Adjustable voltage and current stabilizer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Adjustable voltage and current stabilizer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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In amateur radio practice during experimental work, it is often necessary to have a universal power supply on hand. If we sum up the requirements for a power source in the development and adjustment of analog and digital devices, then, in addition to high requirements for the quality of the output voltage and a wide range of its regulation, it is very important that it combines the functions of high-quality current and voltage sources. We offer one of the options for such a device to the attention of our readers.

The proposed power supply allows you to use it as a voltage source and as a direct current source. The undoubted advantages of this block, in addition to versatility, include the presence of controlled protection against short circuits in the load "by default".

The power supply, the circuit of which is shown in the figure, can satisfy most of the requests of experimental radio amateurs. For more than three years (and during this time the power supply has never failed), the author has been exploiting it, using it in experiments and establishing analog and digital devices and ending with charging car batteries.

(click to enlarge)

Functionally, the power supply unit consists of two mutually independent current and voltage stabilization units operating on a common output signal control element.

Consider the purpose of the elements of the proposed device. A rectifier is assembled on diodes VD1-VD4, and a smoothing filter for the supply voltage is assembled on capacitors C1-C3. Transistors VT1-VT4 are a powerful regulating element that controls the output voltage and current. The use of several transistors connected in parallel, in addition to sharing the load current between them, makes sense for a number of reasons. Firstly, such a solution allows spreading the heating points along the heat sink, which increases its efficiency, making it possible to reduce its size. Secondly, it is possible to use cheap transistors with a maximum allowable collector current less than the maximum load current without compromising the operational reliability of the device. Resistors R4-R7 are matching elements for the emitter circuits of transistors connected in parallel, allowing you to evenly divide the total load current between transistors with a large spread of electrical parameters. Transistor VT5 matches the input resistance of the regulating element and the output transistors VT6 and VT7.

On diodes VD5 and VD6, a zener diode VD7, an integrated stabilizer DA1 and capacitors C4-C7, a bipolar voltage regulator is assembled to power the control unit. Microcircuits DA2 and DA3 act as reference voltage sources for output voltage and current control units, respectively. The choice of integrated voltage regulators of the KR142 series for this purpose is explained by the parameters of these microcircuits, which are quite sufficient for laboratory purposes, such as a voltage temperature coefficient of less than 0,02% / ° C and a ripple smoothing coefficient of more than 30 dB. And the use of sequential stabilization further improves the parameters of exemplary voltage sources. In addition, the simplicity of circuit implementation and the availability of the element base are of great importance.

The follower on the op amp DA4.1 compensates for the voltage drop on the output current sensor R17R18 and eliminates the error in setting the output current associated with the possible flow through these resistors of the total current of the voltmeter PV1, the resistive divider of the output voltage R14R15, the output divider of the reference voltage source R11R12 and the current consumed stabilizer DA2. In addition, the use of a very powerful DA4.1 op amp provides ample opportunities in choosing a reference voltage source circuit. However, the error in setting the output current in this case is insignificant and is less than 20 mA. If such an error is not fundamental, it can be neglected by excluding the DA4.1 op-amp and connecting the conductors going to its inputs. The use of this op-amp may become necessary in case of recalculating the source for other output voltages and currents (and, consequently, recalculating the resistance of resistors R17 and R18), when the error voltage on the current sensor becomes noticeable.

On the op-amp DA4.2 and DA5.1, the nodes for controlling the output voltage and current, respectively, are assembled. Such nodes are well represented and discussed in amateur radio literature and are implemented as standard. Control signals from them are fed to transistors VT6 and VT7, connected in cascade. We will consider the principle of their operation using the example of a current stabilizer. As long as the output current of the power supply is less than that set by the variable resistor R12 (compared to the voltage on the current sensor R17R18), the unit is in voltage stabilization mode, since the VT7 transistor is fully open and does not affect operation. When you try to exceed the set current level, the output voltage decreases, since the op-amp DA5.1 goes into control mode, reducing the base current of the transistor VT7. In this case, the op amp DA4.2 switches from the active mode to the comparator mode, opening the transistor VT6 and thereby disconnecting it from the control circuit.

On the op amp DA5.2 and the LEDs HL1 and HL2, a node for indicating the operating mode of the power supply is assembled. Depending on the voltage level at the outputs of the op-amp DA4.2 and DA5.1, the DA5.2 comparator switches the output voltage, including the corresponding LED. And since the included power supply is always in some mode of operation, as evidenced by the glow of one of the LEDs, there is no need for an on indicator.

The details of the described power supply were calculated and selected for the transformer available to the author. With the element base indicated on the diagram, the unit provides adjustment of the output voltage from 0 to 18 V and the load current from 0 to 14 A. With an output voltage of 15 V and a current of 12 A, the double ripple amplitude does not exceed 5 mV. The elements of the source can be easily recalculated according to your own capabilities or desires.

All parts of the block, with the exception of the mains transformer T1, rectifier diodes VD1-VD4, transistors of the regulating element VT1 - VT4 and VT5, LEDs for indicating stabilization modes HL1 and HL2, variable resistors R10 and R12, current-leveling resistors R4-R7 and filter capacitors C1-C3 , mounted on a printed circuit board 100x80 mm in size, made of double-sided foil fiberglass 2 mm thick. As a heat sink for VT1-VT5 transistors and VD1-VD4 diodes, the original power supply used a device casing made of 1,8 mm thick aluminum sheet. The casing has a U-shape with a top cover. Its dimensions are 190x170x350 mm. Transistors and diodes are fixed on its rear wall through insulating mica spacers 0,05 mm thick, previously lubricated with KPT-8 heat-conducting paste. Current-leveling resistors R4-R7 are installed next to the transistors by surface mounting on mounting sites isolated from the device case. On the front panel there is a power switch SA1, fuses FU1 and FU2, an ammeter PA1 and a voltmeter PV1, LEDs HL1 and HL2 are installed above them, respectively. Under the measuring instruments, regulators of output current and voltage stabilizers are installed - variable resistors R12 and R10. The mains transformer T1 and filter capacitors C1-C3 are installed on the power supply chassis.

Network transformer T1 - factory-made, having a serial number 4.540.176. The magnetic core of the transformer is assembled from W-shaped plates PB 40-80. The primary winding is wound with PEV-2 1,25 wire and contains 296 turns. The secondary winding II is made of copper bus PSD 1,8x5 and consists of two identical windings of 14 turns connected in series. Winding III contains 17 turns of PEV-2 1,0 wire. A homemade transformer is calculated for the maximum power consumed by the load, plus four watts for the control node. It should be noted that in idle mode, the output voltage of winding III must be in the range from 12,6 to 14 V and provide the above power (4 W) under load.

The maximum allowable forward current of the rectifier diodes VD1-VD4 must exceed the maximum load current. With a decrease in current less than 10 A, it is possible to use diodes of the KD213, KD243 series with any letter index. Oxide filter capacitors C1-C3 - K50-18, but other more modern ones are also acceptable. The high capacitance of these capacitors is due to the exceptionally high load current capability. Their capacitance can be changed in proportion to this current.

Transistors of the regulating element KT819AM are interchangeable with KT808 or similar ones with a permissible collector current of 10 A and sufficient power dissipation. The transistor KT818AM (VT5) can be replaced by any of the KT816 series, and KT817V (VT6, VT7) - by any of the KT815, KT807 series. Together with diodes KD212A (VD5, VD6), it is permissible to use KD226 with any letter index or similar. Capacitors C4-C7, C10 - K50-35, C8, C9 -K50-16, C11-C15 - any suitable capacity for a rated voltage of at least 25 V.

The choice of K157UD2 (DA4 DA5) microcircuits is due to their large allowable output current, which is especially important for the DA4.1 op-amp, since the current of the DA2 stabilizer and the R14R15 resistive divider flows through it. If the number of microcircuits is not limited, instead of these microcircuits, K553UD2 with the corresponding correction circuits will do. It is important that, in addition to the permissible output current of at least 20 mA, microcircuits have frequency correction circuits. This is due to the fact that due to the large phase shift in the CNF circuit, it is necessary to reduce the cutoff frequency to increase the stability margin.

Current-leveling resistors R4-R7 and current sensor R17, R18 - wire C5-16M, variables R10 and R12 - SP-1 or any other convenient for installation on the front panel of the power supply. Measuring instruments PV1 and RA1 - any with a total deviation current from 0,05 to 1 mA and a convenient scale. In the author's version, M4248.3 measuring heads with a total deviation current of 0,1 mA are used.

Establishing a device assembled from known good parts comes down mainly to checking the correct installation. After that, the engines of variable resistors R10 and R12 are set to the lower position according to the diagram and the device is checked for the absence of self-excitation at the outputs of the op-amp DA4.2 and DA5.1. Eliminate it in case of occurrence by selecting capacitors C12 and C13 in the direction of increasing their capacitance. Further, using an exemplary voltmeter and ammeter, resistors R9 and R11 set the upper limits of voltage and current regulation, and resistors R13 and R16 calibrate the voltmeter PV1 and ammeter RA1. It is also necessary to make sure that there is no generation on the load in various permissible operating modes.

The device withstands short circuits in the load, but you should not abuse this at limiting currents close to the maximum. It should be noted that the power released on the transistors of the regulating element is directly proportional to the difference between the voltage at the output of the diode bridge VD1-VD4 and the voltage at the output of the power supply (voltage drop across the regulating element) and the load current. If the output voltage is low and the current is close to the maximum, about 300 watts of power is released on the heat sink. To protect against overheating (when the dimensions of the case are insufficient for good cooling), an additional unit should be provided that disconnects the power supply from the mains. This can be either a simple electronic or an electromechanical (thermal relay based on a bimetallic plate) device.

Author: G. Fedusov, Nizhny Novgorod

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