Electronic ignition block. Scheme, description

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Electronic ignition block. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering /Automobile. Ignition

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Automotive ignition systems are now mainly built on thyristors [1], however, transistor systems have not lost their relevance [2, 3]. Recently, many powerful, including composite, transistors have been produced with characteristics that allow them to be used for automotive ignition systems.

The proposed scheme of an automotive electronic ignition unit was developed and tested by the author in a Zhiguli 2108 car, etc., in which transistor switches (3620-3734) with a non-contact Hall sensor (53.013706) are used.

The difference between this design and the standard one [2] is that the K561LA8 microcircuit, connected according to the Schmitt trigger circuit, is used to generate interrupt pulses.

The technical characteristics practically do not differ from the standard ignition unit, but with the use of the Schmitt trigger, the interrupt pulses are formed with a steeper trailing edge, which allows you to almost instantly turn off the current source from the ignition coil, thereby increasing the high voltage on its secondary winding.

The use of capacitor C2 ensures that the ignition coil is disconnected from the current source when the car engine is stopped, thereby preventing useless heating of the coil.

(click to enlarge)

The electronic ignition block diagram shown in Fig. 1 contains:

- a circuit for generating pulses with adjustable duty cycle on a DD1 chip. assembled according to the Schmitt trigger scheme;
- a powerful key on transistors VT1 and VT3 with an active current limiter on the transistor VT2, a voltage divider on resistors R8, R9 and a current-measuring resistor R10;
- voltage regulator for powering the DD1 chip on the zener diode VD4, capacitor C3 and resistor R3;
- a protection circuit against excess impulse voltage in the on-board network on the zener diode VD6, capacitor C4 and resistor R11;
- circuit for protecting the unit from incorrect connection of the battery on the VD7 diode;
- a circuit for protecting the transistor VT3 from impulse overloads during the operation of the ignition coil on the diode VD5. resistors R12, R13.

The scheme works as follows. When the ignition is turned on, the voltage from the battery is supplied to the circuit through the diode VD7 and resistor R 11. The ignition coil is not supplied with voltage at the initial moment, since the starter does not rotate the engine shaft, and there are no pulses at the input of the DD1.2 microcircuit. At the output of DD1, there is a low-level voltage that keeps the transistor VT1 closed, so the transistor VT3 is also closed.

When the starter turns the motor shaft, pulses appear at the output of the sensor, which arrive through C2 at the input of element DD1.1. The latter switches, and a pulse appears at the output of DD1.2, which opens transistors VT1 and VT3. A current flows through the ignition coil, and electrical energy is stored in the magnetic field of the coil. The next moment, when a positive polarity pulse disappears from the sensor output, the Schmitt trigger abruptly switches to the opposite state, a low level appears at the output of the DD1.2 element, which enters the base of the transistor VT1. Transistors VT1 and VT3 quickly close, and the current passing through the ignition coil also quickly disappears. In this case, an EMF of self-induction with a voltage of 400 V is induced in the primary winding of the coil, and a high voltage pulse of 23000 ... 25000 V occurs in the secondary winding of the ignition coil.

In a powerful key, transistors VT1 and VT3 use an active current limiting circuit in the ignition coil, which protects the transistor VT3 from overload and stabilizes the magnitude of the "gap" current during fluctuations in the supply voltage of the car's on-board network, thereby ensuring the stability of the output characteristics of the ignition system [Z].

When the transistor VT1 is unlocked, the output transistor VT3 is saturated, providing a low residual voltage at the output of the electronic ignition unit. As long as the current flowing through the output transistor VT3 and the current-measuring resistor R10 included in its emitter circuit is below the allowable limit level, the transistor VT2 is locked.

When the output current reaches the limit level, the transistor VT2 begins to open, and the potential on its collector decreases, which leads to a decrease in the magnitude of the control current. Transistor VT3 at the same time leaves the saturation mode in the active mode, the output voltage rises to a level at which the specified limiting current mode is maintained. If the impulse voltage in the ignition coil is exceeded, it is fed through the R12-R13 divider to the VD5 zener diode, which, opening, locks the VT3 transistor. The chain C5-R14, connected in parallel to the output transistor, is an element of the shock excitation oscillatory circuit, i.e. determines the magnitude and rate of increase of the secondary voltage developed by the ignition system. Resistor R14 limits the capacitive current through the transistor VT3 at the time of unlocking the latter, if the capacitor C5 is discharged. Structurally, the electronic ignition unit is made on printed circuit board (Fig. 2) from one-sided foil-coated fiberglass with a size of 95x75 mm, on which the elements of the circuit are mounted. The board is installed in a standard case from the switch 3620-3734.

The electronic ignition unit uses the K561LA8 chip and MLT resistors. Resistor R10 - type C5-16 with a power of at least 1 watt. Capacitors - K73-11 for a voltage of at least 63 V. Diodes VD2, VD3 - KD521A or any low-power silicon. Zener diode VD1 - for a stabilization voltage of 8 V, type D814A or KS182A. Zener diode VD4 - for a stabilization voltage of 9 V, type D814B or KS191A. Zener diode VD5 - KS518A or KS508G. Diode VD7 - type KD209A, can be replaced by a diode KD226G. Transistors VT1, VT2 - KT972A; VT3 - KT898A or KT890A (KT8109A). VT3 is installed on a regular radiator made of 4 mm thick aluminum plate, insulated from the body with a double mica gasket with thermal paste.

To establish the block, a sound generator with a frequency of 30 to 400 Hz is used, simulating the operation of the breaker sensor. To obtain an output signal with a voltage of 7 ... 9 V, if necessary, it is necessary to make a power amplifier based on a KT815 transistor [4]. Any oscilloscope is suitable for viewing pulses, preferably a two-beam one. In addition, you need a power supply with voltage regulation from 8 to 18 V with a current of at least 10 A.

At the time of setting up the circuit, you can do without an ignition coil by loading the collector of the VT3 transistor onto a choke with a magnetic circuit made of electrical steel plates with an inductance of 3,8 mH, a resistance of 0,5 Ohm. To do this, you can use a unified low-frequency choke type D 179-0,01-6,3. The generator-simulator of the pulse sensor is connected to the input of the circuit and the shape and amplitude of the output pulses are observed on the oscilloscope.

By changing the resistances in the VD2-R4 and VD3-R5 circuits, you can adjust the duty cycle of the pulses, which allows you to adjust the time for closing and opening the ignition coil.

To set the required limiting current, the oscilloscope is connected to the emitter of the transistor VT2. In this case, it is necessary to temporarily connect a resistor with a resistance of 2 ohm to the emitter circuit of the transistor VT0,1. By changing the voltage on the power supply, observe the appearance of a signal at the emitter. The current limiting level is adjusted by resistors R12 and R13. After pre-configuration, the circuit is installed in the car in accordance with the connection diagram [2] and its final configuration is performed.

Literature:

1. Lomakin L. Electronics behind the wheel. - Radio, 1996, N8, p.58,
2. Starkov V. Transistor ignition systems - Radio, 1991, N9. pp.26-29.
3. Bela Buna. Electronics on the car. - M.: Transport, 1979.
4. Cars "Zhiguli 2108" and their modifications. Device and repair. - M.: Transport, 1987.
5. Yutt V.E. Electrical equipment of cars: Textbook. - M.: Transport, 1989, 175s.
6. Sidorchuk V. Electronic octane corrector. - Radio, 1991, N11, p.26.

Author: G.Skobelev, Kurgan; Publication: N. Bolshakov, rf.atnn.ru

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