Switching voltage stabilizer for a phone with caller ID. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Surge Protectors
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Usually, the power supply of a phone with caller ID on the Z-80 processor contains a network transformer and a voltage regulator on the KR142EN5A chip. Such a block in operation heats up significantly, wasting electricity and increasing the fire hazard, especially since manufacturers are trying to increase the voltage at the input of the stabilizer to increase the phone's resistance to failures.
I modified and applied in my phone the common power supply "Electronics D2-34-2", leaving the case and the mains transformer from it, and replaced the stabilizer with a pulse one. The output voltage of the stabilizer is 5 V. It can also be used with other equipment designed for this voltage and current 200 ... 300 mA.
The scheme of the device is shown in fig. 1. The frequency of the converter is approximately 120 kHz. Due to the high efficiency - about 80%, excluding losses in the network transformer and rectifier - the switching transistor VT2 does not need a heat sink. This is achieved by the fact that when the transistor VT2 is open, it is in a state close to saturation, which is provided by the current transformer T2.
On the transistor VT3, a node is assembled that controls the operation of the switching element. This node does not allow transistor VT2 to open until the current through resistor R5 is less than the reverse current through diode VD6 and inductor L1. In addition, the conversion frequency becomes dependent only on the difference between the values of the input and output voltage, the inductance of the inductor L1 and the current consumed. This feature of the stabilizer greatly reduces the requirements for the accuracy of the throttle.
In my design for the L1 choke, a ferrite magnetic core was used from the power supply of the Samsung VK-30 video player. The drawing of the magnetic circuit is shown in fig. 2. The winding contains 42 turns of PEV-2 0,5 wire.
The current transformer T2 is wound on a ring magnetic circuit of size 10x6x5 made of ferrite with a magnetic permeability of approximately 800. The primary winding contains 10 turns of PEV-2 0,5 wire, and the secondary winding contains 98 turns of PEV-2 0,2 wire. Both windings are evenly distributed throughout the ring.
The output voltage of 5 V is set with a trimmer resistor R10. The Zener diode VD7, closed in normal mode, protects the load from possible surges in the output voltage.
Author: A. Mulyndin, Almaty, Kazakhstan
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