Voltage stabilizer with current protection on the KR142EN19 chip. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Voltage stabilizer with current protection on the KR142EN19 chip. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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Modern voltage stabilizers protect against overload, usually by limiting the current. In the event of an output short circuit, the load current becomes much less than the operating current, so the stabilizers do not overheat.

Short circuit protection protected stabilizers "detect" by the appearance of voltage at the output. However, if this voltage is too low, the regulator does not "recognize" the removal of the short and therefore does not start. This happens when the load of the stabilizer consumes a large current even at a low supply voltage, such as, for example, the incandescence of a kinescope or a collector motor.

A similar situation occurs when the load is powered by a bipolar voltage. If one arm of the stabilizer had time to start earlier, part of its voltage can get through the load to the output of the other and prevent it from starting.

In these cases, it is necessary to use more complex stabilizers, in which the circuit current is significantly increased and the possibility of its adjustment is provided. Since such devices in the form of microcircuits are not yet produced, radio amateurs have to develop them on the basis of discrete elements.

Voltage stabilizer with current protection on the chip KR142EN19

The article describes a protected voltage regulator with an increased and adjustable overload current. The stabilization coefficient of the device, the circuit of which is shown in the figure, is more than 800, the voltage drop across the stabilizer is not more than 0,5 V. The overload current can be set in the range from 30 mA to 1,5 A. Such a large regulation interval is ensured by the fact that during overload, the current enters the load not through the regulating transistor VT3 of the stabilizer, but from the starting node on the transistor VT1, specially designed for operation in the output closing mode.

The main element of the stabilizer is the KR142EN19 chip. It consists of an analogue of a zener diode with a stabilization voltage of 2,5 V and an error signal amplifier. When the voltage at control input 1 of the microcircuit exceeds 2,5 V, the anode current (pin 3) increases very quickly from 1,2 mA to a level limited by an external resistor. The maximum current of an open microcircuit should not exceed 0.1 A, and the power dissipation should not exceed 0,4 W. The voltage on an open microcircuit, determined by its internal device, is approximately 2,5 V. On a closed microcircuit, it should not exceed 30 V.

The described stabilizer works as follows. When the output voltage increases, the voltage on the output voltage regulator engine, the variable resistor R8, also rises. If it exceeds the threshold of 2,5 V, the DA1 chip will open, thereby sequentially closing transistors VT2 and VT3.

Since the voltage at the anode of the microcircuit cannot be less than 2,5 V, the voltage at the emitter of the transistor VT2, so that it can effectively close, must be slightly higher. Therefore, through the diodes VD1 and VD2, a part of the output voltage is applied to the emitter of the transistor VT2.

Resistor R5 limits the base current of the regulating transistor VT3. Therefore, the protection operation current depends on its resistance. The current value increases as the resistance of this resistor decreases.

In the event of an output short circuit, the transistor of the starting node VT1 is open and saturated with current flowing through the resistor R2. The overload current is determined by the resistance of the resistor R1 and therefore practically does not depend on temperature. The voltage at the base of the transistor VT1 during overload does not exceed 0,5 V relative to the negative wire. This level is not enough to open the transistor VT2, and then the transistor VT3. Therefore, in overload mode, no current flows through them and they do not heat up. The transistor VT1 of the starting node heats up very slightly due to the small voltage drop in the collector-emitter section.

After eliminating the cause of the overload, voltage appears at the output of the stabilizer, which leads to an increase in voltage at the base of the transistor VT1, and then at the base of the transistor VT2. First, the transistor VT2, and then the transistor VT3 open, and the stabilizer starts. When the voltage at the output of the stabilizer reaches the nominal level, the DA1 chip opens, partially closes the VT2 transistor and completely closes the VT1 transistor.

If the transistors VT2 and VT3 are replaced with significantly lower frequencies than those indicated in the diagram, generation is possible, which can be prevented by connecting a capacitor with a capacity of several hundred picofarads between pins 1 and 3 of the DA1 chip.

On the emitter junctions of transistors VT1 and VT2, reverse voltage pulses are possible, the amplitude is proportional to the output voltage of the stabilizer. Therefore, GT705D (VT1) in extreme cases can be replaced with a transistor of another series, the emitter junction of which can withstand a reverse voltage of at least 10 V, for example, in KT859A.

Author: S.Kanygin, Kharkov, Ukraine

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