Low voltage voltage converter, 5 volts 120 milliamps. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters
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In devices based on digital microcircuits and microprocessors with autonomous power supply, batteries of galvanic cells must provide a stabilized voltage of 5 V. To achieve this in the simplest way - using six 1,5 V cells and an integrated stabilizer KR142EN5A - is unprofitable both energetically and economically.
The proposed simple stabilized converter allows you to get a voltage of 5 V at a load current of up to 120 mA. Its input voltage can be in the range of 2 ... 3,5 V (two galvanic cells). The efficiency at an input voltage of 3 V and the maximum load current is approximately 75%. The converter circuit is shown in fig. 4.2.
A blocking oscillator is assembled on the transistor VT2. Winding 1 of transformer T1 also performs the function of a storage choke, and from winding II a positive feedback signal is sent to the base of transistor VT2. The pulses that appear on the collector of this transistor charge the capacitors C1, C4 through the diode VD5, the voltage on which is the output. It depends on the repetition rate and duty cycle of the blocking generator pulses, which, in turn, depend on the collector current of the transistor VT1, which recharges the capacitor C3 in the intervals between pulses.
After the blocking generator is supplied with a supply voltage, and as the capacitor C2 is charged through the resistor R1, the collector current of the transistor VT1, the frequency of the generated pulses and the output voltage of the converter increase. But as soon as the latter exceeds the sum of the stabilization voltages of the zener diode VD2 and the opening of the transistor VT3, part of the current flowing through the resistor R1 and the base of the transistor VT1 will branch into the collector circuit of the opened transistor VT3. This will reduce the pulse frequency. Thus, the output voltage will be stabilized.
Trimmer resistor R3 allows you to set it equal to 5 V. Transistor VT2 - KT819 with any letter index, KT805A or KT817 also with any index. In the latter case, the output power of the converter will be slightly less. The efficiency of the device will increase if a germanium diode D1 is used as VD310. Transformer T1 is made of a DPM-1,0 choke with an inductance of 51 μH. The winding on it is used as the primary one. A feedback winding (II) of 14 turns of wire with a diameter of 0,31 mm in enamel insulation is wound on top of it. Capacitor C3 must be metal film series K71, K78. A ceramic capacitor is undesirable here due to low temperature stability. The device is not critical to the types of other parts. The converter is mounted on a board made of double-sided foil fiberglass. The foil on one side of the board is left intact and serves as a common wire.
Author: Semyan A.P.
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