Touch reversing switch. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Often in the manufacture of a particular circuit, embodied at least on a mock-up, when assessing the correspondence of its real work and description, at least three logical options arise:

1. The scheme did not work and was rejected for one reason or another as inoperable.

2. The scheme worked immediately, and the study was not carried out.

Z. The circuit did not work, but after a careful study of circuitry on this topic, constructive study, careful measurement of modes, a rational choice of radio elements and making the necessary adjustments, it worked.

The first option is hardly worth analyzing. The second option, although it gave a positive result, but may have "pitfalls".

Let us dwell on the third option, which is the most time-consuming initially, but, as practice shows [7, 11], which makes it possible to obtain reliable operation of the circuit in the future.

As an example, consider the development of a simple (Fig. 1) and good in its idea scheme [8].

The "carnation" circuit (immediately attracted attention with its simplicity, which suggested good repeatability), so three printed circuit boards were made, on which new radio elements were installed in accordance with the description. However, the "carnation" did not want to work stably. It either required a long primary (after two or three days of a de-energized state) retention of the sensor, or for unknown (at first glance) reasons, the VT1-VT4 transistors began to fail, and they are different on different boards.

Other types of transistors, trinistors were installed on the boards, the boards were even put aside for some time for "ripening", but this did not give a positive result.

Since periodically there was a need for a touch switch of this type, the idea arose to develop a cost-effective unified sensor circuit based on the "stud", which would work equally well in both battery-powered and AC-powered circuits, as well as with galvanic coupling. with or without a network. After a theoretical study of the "carnation" circuitry, it was noted that it has enough unrealized resources.

It was decided to use "popular" transistors of the KT315 type as active elements, and (for better repeatability) with any letter index and without preliminary selection. The sensitivity resource (Fig. 2) was increased by reducing the resistance of the resistor R1 to 1 MΩ and increasing the resistance of the resistor R1 to 2 MΩ (and in some cases by excluding it), since in [8] it together with R1 forms (see Fig. 1 ) a voltage divider coming from the sensor pad, which reduces the input voltage level by about 10 times. To compensate for this, in the circuit [8], amplifying elements (KT3102) with a high current gain are used, which is inappropriate.

(click to enlarge)

To minimize the effect of interference induced in the connecting wires (which was explained in [8] introduction of R2) in real designs, it is desirable to connect the sensor sensor with the circuit with conductors of a minimum length using a shielded wire.

Static modes

Since in [10] for KT315 the maximum voltage e-b, b-k is not specified, then in order to increase the reliability of operation in the sensor circuit, a decision was made instead of the diode VD1 (by the way, its type, as well as the type VD2 in [8] is not specified ) install a KS168 type zener diode connected in the same direction. It should already perform two functions: in the forward direction, for the negative half-wave of the signal, it should work like a conventional diode, protecting the VT1 e-b transition from the effects of reverse voltage through the control circuit, and for the positive half-wave, as a limiter (suppressor), normalizing the maximum value of the control voltage half-wave at the voltage level of its stabilization. The same drawback in the input stage is also present in the sensor circuit [5].

In the process of working out the circuit (see Fig. 1), it was noticed that after finding the circuit (3-4 days) in a de-energized state, for some reason it does not work even when the sensor is touched for a long time, but when the pins of the b-to VT1 on the breadboard are closed with fingers board (which indicates sufficient amplification of the active elements) is triggered. In the future, for one or two days, the circuit works normally, then, after the same period of being in a de-energized state, this phenomenon appears again and is eliminated in a similar way. There was an assumption that the cause of the phenomenon lies in the electrical formation of C2: as soon as C2 is sufficiently charged for the first time (and therefore formed), the circuit operates stably even after a short-term discharge (by closing the terminals) of C2.

For forced initial electrical forming of C3 (see Fig. 2) to a level of 0,4 V, when the supply voltage is turned on, a voltage divider R2R3 and a switching diode VD3 are introduced into the sensor circuit. When this voltage is reached, VD3 closes and in the future the divider does not affect the operation of the sensor. This solution to a certain extent compensates for the C3 leakage current inherent in high-capacity oxide capacitors, and also increases sensitivity, reducing the sensor touch time required for the circuit to operate.

As a result of measurements carried out using an S1-33 ​​oscilloscope with an open input (input resistance 1 MΩ), it turned out that with a sufficiently long holding of the sensor, the voltage across the capacitor C3 increases up to 6 ... b-k VT8 is out of order. Therefore, a resistor R2 is introduced into its base circuit in a similar way to circuitry, which has proven itself well in a trinistor regulator [4]. As a result of this, the time constant of the discharge circuit C4R3 (b-e) VT4 increased significantly, which made it possible to obtain a much longer exposure at a lower (compared to Fig. 2) value of the capacitance of the oxide capacitor C1.

To eliminate overloads in the base circuit VT3 and VT4 for the same reasons, limiting resistors R5, R7 are introduced.

Voltage measurements carried out on C3 showed that their introduction did not affect the on and off parameters of the sensor in any way.

The purpose of the capacitor C3 (see Fig. 1) is not indicated in the description [8]. Practical measurements on a working circuit showed that its presence lowers the turn-on threshold by about 0,1 V and increases the turn-off voltage by the same amount, which increases the total exposure by 10 ... 15 s.

Hence it was concluded that its use is inappropriate. During operation, when the trinistor is turned off and there are inductive loads in the network, a wide range of interference may occur.

Therefore, to reduce the internal resistance of the sensor power supply at high frequency, capacitor C2 was introduced into the circuit (see Fig. 4), which reduced the likelihood of high-frequency noise penetrating into the signal circuit through the power circuits. It is hardly worth using a high-voltage high-power transistor (1 W!) of the KT1 type as a key to control VS10 (see Fig. 940), supplying a current of about 1 mA to the VS55 control circuit in the open state! You can completely get by with the same (see Fig. 2) KT315 by connecting it to a stabilized constant voltage source, from which the rest of the transistors of the sensor circuit are powered. This, in addition to stabilizing the switching parameters of VS1, excludes possible overloads in the circuit of its control electrode, since the current in its circuit when VT4 is fully open is determined by the value of the quenching resistors R10, R11. Since, according to [10], the maximum current of the KT315 collector is 100 mA, this mode is quite safe for it.

In the process of measuring the current (not voltage) through the control electrode VS1 (see Fig. 2) using the Ts4342 avometer, it was noticed that at the moment of switching on there is a jerk of the meter arrow towards a larger value, and then the current is set at 4 ... 5 mA (depending on instances of VT4 and VS1). In the literature, I did not find information on the dependence of the current through the control electrode on the change in the nature of the load, therefore it was assumed that the reason for the phenomenon is the use of a non-linear load - HL1, the resistance of which in a cold state is much less than in a hot one.

The value of the resistor between the control electrode and the cathode (R5 - Fig. 1, R9 - Fig. 2, R7 - Fig. 3, R10 - Fig. 4, 5), recommended in the literature, to minimize the influence of destabilizing factors on the parameters of switching on the SCR in the circuit control electrode should not exceed 1 kOhm.

(click to enlarge)

It is not advisable to power the sensor directly from the mains (see Fig. 1), it is better to connect its power supply in parallel (a-c) to the trinistor, for example, as recommended by [6]. According to its current-voltage characteristic (Fig. 8), after VS1 has turned on, it can be switched to the closed state by reducing the current through it to a value less than Ioff. In direct current devices, either a switching capacitor or special series resonant circuits are used for this purpose, the overcharge voltage or back EMF of which, briefly applied to the trinistor in the opposite direction, turns it off. In circuits of alternating and pulsating current, the trinistor closes on its own when the value of its anode current passes through zero automatically.

In this scheme, a key amplitude control method is used, which is inferior to the pulse one in terms of energy consumption for control. Therefore, shunting the control circuit for the time the trinistor is in the open state, which takes place in our case, is optimal. In addition to reducing the average current consumption of the control circuit, such a connection, of course, will also reduce heat generation at R10, R11 (see Fig. 2). In this case, the VD5 diode is no longer used for rectification, but for separating the direct current supply to the sensor (smoothed C2) and the pulsating voltage source supplying VS1.

Dynamic modes

It is convenient (and safe!) to check the operation of the sensor circuit elements on a breadboard, using a 9 ... VD10. Since in this mode the circuit is a driver of the control voltage from the pickup voltage coming from the touch pad E2, an oscilloscope is used to observe the processes occurring in it.

The amplitude value of the pickup voltage at the sensor site is 15 V (of course, in the specific place where the measurements were taken). The voltage at the base of VT1 is 6 V (serves as an amplifier for the pickup signal power), at the emitter - 6 V, at the base of VT2 - about 6 V (serves as a voltage amplifier and signal limiter from above), at the collector - 0,8 V, with a clear limit above. On the VT3 collector, the signal has a level of 8 V, has already been formed (limited from below) and is ready to enter the output key (Fig. 3, 4) or the VS1 control key (Fig. 2, 5), the function of which in all circuits is performed by VT4, the signal voltage based on which is about 1,5 V. When C2 is connected (see Fig. 3) and the voltage across it is measured using an C1-33 oscilloscope with an open input (input resistance 1 MΩ), it turned out that the circuit turns on when voltage of about 0,8 V, and turns off at a voltage of 0,7 V. Additionally, it turned out that an attempt to connect to the same point with the same oscilloscope, but with a closed input, turned on the circuit, since the delay capacitance was the input capacitance of the oscilloscope.

To test the operation of the sensor on alternating current with galvanic isolation from the network, a transformer was used from an electric soldering kit 2.940.005 TU, manufactured by the Mayak Vinnitsa plant. The sensor circuit was connected to its lower connector, the value of the alternating voltage on which was about 24 V. All elements of the circuit in Fig. 2 were left unchanged, only resistors R10, R11 to obtain a current of 1 mA through the Zener diode VD20 were shunted by a resistor of the MLT-0,5 type resistance 470 Ohm. An incandescent lamp for a voltage of 28 V and a power of 20 W was used as a load.

While checking the operation of the circuit, the common wire from the needle probe of the oscilloscope broke inside the insulating shell, and the fact itself remained unnoticed ... The circuit stopped working. Touching the sensor either gave a flash, or the lamp shone, blinking half-heartedly, and with each touch everything happened differently. The type of inclusion was affected by the contact area, the pressing force, how the touch was carried out - sitting or standing, with the left or right hand, etc. Elements of the circuit are no longer out of order.

After checking the cascading passage of the pickup with an oscilloscope, I noticed that the signal was the same everywhere, and realized that there was no connection with the case. I soldered the common wire, and the circuit was fully restored!

I started looking for the reason for the strange behavior of the circuit. I disconnected the input probe C1-3Z from C2 - the circuit worked, disconnected the common wire of the oscilloscope - it stopped working, connected the common wire - it worked again. It became clear that there was interference with the mains frequency through the oscilloscope case, which, of course, is not grounded in the home workshop.

I checked the noise level on the oscilloscope case with a phase probe with a neon lamp - it glows a little, I checked it with a Chinese "miracle" probe with a digital indication - 60 V! I checked the amount of pickup on the case of the included power supply - the same figure! It became clear why when testing the sensor circuit on direct current powered by this source, the circuit worked normally.

I connected the circuit (see Fig. 2) in compliance with the phasing specified in [8]. The upgraded "carnation" worked fine.

In addition to the special K145AP2 microcircuit [9, 11], nowhere, and even more so in serious industrial equipment, for example, in the SVP-3 program selector [2], pickup was not used as a control signal. Whatever type of sensor is used - resistive, capacitive for disruption or excitation of generation - the level of the control signal (despite the difference in physical principles and circuitry) is always stable, which is not easy to obtain using a simple circuit from a pickup signal with a network frequency.

Based on the analysis, I decided not to complicate the circuit, but to use the available sensor resources - high gain and a stabilized supply voltage, using a resistive sensor that connects the input of the DC amplifier to VT4-VT5 with the positive pole of the source using the resistance of the skin of the finger and resistors R1, R4 nutrition. The scheme of variants of the unified sensor is shown in Fig. 4-5. The sensor works equally well from any (from the task set at the beginning of the article) power sources, it is quite safe when working from a 220 V network, since the human body is connected from both sides of the contacts through 1 MΩ resistances. For example, the value of the current-limiting resistor, which is part of the single-pole voltage indicator (with a neon lamp) type INN1, used in industry, is 910 kOhm.

As a result of the changes made, the circuit (see Fig. 4), which is in the "standby" mode, consumes only 9 mA from the 1 V power supply! In the on mode, after touching the sensor, the current consumption is 8 mA.

The only check that is desirable to carry out for the selection of the installed transistors VT1-VT4 is the "ringing" of the transitions with an ohmmeter at the limit of 100 kOhm. When checking the resistance of transitions in the opposite direction, the meter needle should not deviate even slightly.

Adjustment. In some cases, with high gains VT1-VT4 (and the absence of R2), when the sensor is connected to a power source, HL1 immediately lights up, although their repeated check with an ohmmeter, even at the limit of 1 MΩ, does not cause the meter to deviate, which indicates their serviceability. In this case, proceed as follows. In parallel with the e-b VT1 transition, an avometer is connected, turned on by a voltmeter at the limit of 5 ... 10 V. If VT1 is working, HL1 should go out. Switch the avometer to higher measurement limits until HL1 lights up again. After that, the avometer is switched to a lower limit, the lamp should go out. This technique allows you to use the avometer as a resistance store, since avometers (in the author's version of Ts4342) have an "open" input and an input resistance of about 20 ... 25 kOhm / V, which makes it possible to approximately estimate the required value of R2, which reduces the overall gain of the circuit, up obtaining clear work for specific transistors.

If necessary, instead of current-limiting resistors R10, R11 (see Fig. 2) of the MLT-2 type, on which a thermal power of about 4 W is released, you can install a reactive ballast - a capacitor of the K73-17 type with a capacity of 0,22 μFCh630 V. This will change somewhat rectifier circuit (Fig. 6).

(click to enlarge)

The KTs5V diode assembly is excluded from the circuit shown in Fig. 405. The VD5 zener diode in the circuit performs two functions: for the negative half-wave it serves as a rectifier diode, and for the positive half-wave it serves as a limiter at the stabilization voltage level. Resistor R11 serves to limit the inrush current when charging C5. Trinistor VS1 operates as a half-wave rectifier, which favorably affects the service life of HL1.
Figure 7 shows the printed circuit board of the unified sensor.

The board is designed to accommodate circuit parts from Fig. 2 to Fig. 6. Depending on the desired option, install the appropriate components. Places for parts not used in this circuit are either closed with wire jumpers or left free. The same applies to the interconnection of pads for setting jumpers JP0, JP1, JP2 with the circuit.

References:

1. Abdeeva N., Grishina A. Thyristors//Radio. - 1972. - No. 1. - P.54.
2. Bulych V.I. etc. Repair and adjustment of unified color television sets. M.: DOSAAF, 1979 - 23 p.
3. Butov A. Touch power controller//Radio. - 2002. - No. 1. - P.32.
4. Elkin S.A. Power regulator for a soldering iron - automatic light illumination//Electrician. - 2000. - No. 11. - p.7.
5. Kashkarov A. Magic flower// Radio amateur. - 2000. - No. 1. - p.9.
6. Krylov V. Methods and control devices for thyristors // To help the radio amateur. - M.: DOSAAF. - 1973. - Issue 43.
7. Kulsky A.L. On simple cases and elementary schemes//Radioamator. 2002. - No. 11. - P.39.
8. Lovchuk V.B. Instead of a carnation switch//Electrician. - 2002. - No. 6. - P.13.
9. Malyshev S.Yu. Touch dimmer//Radioamator. - 2002. - No. 1. P.27.
10. Tereshchuk R.M. and others. Semiconductor receiving-amplifying devices: Handbook of a radio amateur. - K .: Naukova Dumka, 1981.
11. Titarenko Yu.M. Touch switch//Radioamator. - 1997. - No. 12. P.35.

Author: S.A. Elkin

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