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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Electronic wiper interrupter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices Modern domestic cars are equipped with a wiper that provides two speeds of its operation, as well as both continuous and pulsating modes. This creates comfort when driving in difficult weather conditions. Meanwhile, for many old car models, and even for some relatively new ones, the wiper works in only one - continuous - mode. The addition of their windshield wiper control unit with a simple electronic interrupter allows for controlled intermittent operation. Most of the previously published electronic wiper control units [1] have a significant drawback. The fact is that when the driver turns on the wiper, the moment of supplying current to its electric motor is delayed by a time that depends on the position of the variable resistor engine that sets the duration of pauses between brush movement cycles [2, 3]. This creates certain difficulties during operation, distracts the driver for additional manipulations with the pause regulator. Further improvement of these blocks and the experience of their operation have shown that one cycle of the movement of the brushes when the wiper is initially turned on is not always enough to clean the windshield. As a rule, this requires from three cycles of movement under normal conditions to five under the most unfavorable conditions. The breaker described below (see the diagram in Fig. 1), connected to the wiper, provides an adjustable intermittent mode and simultaneous switching on of the M1 electric motor for four to five continuous cycles of brush movement with each successive switching on, after which the device automatically switches to single cycle mode with pauses in between. The speed modes provided by the design of the wiper - fast or slow - remain unchanged, you can only set the duration of pauses between cycles in these modes. Pauses are set by a variable resistor, the handle of which is displayed on the dashboard of the car.
The device is designed to work with the existing wiper mode switch, and the connection diagram is shown on the example of the M-2140 car. The numbering of the connector conductors and the connection of the switch to them correspond to the factory wiring diagram of the vehicle. Conductor A, connecting pin 1 of connector X2 with pin 1 of switch SA2 (see Fig. 1), must be removed when connecting a breaker. The breaker consists of a trinistor switch (VS1), a unijunction transistor opening pulse generator (VT2), a trinistor initial turn-on unit (VT1), self-induction EMF protection elements (VD1, C3). In the initial state, the wiper mode switch SA2 is in the zero position ("Off"). The contacts of the limit switch SF1, mechanically connected to the motor reducer, are open. When the contacts SA1 of the ignition lock are closed, the voltage of the on-board network is supplied to pin 1 of the breaker and through the motor windings, pin 4 of connector X2 to pin 2. Diode VD1 is closed, and capacitor C1 starts charging through diode VD2 and resistor R1. The charging time constant is small (0,5 ... 1 s), and the capacitor is quickly charged to the voltage of the on-board network. The breaker is ready to go. If we now move switch SA2 to position "1" - low speed of the brushes - its contacts 1, 4 and 2 will close, which means that conclusions 2 and 3 of the breaker will also close. The charging circuit of capacitor C1 is turned off; the positive plate of the charged capacitor C1 is connected through the resistor R3 to the emitter of the transistor VT1, and the negative plate through the resistor R2 to its base. Therefore, the capacitor C1 begins to discharge through the resistor R2, the emitter junction of the transistor VT1 and the resistor R3. There is no other discharge circuit, since the VD2 diode is closed. The transistor opens and opens the trinistor VS1, which is connected in parallel with the contacts SF1. As a result, the motor shaft M1 begins to rotate, the contacts SF1 close, closing the conclusions 3 and 4 of the breaker. This leads to the closing of the trinistor VS1, and the engine continues to work until the contacts SF1 open. At the same time, the capacitor C1 continues to discharge along the above circuit. The time constant of its discharge is chosen larger - 7 ... 9 s. When the wiper blades complete a full cycle of movement and the SF1 contacts open, the supply voltage will again be supplied to the trinistor anode. Since the discharge of the capacitor C1 is still ongoing, the open transistor VT1 will reopen the trinistor. Not having time to stop, the electric motor turns on again and the cycle repeats. Such a cyclically continuous switching on of the electric motor will continue until the capacitor C1 is completely discharged and the transistor VT1 remains closed at the next appearance of voltage at terminal 3 of the device. From this moment, the capacitor C2 of the pulse generator begins to charge. When a certain threshold voltage is reached on this capacitor, the transistor VT2 will open and a pulse will form on the resistor R5, opening the trinistor VS1. The electric motor is turned on again, and the cycle is repeated, but now with a frequency set by the charging circuit R6R7 of capacitor C2. At the minimum resistance of the resistor R6, there is practically no pause between cycles, at the maximum, the pause is approximately 15 s. If you turn the SA2 switch to the "0" position, the device will return to its original state - the capacitor C1 is quickly charged again to the supply voltage, no current flows through the remaining circuits. The breaker is ready for the next wiper operation. When the switch SA2 is set to position "2" (the wiper is switched on in the mode of fast movement of the brushes and the headlight washer motor) and to position "3" (the windshield washer motor is added), all processes in the device proceed similarly. All elements of the interrupter, except for the variable resistor R6, are placed on a printed circuit board made of foil fiberglass with a thickness of 1,5 mm. The drawing of the board is shown in fig. 2. Any 0,125W or 0,25W resistor can be used in the device. When choosing oxide capacitors C1 and C2, which are an integral part of timing circuits, it should be borne in mind that as the temperature decreases, their capacitance decreases, in some types - significantly. For this reason, the use of K50-6 capacitors should be deliberately abandoned.
Instead of KT3107G, any low-power pnp transistor with a collector pulse current of at least 100 mA and a static base current transfer ratio of at least 100 is suitable. With the values of resistors and capacitors indicated in the diagram, the number of continuous cycles at the moment the wiper is turned on is 4-5, and the duration of the pause can be adjusted within 0 ... 15 s. The handle of the variable resistor R6 is placed on the instrument panel near the mode switch handle. Literature
Author: A.Kuzema, Gatchina, Leningrad Region
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