Fan switch. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices

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It is known that many of its characteristics significantly depend on the temperature regime of the car engine. Both an underheated engine and an overheated one are a source of additional problems.

Now drivers who have to drive through the streets of big cities are increasingly finding themselves in situations where for a long time they can only move at walking speed, or even stand more. In summer, in such traffic jams, the car engine usually overheats quickly and needs to be stopped to cool down.

The author of this article talks about how to make life easier for yourself and the car in such cases.

It's a sad joke: a motorist who happened to drive a domestic car has no shortage of difficulties. Indeed, he always has a wide range of them at hand - from starting a cold engine in frost to, paradoxically, starting a hot engine in hot weather. I propose to discuss some features of the operation of an overheated engine.

Most modern cars are equipped with an electric fan equipped with the simplest electromechanical automation (see the diagram in Fig. 1). The assembly is connected to terminal 15/1 of the ignition switch. Note that the designation of the electrical system clamps corresponds to the international one, which is also accepted by all leading domestic car manufacturers.

The sensor for turning on the fan motor M1 is the SF1 thermal switch, which is usually installed on the radiator. If the temperature of the engine of the car increases, but has not yet reached the upper threshold value (99 & No. 176; C for VAZ cars and 92 C - AZLK), the SF1 contacts will be open, and the electric motor will be de-energized.

As soon as the engine warms up to the upper temperature threshold, the contacts of the SF1 sensor will close, the relay K1 will work and the contacts K1.1 will turn on the fan motor M1. Intensive cooling of antifreeze in the cooling system will begin.

At the moment when the engine temperature drops below the lower temperature threshold (94 & No. 176; C for VAZ cars and 87 & No. 176; C - AZLK), the SF1 contacts will open and the fan will again be de-energized. Thus, the temperature operating mode of the engine is set. The automatic cooling system described works quite satisfactorily while driving and even when parked if the weather is moderately hot. However, as soon as you get stuck in traffic on a hot summer day, you will soon have to make sure that the car's fan is running without turning off, and the engine temperature rises menacingly.

Under such conditions, attempts to turn off the engine at least for a short time in order to cool it down will most likely not only not lead to the desired result, but even vice versa. After all, when the ignition is turned off, the fan will also be completely de-energized, and the hot engine will create a real “sauna” under the hood, the carburetor and fuel pump will quickly overheat, and this can lead to the fact that you may not be able to start the engine again.

How can that be?

To a certain extent, the situation can be alleviated by using an automatic electronic fan switch. It is connected to the existing automation node as shown in the diagram in Fig. 2.

The automation unit, regardless of whether an electronic switch is connected to it, it is advisable to modify it by introducing two protective diodes into it - VD1 and VD2. These diodes will significantly reduce the electrical erosion of contacts K1.1 of relay K1 and thermal contact sensor SF1, respectively.

The fan switch (see diagram in Figure 3) only starts to operate when the motor is overheated. Under nominal temperature conditions, the operation of the fan is controlled by the automation unit described above, which is powered from terminal 15/1 of the ignition switch. The 12V voltage at this terminal is present only in two (out of four) positions of the ignition key - "Ignition" and "Start".

(click to enlarge)

The commutator is powered by terminal 30, i.e. actually from the positive terminal of the battery. Capacitors C1, C2 and diode VD4 smooth out the supply voltage ripple. Diode VD4 together with diode VD1 also protect the low-current part of the device from erroneous supply voltage in reverse polarity.

The voltage from the ignition switch - from its terminal 15/1 - is supplied to the shaper, assembled on the element DD1.1, resistors R1, R2, capacitor C3 and zener diode VD2. This shaper suppresses both high-frequency voltage ripples and high-voltage impulse noise.

In addition, the switch has three time interval generators. The first of them, consisting of capacitor C4, resistor R4 and element DD1.2, forms a single low-level pulse with a duration of about 100 ms. The second - on the element DD1.3 and the differentiating circuit C5R8 - generates an interval of approximately 1 ms. Finally, the third time interval of 60 s is formed by the elements DD2.3, DD2.4 and the differentiating circuit C6R9.

When the ignition is on, a high level voltage is applied to the inputs of the DD1.1 element, which means that the output of this element is low. Therefore, the capacitors C4-C6 are discharged and a low level operates at the inputs of the elements DD1.2, DD1.3 and the lower inputs of the elements DD2.3, DD2.4 according to the circuit.

A high level from the output of the element DD1.2 keeps the transistor VT1 closed. The RS flip-flop assembled on the elements DD2.1, DD2.2 can be in any state, its inputs are high. At the output of the elements DD2.3, DD2.4, connected in parallel, there will be a high level, so the transistor VT2 is closed, the relay K1 of the switch is de-energized, the contacts K1.1 are open (they are not shown in Fig. 3).

After the ignition is turned off, a low level appears at the input of element DD1.1, and a high level appears at the output. The output current flowing through the relatively low-resistance resistor R3 begins to charge the capacitors C4-Cb. The transistor VT1 opens, and a current begins to flow through the diode VD3 and the thermistor circuit, determined by the resistance of the resistor R6 and the thermistor.

It is necessary to consider two cases: the first - the engine is cold, the resistance of the thermistor circuit is high, the second - the engine is hot, the resistance is low.

With a cold engine with the ignition off, a low level will appear at the output of element DD1.3 for 1 ms. Since the resistance of the thermistor is large, the voltage level across the resistor R7 element DD1.4 determines as high. Thus, there will be a low level at the lower trigger input according to the circuit. Therefore, a unit voltage will be established at the output of both elements.

At the lower input of the elements DD2.3, DD2.4 for 1 min (while the capacitor C6 is charging), a high level also acts. This means that the output of these elements will be low and the transistor VT2 will open.

But after 1 ms, the low level at the output of the DD1.3 element will change to a high one. This will set the trigger on the lower input to state 0 and close the transistor VT2. During the time of 1 ms, the relay will not have time to operate, since its speed is in the range of 7 ... 10 ms.

After about 100 ms, the capacitor C4 will charge, the transistor VT1 will close and the low level will be set again at the input of the element DD1.4 - the trigger state will not change. A minute later, the capacitor C6 will be charged and at the lower input of the elements DD2.3, DD2.4 the high level will change to a low one. The switch will go into a stationary state, in which it can stay indefinitely.

If the ignition is turned off with a hot engine, then at the output of element DD1.3, as in the first case, a low level will appear, and at the output of element DD1.4 - high, since the resistance of the thermistor has decreased and the voltage across resistor R7 element DD1.4 determines now as a low level.

As a result, the trigger will immediately switch to state 1 via the upper input. After 1 ms, a high level will appear at the upper input of the trigger, which does not change the state of the trigger. Another 100 ms will pass - transistor VT1 closes. In this case, the voltage across the resistor R7 will decrease to almost zero (low level), and the flip-flop remains in a single state. Therefore, within 1 min, the transistor VT2 will be open, and the relay K1 will be turned on. This means that the fan is working, cooling the liquid in the car's radiator and providing air exchange in the engine compartment.

At the end of the minute exposure, the fan will turn off and the switch will again go into a stationary state. This mode of operation allows, if necessary, to give the car engine a certain margin of thermal stability. After turning on the ignition and starting the engine, the existing automation unit with the contact temperature sensor SF1 starts to control the fan again.

The duration of the time period during which the fan is turned on after the switch is activated can be changed by selecting resistor R9. The greater the resistance of this resistor, the longer the fan will run. The required duration should be determined experimentally. Excessively long exposure leads to a useless loss of heat, electricity, fuel, and the resource of the fan motor. However, if a "hot" start of the car engine gives you too much trouble, consider these costs justified.

Approximately the same can be said about the temperature threshold of the switch. The value of this threshold is best determined empirically, based on the specific conditions and features of your car engine. So, if a hot engine does not start well, the threshold should be chosen quite low - about 80 ° C, and sometimes even 60 ° C. The threshold is set by a selection of resistor R6; a higher threshold corresponds to a lower resistance.

We note here that one should not be guided by the car's thermometer because of its too large error. It is better to use a homemade thermometer, described in [1].

The switch can use microcircuits of the K561, K564, K1561 series (K176 is better not to use, since they require a more stable supply voltage). Elements DD1.3, DD1.4, DD2.1, DD2.2 can be replaced with one trigger (two in one case) K561TM2 or 564TM2, K1561TM2.

We will replace the KT502E (VT1) transistor with KT814G or KT816G, and the KT814G (VT2) transistor with KT816G.

Diodes VD1 and VD4 can be almost any silicon small-sized, and VD3 and VD5 - any of the series KD102, KD103, KD105, KD106, KD208, KD209. The VD2 zener diode is suitable for any low-power stabilization voltage from 8 to 15 V (in extreme cases, you can do without it). Oxide capacitors - from the K52, K53, IT series; the rest are ceramic. Relay K2 - 111.3747, 112.3747, 113.3747, 113.3747-10 or any other suitable one, for example, described in [2].

Literature

1. Bannikov V. Engine temperature gauge. - Radio, 1996, No. 7, p. 47.
2. Bannikov V. Small-sized automobile electromagnetic relays. - Radio, 1994, No. 9, p.42; No. 10, p. 41.

Author: V.Bannikov, Moscow

See other articles Section Automobile. Electronic devices.

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