Voltage stabilizer with protection, 14-20/12 volts 0,5 amps. Scheme, description

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Voltage stabilizer with protection, 14-20/12 volts 0,5 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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The schematic diagram of the stabilizer is shown in fig. 1.6. The source of exemplary voltage is a thermally stabilized zener diode VD1. To eliminate the influence of the input voltage of the stabilizer on the mode of the zener diode, its current is set by a stable current generator built on a field-effect transistor VT1. Thermal stabilization and stabilization of the current of the zener diode increase the coefficient of stabilization of the output voltage.

The exemplary voltage is supplied to the left (according to the circuit) input of the differential amplifier on transistors VT2.2 and VT2.3 of the K125NT1 microassembly and resistor R7, where it is compared with the feedback voltage taken from the output voltage divider R8, R9. The voltage difference at the inputs of a differential amplifier changes the balance of the collector currents of its transistors. The regulating transistor VT4, controlled by the collector current of the transistor VT2.2, has a large base current transfer coefficient. This increases the depth of feedback and increases the stabilization factor of the device, and also reduces the power dissipated by the differential amplifier transistors.

(click to enlarge)

For short-term protection of the device with its return to its original state, a collector current limiter of the regulating transistor is introduced, made on the transistor VT3 and resistors R1, R2. Resistor R1 performs the function of a current sensor flowing through the regulating transistor VT4. If the current of this transistor exceeds the maximum value (about 0,5 A), the voltage drop across the resistor R1 will reach 0,6 V, i.e. threshold voltage opening transistor VT3. When opened, it shunts the emitter junction of the regulating transistor, thereby limiting its current to approximately 0,5 A, which causes the output voltage to drop without tripping the overcurrent protection. After some time, proportional to the time constant of the circuit R5, C1, this leads to the opening of the transistor VT2.1 and the further opening of the transistor VT3, which closes the transistor VT4. This state of the transistors is stable, therefore, after eliminating the short circuit or de-energizing the load, it is necessary to disconnect the device from the network and turn it on again after discharging the capacitor C1.

The short-circuit current of the device is zero, which means that it excludes overheating of the control transistor when the protection is triggered. Resistor R3 is necessary for the reliable operation of the transistor VT4 at low currents and elevated temperatures. Capacitor C2, shunting the output of the stabilizer, prevents self-excitation of the device, which can be caused by deep OOS in voltage. Resistor R6 in the collector circuit of the transistor VT2.1 limits the current during transients when the protection is turned on, and the HL1 LED acts as an overload indicator.

The main parameters of the stabilizer:

Input voltage, V ..... 14.,.20;
Output voltage, V.....12;
Load current, A ..... 0 ... 0,5;
Change in output voltage (load current up to 0,5 A), V.....< 0,1;
Quiescent current, mA.....15;
Short circuit current, mA.....< 0,1.

The stabilizer is not critical to the layout of the printed circuit board and the placement of parts on it. Therefore, its installation depends mainly on the experience of the designer himself and the dimensions of the pre-selected parts. The static current transfer coefficient of the base of the transistor VT3 must be at least 20, and the transistor VT4 - at least 400. On the regulating transistor VT4, the allowable collector current of which must be at least 1 A, significant power is released, so it should be installed on a heat sink with a power of about 5 Tue Resistors and capacitors - any types for the ratings indicated on the diagram.

Starting to test and adjust the stabilizer, the resistor R5 is temporarily removed so that the protection system does not work, and by selecting the resistor R8, the output voltage is set to 12 V. After that, the resistor R5 is turned on and the required value of the tripping current of the device protection by current is achieved by selecting the resistor R1.

Author: V. Kozlov

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