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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Powerful voltage stabilizers with current protection. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Surge Protectors To power some radio devices, a power supply with increased requirements for the level of minimum output ripple and voltage stability is required. To provide them, the power supply has to be performed on discrete elements. Shown in fig. 4.7 the circuit is universal and on its basis it is possible to make a high-quality power supply for any voltage and current in the load.
The power supply is assembled on a widely used dual operational amplifier (KR140UD20A) and one power transistor VT1. In this case, the circuit has current protection, which can be adjusted over a wide range. A voltage regulator is made on the DA1.1 operational amplifier, and DA1.2 is used to provide current protection. Chips DA2, DA3 stabilize the power supply of the control circuit assembled on DA1, which improves the parameters of the power supply. The voltage stabilization circuit works as follows. Voltage feedback is removed from the source output (X2). This signal is compared with the reference voltage coming from the zener diode VD1. A mismatch signal (the difference between these voltages) is supplied to the input of the op-amp, which is amplified and fed through R10-R11 to control the transistor VT1. Thus, the output voltage is maintained at a given level with an accuracy determined by the gain of the op-amp DA1.1. The desired output voltage is set by resistor R5. In order for the power supply to be able to set the output voltage to more than 15 V, the common wire for the control circuit is connected to the "+" (X1) terminal. In this case, to fully open the power transistor (VT1), a small voltage is required at the output of the op-amp (based on VT1, Ube = + 1,2 V). Such a construction of the circuit allows you to make power supplies for any voltage, limited only by the allowable collector-emitter voltage (Uke) for a specific type of power transistor (for KT827A, the maximum Uke = 80 V). In this circuit, the power transistor is composite and therefore can have a gain in the range of 750 ... 1700, which allows you to control it with a small current - directly from the output of the op-amp DA1.1. This reduces the number of required elements and simplifies the circuit. The current protection circuit is assembled on the op-amp DA1.2. When current flows through the load, voltage is generated across resistor R12. It is applied through the resistor R6 to the connection point R4-R8, where it is compared with the reference level. As long as this difference is negative (which depends on the current in the load and the resistance value of the resistor R12) - this part of the circuit does not affect the operation of the voltage regulator. As soon as the voltage at the specified point becomes positive, a negative voltage will appear at the output of the op-amp DA1.2, which, through the VD12 diode, will reduce the voltage at the base of the power transistor VT1, limiting the output current. The level of output current limitation is adjusted using resistor R6. Diodes connected in parallel at the inputs of operational amplifiers (VD3 ... VD7) protect the microcircuit from damage if it is turned on without feedback through the VT1 transistor or if the power transistor is damaged. In operating mode, the voltage at the inputs of the op-amp is close to zero and the diodes do not affect the operation of the device. The capacitor C3 installed in the negative feedback circuit limits the band of amplified frequencies, which increases the stability of the circuit, preventing self-excitation. A similar power supply circuit can be performed on a transistor with a different conductivity KT825A (Fig. 4.8).
When using the elements indicated in the diagrams, these power supplies allow you to obtain a stabilized voltage of up to 50 V at a current of 1.5 A at the output. The technical parameters of a stabilized power supply are obtained no worse than those indicated for a circuit similar in principle to operation, shown in fig. 4.10.
The power transistor is mounted on a radiator, the area of which depends on the current in the load and the voltage Uke. For normal operation of the stabilizer, this voltage must be at least 3 V. When assembling the circuit, the following parts were used: tuning resistors R5 and R6 of the SPZ-19a type; fixed resistors R12 of the C5-16MV type for a power of at least 5 W (the power depends on the current in the load), the rest are from the MLT and C2-23 series of the corresponding power. Capacitors C1, C2, C3 type K10-17, oxide polar capacitors C4 ... C9 type K50-35 (K50-32). The DA1 dual operational amplifier chip can be replaced by an imported analog maA747 or two 140UD7 chips; voltage regulators: DA2 on 78L15, DA3 on 79L15. The parameters of the network transformer T1 depend on the required power supplied to the load. For voltage up to 30 V and current 3 A, you can use the same one as in the circuit in Fig. 4.10. In the secondary winding of the transformer, after rectification on the capacitor C6, a voltage of 3.5 V must be provided more than is required to be obtained at the output of the stabilizer. In conclusion, it can be noted that if the power supply is supposed to be used in a wide temperature range (-60 ... + 100 ° C), then additional measures must be taken to obtain good technical characteristics. These include improving the stability of reference voltages. This can be done by choosing zener diodes VD1, VD2 with a minimum. TKN, as well as stabilization of the current through them. Usually, current stabilization through a zener diode is performed using a field-effect transistor or using an additional microcircuit operating in the current stabilization mode through a zener diode, fig. 4.9.
In addition, zener diodes provide the best voltage thermal stability at a certain point in their characteristic. In the passport for precision zener diodes, this current value is usually indicated and it is it that must be set with tuned resistors when setting up the reference voltage source node, for which a milliammeter is temporarily included in the zener diode circuit. Author: Shelestov I.P.
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Comments on the article: Vyacheslav Dear radio amateurs! I have been interested in amateur radio (mainly power amplifiers) for over 25 years. Over the past time, I have made quite a lot of power supplies, however, without boasting, I want to say that this circuit is the most efficient, convenient, practical, high-quality, namely, excellent current and voltage stabilization (for example, I loaded a light bulb for 12 V, 15 W, current consumption amounted to 0,92 A, heated up the output transistor on a medium-sized radiator with an area of \u500b\u2babout 20 cm12 for 000 minutes, the voltage all the time, without exception, showed 16 V on the multimeter, believe me, the accuracy was exactly up to thousandths and never changed. an imported bridge for 600A, 10000 V, a capacitor of 63 microns for 20 V, parallel to it 160 microns for 2 V MBGO-827. There are two channels on one printed circuit board (with KT3A - 2 pcs., and 825T2A - 825 pcs. + KT1G - 5 pc. , I added 0,22-watt resistors of 0,3-5 ohms to the emitters), i.e. two unipolar blocks with a load current of at least 2 A, the output + of one block, connecting with - the other, we get a bipolar block with a common point , but you need two separate 15 x 140 V supplies so that each circuit Ema KR20UD400A was powered by her own personal food. I used a 37 W OCM as a transformer, 1,7 V secondary windings with a powerful wire of about 2-2 mm0 in diameter of the wire, I wound it in two wires of 85 each. As a resistor-voltage regulator, R6 used a 10 kOhm imported multi-turn wire with increased accuracy. But when applying exactly 37 V AC. voltage this resistor must be replaced with a 47 kΩ resistor, only then the adjustment limit will be expanded from 0 to almost 50 V, initially with 10 kΩ it was from 0 to 30 V. The minimum voltage will be at the minimum resistance R5. Perfectly adjusts R6 for the current limit you need. Great! Yes, and only. A radiator for transistors for high currents needs a powerful one of about 1500 cm2 or more, and possibly with the use of coolers where the area is insufficient. Until the end of the study is still not carried out. But very pleased. There is an error in the circuit: the power supply of the microcircuit is indicated as + 15 V on legs 9 and 14, correctly on legs 9 and 13 !!! Good luck. Sorry, the handprint has not been preserved. I will answer questions as much as possible. Sincerely, Vyacheslav. Vyacheslav I ask those fans who repeated the scheme of this power supply to respond. Have questions. Thank you. Vladimir Since when did KT815 become PNP? Edik [up] The scheme works great! How to change the circuit to work on a powerful FET at the output of the circuit?
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