2, TR3, and r e s i s t o r s R3 to R6 T h e Schmitt trigger is e s s e n t i a l to p r o d u c e the fast edged switching waveform from the slower changing input, provided by

s h a p e d output from the s e n s o r coil and you can s e e now that while the amplitude of t h e ramp is variable, the in-put t h r e s h o l d is c o n s t a n t Dl also b l o c k s the negative going part of t h e input waveform, which is superfluous, while R l is a current limiter to p r o t e c t Dl and T R I in the event that for e x a m p l e t h e input is a c c i d e n t a l l y con-

n e c t e d to t h e supply while t h e power is on

P r o t e c t i o n for the engine's m e c h a n i c a l bits can be

pro-vided by including C I , which a c t s as a rev limiter While

it is charged quickly v i a D l , this charge leaks away slowly via the b a s e emitter of T R I due to this d e v i c e ' s current gain offering a relatively high impedance, and in c o n s e -

q u e n c e the waveform at T R l ' s emitter takes on a more triangular s h a p e As engine s p e e d i n c r e a s e s the mean average d.c voltage drop a c r o s s R2 also i n c r e a s e s until

a point is r e a c h e d where even the lowest level of the waveform e x c e e d s the low t h r e s h o l d of the Schmitt trig-ger; the amplifier c e a s e s to o p e r a t e and no sparks are generated

CI also affords s o m e RF filtering, but it might b e ing to learn that the input leads are rarely s c r e e n e d T h e

surpris-s e n surpris-s o r coil isurpris-s of surpris-s u c h low i m p e d a n c e that thisurpris-s isurpris-s

unnec-e s s a r y and in any c a s unnec-e s i n c unnec-e both t h unnec-e s unnec-e wirunnec-es arunnec-e run

t o g e t h e r as a pair, any externally induced current will

be equally p r e s e n t in both, cancelling e a c h o t h e r out

A real working amplifier

Figure 2.7 shows a circuit which is the culmination of six

m o n t h s development including testing in the field

on-board a real m o t o r v e h i c l e which, for earlier v e r s i o n s , proved to be d e s t r u c t i v e (to the circuit, not the vehi-

c l e ) Such is the way of r e s e a r c h and development, and

t h e s e e v e n t s m a d e definite i n d i c a t i o n s that t h e unit should be:

• e l e c t r i c a l l y robust,

• m e c h a n i c a l l y robust; and,

• utterly weatherproof

Referring to Figure 2.7, the input stage is as d e s c r i b e d for the hypothetical amplifier earlier, with the c o m b i n e d diode j u n c t i o n s of both Dl and T R I forming the input

t h r e s h o l d level, and having R l as a p r o t e c t i v e current limiter Cl is merely an HF filter in c o n j u n c t i o n with R l and does not provide any rev limiting

To r e d u c e c o m p o n e n t count, the fast switching action needed to sharpen the pulse p r o d u c e d by T R I is pro-vided by ICI, a 5 5 5 timer IC used in an unusual way Instead of being employed in a c o n v e n t i o n a l (for the IC) manner as a m o n o s t a b l e e t c both trigger and t h r e s h o l d inputs (pins 2 and 6 ) are tied t o g e t h e r to exploit the be-haviour of the internal b i s t a b l e , forcing a Schmitt trigger action T h e 555 was c h o s e n b e c a u s e the output struc-ture can s o u r c e the driver stage, TR2, directly without the need for any more t r a n s i s t o r amplifiers

While t h e r e is no input and T R I is off pins 2 and 6 of ICI are high and the output pin 5 is low, so that TR2 is also off, allowing the bias r e s i s t o r , R4, to s a t u r a t e the main

t r a n s i s t o r switch for the ignition coil, T R 3 , and the coil

is on

Upon an input ramp voltage from the timing s e n s o r ceeding the c o m b i n e d t h r e s h h o l d levels of Dl and T R I ,

ex-T R I c o n d u c t s and quickly pulls the trigger input down

and switch on TR2, which c l a m p s R4 to ground and prives TR3 of b a s e drive current T h e coil is switched off, ICI is r e s e t when the ramp is c o m p l e t e d as T R I col-

de-l e c t o r g o e s high again, and 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

Note that all s t a g e s use the 0 V rail as the s o l e r e f e r e n c e and are thus immune to supply rail fluctuations, which will o c c u r often in the range of 1 2 - 1 3 8 V e s p e c i a l l y if an

e l e c t r o - m e c h a n i c a l regulator is employed, and can be less than 9 V while the s t a r t e r m o t o r is giving the bat-tery a hard time

Electrical safeguards

T h e o t h e r a r e a of e l e c t r i c a l w e a k n e s s is c o n c e n t r a t e d

on T R 3 This is b e c a u s e of s o m e h o r r i b l e punishments that the ignition coil will try to inflict on this device From the range of high voltage power t r a n s i s t o r s readily avail-able the only one to prove itself electrically tough enough

to be truly reliable is the long standing, T 0 3 packaged

B U 2 0 8 d e v i c e d e s i g n e d for u s e in c o l o u r T V l i n e

t i m e b a s e s and s w i t c h e d - m o d e p o w e r s u p p l i e s T h e

essential to ensure maximum voltage drop a c r o s s the coil and r e d u c e power dissipation in the t r a n s i s t o r itself to a minimum — it is t h e m o r e e x p e n s i v e version, but that

dissi-pation and power l o s s , as this c o m p o n e n t ( R 4 ) has a

c o n s e r v a t i v e value of 22 Ω(!) However T R 3 still n e e d s two e s s e n t i a l p r o t e c t i o n s c h e m e s

One of t h e s e must c o p e with ignition coil b a c k e.m.f., which, without a power sapping c o n d e n s e r ( s e e e a r l i e r )

is e x c e s s i v e But surely this can only o c c u r without a spark plug as a load, else how can this happen where

t h e r e is an air gap which must strike and c o n d u c t and

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t h u s l i m i t b o t h t h e c o i l ' s p r i m a r y a n d s e c o n d a r y voltages? T h e truth is that, c o m p a r a t i v e l y speaking, the air gap takes a long time to respond Until this happens

it is as if t h e r e were no load at all and the coil s h o v e s up the potential enormously A very simple calculation can

be made to get s o m e idea of the t h e o r e t i c a l magnitude

of b a c k e.m.f from a coil by:

voltage drop across coil

commutation time

where commutation time is the time taken for the ing d e v i c e to switch off, which is of c o u r s e not truly

switch-i n s t a n t a n e o u s Assumswitch-ing for e x a m p l e a c o m m u t a t switch-i o n time of 100 ns which even for a BU288 is very much on the slow side, we get (in t h e o r y ) :

12V = 120,000 V!

100 ns

This is what we get on the primary side In p r a c t i c e

how-ever it will b e p r e c i s e l y 1,400 V Why so? B e c a u s e this is the designed c o l l e c t o r to b a s e ( Vc b) limit of a BU208, never mind that this value is double the maximum V ! ce

T h e b a s e / c o l l e c t o r junction is breaking down in the

re-v e r s e direction like a Zener diode, and it is not supposed

to be used in this way Damage is cumulative and the device may fail after even s o m e tens of hours of appar-ently fault free operation

The voltage limiting protection s c h e m e in Figure 2.7

com-p r i s e s i d e n t i c a l c o m com-p o n e n t s S R I and SR2, which are nothing more e l a b o r a t e than two mains transient sup-

p r e s s e s in s e r i e s T h i s c o m p o n e n t is a Metal Oxide Varistor (MOV), the r e s i s t a n c e of which is voltage de-pendent It has a knee voltage of 340 V (that is, 1.414 χ

240 V), which is the peak value of the mains supply Up