Ignition system with overvoltage and excess current protection

In a known ignition system, the emitter-collector circuit of the ignition transistor is connected in series with the primary winding of the ignition coil and with a precision resistor. When the voltage across the latter exceeds a predetermined value, an auxiliary transistor is switched to a conductive state. The emitter-collector circuit of the auxiliary transistor is connected to the base of the ignition transistor and, when conductive, prevents further increases of current through the primary winding of the ignition coil. To protect this circuit, a series circuit including two Zener diodes is connected between the base and collector of the ignition transistor. Further, a voltage divider is connected in parallel with the emitter-collector circuit of the ignition transistor and an additional resistor is connected between the base and the emitter thereof. When overvoltages across the primary winding cause breakdown of the Zener diodes, the current through the Zener diodes is blocked from the emitter-collector circuit of the auxiliary transistor either by a diode or by a blocking transistor having an emitter-collector circuit connected in parallel with the base-emitter circuit of the auxiliary transistor. The blocking transistor is maintained in the conductive state blocking the auxiliary transistor throughout the application time of an externally applied blocking signal which is adapted to maintain the ignition transistor in the blocked state.

CROSS REFERENCE TO RELATED APPLICATIONS AND PUBLICATIONS: 
DE-OS No. 25 49 586 
The present invention relates to ignition systems for internal combustions, 
and, in particular, to ignition systems wherein the ignition transistor 
has an emitter-collector circuit connected in series to the primary 
winding of the ignition coil and a current limiting resistor is connected 
in series with the emitter-collector circuit of the ignition transistor. 
The so-formed series connection is connected to a source of DC voltage. 
More particularly, the present invention relates to an ignition system 
wherein an auxiliary control transistor is provided which becomes 
conductive when the current through the current limiting resistor reaches 
a predetermined current value and which, when conductive, controls the 
ignition transistor to prevent further increases of current through the 
primary winding of the ignition coil. 
BACKGROUND AND PRIOR ART: 
The present invention relates to an ignition system as disclosed, for 
example, in DE-OS No. 25 49 586. This type of circuit operates 
satisfactorily, but it would be desirable to provide some protection for 
the ignition transistor against overvoltages while it is in the blocked 
state, that is while a spark is being generated. Such a protection could 
comprise a clamping circuit which allows a switching of the ignition 
transistor to the conductive state for a short time while the spark is 
being generated, so that dangerous overvoltages would be substantially 
decreased. This is, however, difficult to accomplish in the known circuit 
since the current limiting resistor would then immediately cause the 
auxiliary transistor to become conductive causing the current flowing 
through the clamping circuit to flow through the emitter-collector circuit 
of the auxiliary transistor. Under these conditions, a collector-base 
clamping circuit for the ignition transistor would no longer be effective 
and the auxiliary transistor could be destroyed by excessive currents. 
THE INVENTION 
It is an object of the present invention to furnish protective circuits 
which not only protect the ignition transistor from overvoltages but also 
protect the auxiliary transistor from excessive currents which could 
result during operation of the overvoltage protective circuit. 
In accordance with the present invention, a clamping circuit is connected 
between the output circuit and the control electrode of an ignition 
control element (ignition transistor). The clamping circuit includes at 
least one threshold element (e.g. a Zener diode) adapted to switch from a 
blocked to a conductive state when the voltage across the primary winding 
of the ignition coil exceeds a predetermined voltage. The clamping circuit 
furnishes a control signal allowing a selected current to flow through the 
primary winding in response to a voltage across the primary winding 
exceeding a predetermined voltage. Further, a blocking circuit (e.g. a 
diode or a transistor) is provided for blocking current flowing through 
said threshold element when said threshold element is in the conductive 
state from said output circuit of said auxiliary control element. In a 
first preferred embodiment, a diode is connected between the 
emitter-collector circuit of the auxiliary transistor and the clamping 
circuit with a polarity so as to block any current flowing through the 
clamping circuit from the emitter-collector circuit of the auxiliary 
transistor. 
In a second preferred embodiment, a blocking transistor has an 
emitter-collector circuit connected from the base of the auxiliary control 
resistor to a reference potential. This transistor is maintained in the 
conductive state thereby blocking the auxiliary control transistor 
throughout the whole time that an ignition signal adapted to switch the 
ignition transistor to the blocked state and maintain it in said block 
state is applied to the ignition system.

In the ignition system shown in FIG. 1, the primary winding 10 of an 
ignition coil has a first end connected to the positive terminal of the 
battery (U.sub.B) and a second end connected to the collector of an 
ignition transistor 12. The emitter of ignition transistor 12 is connected 
through a precision resistor 14 to a reference potential (e.g. chassis). 
To protect ignition transistor 12 against overvoltages, it would be 
possible to connect a single Zener diode with a correspondingly high 
threshold voltage in parallel with the collector-base circuit of ignition 
transistor 12. However, in the circuit shown in FIGS. 1 and 2, a voltage 
divider including resistors 16 and 18 is connected in parallel with the 
emitter-collector circuit of transistor 12. The common point of resistors 
16 and 18 is designated by reference numeral 20 and is connected to the 
base of transistor 12 by a series circuit including two Zener diodes 22, 
24. An additional resistor 26 is connected between the base of transistor 
12 and its emitter. The base of an auxiliary transistor 28 is connected 
through a voltage divider including resistors 40, 42 and 44 to the 
terminal of precision resistor 14 which is connected to the emitter of 
transistor 12. The voltage divider 38 is theoretically not required, but 
is generally used in practice. The emitter of auxiliary transistor 28 is 
connected to the reference potential, while its collector is connected 
through a load resistor 46 and the emitter-collector circuit of an input 
transistor 48 to battery terminal U.sub.B. 
In the known ignition system, transistor 12 is the output stage of a more 
or less complicated control circuit as, for example, described in the 
above-mentioned DE-OS No. 25 49 586. As indicated by an arrow in FIGS. 1 
and 2, an ignition signal, adapted to maintain transistor 12 in the 
blocked state throughout its duration, is applied to the base of 
transistor 48 at a terminal 48' by the remainder of the ignition system 
(not illustrated). 
A current blocking diode is connected between the collector of transistor 
28 and the base of transistor 12. 
OPERATION: 
In the absence of the ignition signal at terminal 48', transistor 12 is in 
the conductive state. A current flows from the battery U.sub.B through the 
primary winding 10 of the ignition coil, the collector-emitter circuit of 
transistor 12 and precision resistor 14 to the reference potential. 
Because of the inductance of the ignition coil, the current in the series 
circuit of primary winding of the ignition coil, emitter-collector circuit 
of transistor 12 and precision resistor 14 increases in accordance with a 
predetermined exponential function. This increases the voltage across 
resistor 14. As soon as this voltage has reached a predetermined voltage 
corresponding to a predetermined current through primary winding 10, 
auxiliary transistor 28 is switched to the conductive state. This causes a 
decrease in the conductivity of the emitter-collector circuit of 
transistor 12 so that further increases of current through the primary 
winding 10 beyond the predetermined maximum current value do not occur. 
This current limited condition, which in modern transistorized ignition 
systems is kept to as short a time period as possible, is maintained until 
the externally applied ignition signal is applied at terminal 48'. This 
causes transistor 12 to block which, in known fashion, results in a steep 
increase in voltage across primary winding 10 and therefore across the 
secondary winding of the ignition coil. A spark plug in the secondary 
winding then creates a spark due to the high voltage across it. 
Let it now be assumed that, for any reason, the voltage across primary 
winding 10 increases beyond a predetermined value while transistor 12 is 
blocked, thereby creating the danger that the reverse voltage of 
transistor 12 is exceeded causing transistor 12 to be destroyed. Under 
these conditions, Zener diodes 22 and 24 break down, causing a voltage to 
be applied to the base of transistor 12 which causes it to be switched 
briefly to the conductive state. Since Zener diodes 22, 24 immediately 
reblock following a decrease of voltage to an allowable value, a brief 
switching to the conductive state of transistor 12 does not interfere with 
the generation of the spark. It should also be noted that, since the 
voltage across primary winding 10 is to be decreased very rapidly, 
ignition transistor 12 is switched to a sufficiently conductive state upon 
breakdown of Zener diodes 22, 24 that the voltage across resistor 14 is 
sufficient to cause transistor 28 to switch to the conductive state. 
Therefore, in the absence of diode 50, the current flowing through Zener 
diodes, 22, 24 could flow through the collector-emitter circuit of 
transistor 28. The resultant decrease in base potential of transistor 12 
would cause this transistor to switch to the blocked state thereby 
preventing proper operation of its clamping circuit. Transistor 12 would 
thus again be exposed to the dangerous overvoltages. However, in the 
presence of diode 50, no current can flow from Zener diodes 22, 24 through 
the emitter-collector circuit of transistor 28. The latter is thus 
protected from excessive currents and, also, unable to interfere with the 
proper operation of transistor 12. 
The embodiment shown in FIG. 2 is similar to that in FIG. 1; however diode 
50 has been omitted. Instead of diode 50 a transistor 52 is provided. 
Transistor 50 has a collector connected to the base of transistor 28 and 
also connected through a resistor 54 to battery U.sub.B. The emitter of 
transistor 52 is connected to a reference potential. As indicated by an 
arrow, an inverted ignition signal, that is the signal applied at terminal 
48' after inversion by an inverter, is applied to the base of transistor 
52. The inverted ignition signal maintains blocking transistor 52 is the 
conductive state during the time ignition transistor 12 would normally be 
blocked. The base of transistor 28 is therefore substantially at the 
reference potential independent of the voltage across precision resistor 
14 while the ignition signal is applied at terminal 48'. Transistor 28 
remains blocked and no current can flow through its emitter-collector 
circuit even if an overvoltage appears across primary winding 10 of the 
ignition coil. The clamping circuit can thus maintain transistor 12 in the 
conductive state for the desired brief time interval. Since the ignition 
signal applied at terminal 48' is present in any case in the ignition 
system, the use of the complementary (inverted) ignition signal at 
terminal 52 presents no difficulties. 
It will be noted that the protective circuits of the present invention are 
readily incorporated into the present day ignition systems and operate 
reliably to prevent damage to the ignition transistor and the auxiliary 
transistor of such systems. 
Various changes and modifications may be made within the scope of the 
inventive concepts.