Adjustable voltage regulator with current limit. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Adjustable voltage regulator with current limit. 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 the article brought to the attention of readers, an adjustable switching voltage regulator with current limitation is described. The device allows not only to power various equipment with a stable voltage from 2 to 25 V, but also to charge various batteries with a stable current of up to 5 A.

The described power supply allows you to adjust the stabilized output voltage and the maximum current in the load. It can be used both for powering and adjusting radio equipment, and for charging various batteries. The device operates in two modes: in the case of equipment power supply - as a voltage stabilizer with overload protection, and when charging batteries - as a current stabilizer with voltage limitation. The power supply is easy to use, is not afraid of overloads and output short circuits, has a light indication of the operating mode and high efficiency. The scheme of the device is shown in fig. 1.

Main Specifications

Output voltage, V......2...25
Load current, A......0...5

(click to enlarge)

Parameters such as instability, ripple and efficiency are largely determined by the mode of operation and therefore are not shown. If desired, the characteristics can be changed without significant changes to the device. For example, if you need to get a larger output current, you should put a current sensor - resistor R14 of greater power, and also increase the resistance of the variable resistor R5. To reduce ripples, it is advisable to install an LC filter at the output, but this will lead to a decrease in efficiency.

The power supply contains the following components: internal "negative" voltage regulator VT1VD1R1 with C4 filter; internal "positive" voltage stabilizer VT2VD2R2 with C5 filter; current limiting unit DA1.1R3-R7R10R 14; voltage limiting unit DA1.2VD3R15-R18; pulse shaper DD1.2DD1.3; status indicators DD1.1HL1R12 and DD1.4HL2R13; switching transistor VT3; capacitors of the input C1-C3, intermediate C7, C8 and output C6 filters.

Consider the operation of the device in the voltage stabilization mode. When turned on, a voltage appears on the zener diode VD3, part of which is supplied from the variable resistor R16 engine (which regulates the output voltage) to the inverting input of the op-amp DA1.2. Since the switching transistor VT3 is closed, the capacitors C6-C8 are discharged and the voltage at the non-inverting input of the op-amp DA1.2, taken from the engine of the tuned resistor R18, is close to + UBX. A high level appears at the output of the op-amp, which leads to the switching on of the emitting diode of the optocoupler U1.4. As a result, the phototransistor of the optocoupler U1.2 will open and a high level will appear at the lower input of the DD1.2 element according to the circuit. Therefore, the output of the element DD1.3 is also a high level, which will open the switching transistor VT3.

Through the inductor L1, the load current and the charging of capacitors C6-C8 begin to flow. The voltage on the capacitors and on the trimmer resistor R18 begins to increase. At some point, the voltage at the non-inverting input of the op-amp DA1.2 will become less than at the inverting one. The output of the op-amp DA1.2 will appear low. The emitting diode U1.4 and the phototransistor U 1.2 of the optocoupler will close. At the lower circuit of the element DD1.2 and at the inputs of the element DD1.4, the high level will change to a low one. The switching transistor will close, and the HL2 LED that turns on will signal that the device is operating in voltage stabilization mode. As it discharges to the load, the voltage across the capacitors C6-C8 and, accordingly, on the trimmer resistor R18 will decrease. And as soon as the voltage at the non-inverting input becomes greater than at the inverting one, the process will repeat.

The voltage from the current sensor - resistor R14 is fed to the inputs of the op-amp DA1.1. As soon as the load current exceeds the set value, the voltage at the non-inverting input of the op-amp DA1.1 will be less than at the inverting one. A low level will appear at its output, and the enabled emitting diode of optocoupler U1.3 will turn off. The phototransistor of the optocoupler U1.1 will close. At the top input of the DD1.2 element according to the scheme and at the inputs of the DD1.1 element, the high level will change to a low level. As a result, the switching transistor will close, and the HL1 LED that turns on will signal the operation of the power supply in current stabilization mode. As the capacitors C7, C8 are discharged, the current through the resistor R14 will decrease, which will lead to an increase in the voltage at the non-inverting input of the op-amp DA1.1 and then to the opening of the transistor VT3. When the load current increases again, the process will be repeated. The stabilization current is set by a variable resistor R5.

Most of the parts of the power supply are mounted on a board made of one-sided foil-coated fiberglass, the drawing of which is shown in fig. 2. The switching transistor VT3 and the diode VD4 are placed on a heat sink measuring 60x90x7 mm.

(click to enlarge)

The device can be powered from a network transformer with an effective voltage on the secondary winding of 20 ... 25 V, which will provide the necessary load current. In the author's version, diode assemblies KD227GS are used in the rectifier.

The L1 inductor is made on the basis of the B36 magnetic circuit. The winding contains 20 turns of PEV 1,35 wire. The finished coil is filled with epoxy resin. When assembling the magnetic circuit, a non-magnetic gasket 0,3 ... 0,5 mm is installed between the cups.

If the supply voltage of the device differs significantly from that indicated in the diagram, it should be noted that the resistance of the resistors R1 and R2 is calculated from the condition that the current of the zener diodes VD1 and VD2 is within 3 ... 10 mA. With a significant increase in the supply voltage, a significant increase in the power dissipated by transistors VT1 and VT2 is possible - they should be installed on heat sinks. If the filter capacitors cannot be placed on the board (due to large dimensions), it is advisable to place them separately, increasing the total capacitance of capacitors C1-C3 to 10000-15000 uF, and capacitor C6 to 4700 uF.

Capacitor C7 - niobium or tantalum (K52-9, K53-27) for a nominal voltage of at least 32 V. It is permissible to replace the IRFZ44N transistor with an IRF540N, although it requires more intensive cooling. LEDs HL1 and HL2 - any that provide the necessary indication. It is desirable that they be of different colors.

The establishment of the power supply begins with the transistor VT3 turned off. First, voltage is applied to the input and the operation of the internal stabilizers is checked. The voltage on the capacitor C4 should be within 15 ... 16 V, and on the capacitor C5 - 8 ... 9 V. Minor deviations will not have a noticeable effect on the operation of the device. Transistors VT1 and VT2 in any mode should not get very hot.

After that, a current limiting node is established. The engine of the variable resistor R5 is set to the left according to the scheme, the position corresponding to the minimum current. Then, with a trimmer resistor R3, the voltages at the inputs of the op-amp DA1.1 are equalized: you should find a position in which, with the start of turning the resistor R5 slider, the HL1 LED turned off, and turned on in the leftmost position according to the diagram. With this setting, the variable resistor R5 can change the maximum output current from 5 to 5 A. If you still cannot get the maximum current of 5 A, you should increase the resistance of the resistor RXNUMX and repeat the adjustment.

Then the switching transistor VT3 is connected and the voltage limiting unit is set up. The slider of the variable resistor R5 is set to a position in which the HL1 LED is off. The engine of the tuning resistor R18 is set to the top, and the engine of the variable resistor R16 is set to the middle position according to the scheme, corresponding to half the maximum voltage. The trimming resistor R18 sets half of the maximum output voltage that the power supply should provide. In this case, a load must be connected to the output, for example, a resistor with a resistance of 100 ohms and a power of 2 watts.

It should be remembered that the maximum output voltage should not differ greatly from the effective alternating voltage on the secondary winding of the mains transformer.

At the end of the adjustment, it is advisable to calibrate the resistors R5 and R16. To do this, when the power supply is turned off, the engine of the resistor R16 must be set to the middle, the engine of the resistor R5 to the leftmost position, connect an ammeter to the output and apply the supply voltage. Next, moving the slider of the resistor R5, increase the current in the circuit to some value, for example 1 A, and set the corresponding risk opposite the arrow of the resistor knob, etc. Then, replacing the ammeter with a voltmeter, calibrate the resistor R16. With some skills, using the obtained scales and indicators HL1 and HL2, it is possible without measuring instruments to accurately set the voltage and current of the load, the charging current of the batteries and determine the voltage on them, set the limit operating modes, limiting the current and voltage in specified intervals.

In conclusion, I would like to note that the maximum drain-source voltage of the IRFZ44N (VT3) field-effect transistor is 55 V, the maximum drain current is 49 A, and the open channel resistance is 0,022 Ohm. So, in principle, the described power supply has the ability to "overclock". In addition, by supplementing the device with an RS-trigger, we get an automatic machine that will turn off when an overload occurs or when the required voltage is reached when the unit is used as a charger.

Author: A.Antoshin, Ryazan

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