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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING SHI-current stabilizer
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer The device discussed in this article provides a stable current to the load (average value). Its output current is pulses with constant amplitude and variable duty cycle. Such devices, according to the authors, can be used, for example, for charging batteries and in electrochemistry. Currently, pulse stabilizers, due to their high efficiency and optimal weight and size indicators, are replacing linear control devices. One of the effective ways to regulate voltage and power at the load is pulse-width (PW) control, when the pulse frequency remains unchanged, but their duty cycle varies. This is how the output voltage is regulated in most switching power supplies, including the most modern television receivers and other equipment. Nevertheless, there are devices where it is necessary to stabilize not the voltage, but the current in the load - filaments (heater) in the kinescope and lighting fixtures, when controlling the processes of galvanization and electrolysis, and for charging car batteries. The described SHI-current stabilizer can be used in solving the above problems.
Main technical parameters
The principle of operation of such a stabilizer, the functional diagram of which is shown in Fig. 1 is extremely simple. The DC generator G1 is connected to the load Rl through the measuring element E1 and the switch S1. The commutator is controlled by the pulse width shaper E2. The turn-on signal of the shaper (and, consequently, the switch) is generated by the pulse generator G2. When the required value of the output current is reached, the signal from the measuring element E1 through the amplifier A1 acts on the shaper E2, which turns off the switch. The generator G2 controls the frequency of the pulses, and the shaper E2 controls their duty cycle. Thus, by changing the duty cycle of the switching pulses, it is possible to adjust the average value of the output current in the load circuit. As can be seen from fig. 1, SHI current stabilizer consists of only five elements. But the need for some service functions (protection against short circuits in the load circuit, indication of operating and emergency modes) somewhat complicates the device (Fig. 2).
The impulse noise of the input voltage is smoothed by the filter capacitor C1. Since the input voltage exceeds the allowable power supply for the DD1 microcircuit, the resistor R22 and the zener diode VD1 form the required voltage, which is additionally filtered by capacitors C2 and C3. The unijunction transistor generator VT1 generates exponential pulses with a repetition rate of about 200 Hz (Fig. 3, diagram 1). The pulse frequency can be adjusted by selecting the resistor R1, capacitor C4, as well as changing the resistance of the resistor R2. Transistors VT2, VT3 form steeper rises and falls of these pulses and bring their amplitude to the supply voltage of the microcircuit (Fig. 3, diagram 2) to control the trigger (inputs S1 and R1 of the DD1 microcircuit). Since, when the supply voltage is turned on, the pulse delayed for a short time by the C5L1 circuit, applied to the inputs S1, S3, S4 of the flip-flops, sets a high level at their outputs 1, 3, 4, the transistor VT7 is closed, and the open transistor VT8 through the resistor R20 connects to the minus of the secondary power supply base transistor VT9. The current from the power supply begins to pass through the circuit: measuring resistor R11, transistor VT9, load. After charging the capacitor C4, the first pulse from the generator at the input S1 will not change the state of the trigger (S1-R1), a high level remains at the output 1 of the microcircuit. The load current creates a voltage drop across the measuring resistor R11, which is applied through the resistors R12, R13 to the emitter junction of the transistor VT6 shunted by the capacitor C5. The voltage shape at its base is shown in Fig. 3, diagram 3. At the initial moment, the capacitor is discharged, and the transistor VT5 is closed. Some time after the start of charging, the voltage at the emitter junction of the transistor VT5 reaches the level of its opening. Capacitor C6 is discharged. On the resistor R9, and consequently, at the input R1 of the DD1 microcircuit, a voltage pulse is formed (Fig. 3, diagram 4). At output 1, a low level is set, transistor VT7 opens and closes the emitter junction of transistor VT9. The current through the load stops. With the arrival of the next pulse from the generator on the transistor VT1, the process is repeated. The trimming resistor R13 changes the opening moment of the transistor VT5 and, therefore, regulates the average value of the load current, the pulse shape of which is shown in Fig. 3, diagram 5. Since the selected amplitude value of the output current is 6 A, for a pulsed current with a duty cycle of 2, its average value should be adjusted to 3 A. Current stabilization is carried out as follows. As the load resistance decreases, the output current increases. This will cause an increase in the voltage drop across the measuring resistor R11, which will lead to an earlier opening of the transistor VT5 and a decrease in the duration of the output current pulses. As a result, the average value of the load current will remain constant, equal to 3 A. Similarly, stabilization occurs with an increase in the output current caused by an increase in the supply voltage at the input of the device. With a decrease in the amplitude value of the load current, due either to a decrease in the supply voltage or an increase in the load resistance, the duty cycle of the current pulses decreases, and its average value remains the same. The function of protecting the stabilizer from short circuits in the load is performed by the node on the transistor VT4. In the case of an increase in the output current to 20 A, the voltage drop across the resistor R11 becomes sufficient to turn on the zener diode VD2. The opened transistor VT4 forms a voltage pulse on the resistor R14, applied to the inputs R3, R4 of the DD1 microcircuit. Capacitor C7, shunt resistor R14, attenuates impulse noise in the protection circuit. A low level appears at the output of 3 microcircuits. The previously open transistor VT8 closes, excluding the passage of the base current of the transistor VT9. Subsequent pulses at the input S1 of the microcircuit fix a high level at its output 1 and the closed state of the transistor VT7, so the transistor VT9 remains closed. The current in the load stops and becomes possible only after the stabilizer is turned off and on again. Since the inputs of the S3, S4 and R3, R4 microcircuit are combined in pairs, at its outputs 3 and 4, the single and zero signals appear synchronously. The open state of the transistor VT8 corresponds to a high level at output 4; HL1 LED is off. When the protection is triggered, current flows through the circuit HL1, R18 and the LED signals an emergency mode. The transistor VT6 is used to indicate the operating mode: the current passes through its collector circuit - a series-connected current-limiting resistor R21 and an LED HL2, the glow of which indicates the flow of load current. The current stabilizer uses MLT fixed resistors; tuning resistors R2 and R13 - SP3-38b. Resistor R11 can be either homemade wire or factory-made with a power of at least 4 watts. Capacitor C2 - K50-35, the rest - ceramic K10-17-1b, they can be replaced with KM, KLS, etc. Inductor L1 - high-frequency - DM-0,2 with inductance from 60 to 200 μH. Zener diode VD1 - any with a stabilization voltage of 12 ... 14 V. It is advisable to choose the HL1 LED with a red glow color: AL307A, AL307AM, AL307B, AL307BM or the AL102 series, and the HL2 LED - green or yellow: AL307V-AL307E. Instead of the K561TP2 chip, you can install the K564TP2 if you pre-form its conclusions with tweezers. Unijunction transistor - KT117 with any letter index; in extreme cases, it can be replaced by a well-known analogue of two low-power silicon transistors of various structures. Transistors KT208A and KT312V are interchangeable with devices of the KT361, KT3107 and KT315, KT3102 series, respectively, with any letter index. By gain, a selection of transistors is not required. A powerful composite transistor KT825 can also be with any index, but if there are several of them, it is advisable, after measurements, to select the collector-emitter with the lowest saturation voltage at a collector current of 3 ... 6 A. All elements, with the exception of the KT825 transistor, are mounted on a printed circuit board made of one-sided foil fiberglass with a thickness of 1 ... 1,5 mm and dimensions of 80x45 mm. The KT825 transistor is mounted on a heat sink with a cooling surface of about 200 cm2. To set up the device, you will need a powerful laboratory power source with a permissible current of at least 10 A, for example, B5-21. Let us assume that at a current in the load I = 6 A, the voltage across it reaches 15 V or more, depending on the temperature of the ambient air (solution) and the concentration of the solution. From Ohm's law, it is easy to calculate the resistance of the equivalent load R \u2,5d U / I \u90d 25 Ohm. Resistor power P = I ( U = 10 W. This condition is satisfied by four parallel-connected PEVT-2 resistors with a resistance of 100 Ohms. To avoid damage to the elements of the device by high current, the adjustment should be carried out in two stages. At the first, a load equivalent is connected - the MLT-150 resistor resistance of 11 ohms, the load current in this case will be about 1 mA.To create a voltage drop of about 6,8 V on the measuring resistor R0,25, its resistance should be chosen equal to XNUMX ohms, power - XNUMX watts. After connecting the calculated elements (R11=6,8 Ohm, Rn=100 Ohm), the first stage of adjustment begins. Turn on the power and measure the voltage at the zener diode VD1, which should be 12 ... 14 V. Using an oscilloscope, control the pulses based on the transistor VT2, if necessary, adjust the period of their repetition T = 2 ms with the resistor R5. In the absence of amplified pulses on the collectors of transistors VT2 and VT3, you will have to select a resistor R5. Then the pulses on the collector of the transistor VT5 are controlled and the regulation interval is determined by the resistor R13. The oscilloscope checks the presence and shape of the current pulses on the dummy load: resistor R13 sets the shape of the "meander" pulses, while the HL2 "Work" LED should light up. Changing the voltage from the power supply should lead to a change in the duty cycle of the pulses accordingly. For a short time, a load equivalent is shunted with a 18 Ohm resistor (such a load creates a current in the output circuit of 0,6 A and a corresponding voltage drop across the measuring resistor of 4 V, which is equal to the voltage drop across the resistor R11 with a resistance of 0,2 Ohm at a current of 20 A). The pulses on the load should disappear, and the HL1 "Emergency" LED will turn on. After turning off the power supply and turning it back on, normal operation of the device should be restored. If the short circuit protection does not work, it is necessary to select the VD2 zener diode and the R10 resistor. This completes the first stage of development. At the second stage, a resistor R11 is installed with the one indicated in fig. 2 resistance and connect the equivalent load with a resistance of 2,5 ohms. Resistor R20 is temporarily switched from the collector of the transistor VT8 to its emitter. After turning on the power supply, the voltage drop across the resistor R11, the load, the emitter-collector section of the transistor VT9 is measured. It should be 1,2, 15 and 1,5 ... 2,5 V, respectively. By changing the voltage at the output of the power supply at the moment the transistor VT9 enters saturation mode, the minimum required supply voltage of the device is determined. The power supply (to increase efficiency, it is desirable to use a pulsed one), with which the SHI stabilizer is supposed to be operated, should be adjusted to this voltage, and then connected instead of the laboratory one: the voltage drop on the listed elements should remain the same. Its discrepancy indicates insufficient power of the switching power supply. If the power of the block is sufficient, the connection of the resistor R20 is restored, instead of the equivalent load, a real load and a 5 A ammeter are connected. The load current is set to 13 A with the resistor R3, after which the ammeter can be turned off. The device is ready for use. Authors: V.Zhukov, V.Kosenko, S.Kosenko, Voronezh
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