Powerful stabilizer with current protection, 50 volts 5 amps. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Powerful stabilizer with current protection, 50 volts 5 amps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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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. 3.23 the scheme 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.

Powerful stabilizer with current protection, 50 volts 5 amps

The voltage stabilization circuit works as follows. A voltage feedback signal is taken 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 applied to the input of the op-amp, which is amplified and fed through resistors 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 above 15V, the common wire of the control circuit is connected to the "+" (X1) terminal. In this case, to fully open the power transistor (VT1) at the output of the op-amp, a small voltage is required (based on VT1 ibe = +1,2 V). This design of the circuit allows you to make power supplies for any voltage, limited only by the allowable collector-emitter voltage (UK3) for a specific type of power transistor (for KT827A, the maximum UK3 = 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, which reduces the number of necessary elements and simplifies the circuit.

The current protection circuit is assembled on the op-amp DA1.2. When current flows in the load, a voltage is generated on the resistor R12, which 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 ... VD6) 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. Capacitor C3 installed in the negative feedback circuit limits the band of amplified frequencies, which increases the stability of the circuit, preventing self-excitation.

When using the elements indicated in the diagrams, these power sources allow the output to receive a stabilized voltage of up to 50 V at a current of 1 ... 5 A.

The power transistor is mounted on a radiator, the area of which depends on the current in the load and the voltage UK3. 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 type C5-16MV for a power of at least 5 W (power depends on the current in the load), the rest are from the MLT and C2-23 series of the corresponding power Capacitors CI, C2, C3 of 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 μA747 or two 140UD7 microcircuits; voltage regulators: DA2 on 78L15, DA3 on 79L15. The parameters of the network transformer T1 depend on the required power supplied to the load. 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 increasing the stability of the reference voltages. This can be done by choosing zener diodes VD1, VD2 with a minimum TKN, as well as stabilizing 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. 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 trimming resistors when setting up the reference voltage source node, for which a milliammeter is temporarily included in the zener diode circuit.

Author: Semyan A.P.

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