In principle, many other peripheral devices that are not designed for this can be powered from USB, but there are some limitations. In particular, according to the supply voltage, which is only 5V. Despite the fact that peripherals powered by their own network adapters usually require a higher voltage, and even 5V is not always what you need to power many homemade products.
The figure shows a diagram of a simple adapter that allows you to get a stable DC voltage from a USB port adjustable between 1.4 and 35V, provided that the load current does not exceed 350 mA. Using such an adapter, you can power a variety of circuits and devices from a USB port, and even use it as a laboratory source, which is most important when working with a USB laboratory or an affordable set of programs such as those that allow you to turn a PC with a sound card into low-frequency oscilloscope, millivoltmeter, low-frequency generator, frequency meter (such programs are usually available on the Internet for free).
The circuit is built on IC LT1372, designed to build circuits of DC-DC voltage converters. The built-in generator generates pulses with a frequency of about 500 kHz. The stabilization circuit adjusts the latitude of these pulses and feeds them to the output switch on the output transistor, which is part of the microcircuit. The chip has protection for the output transistor against overcurrent through it. With a current through it of more than 1.3A, protection is triggered. The protection is based on the principle of measuring current by measuring the voltage across the resistance in the emitter circuit of the output transistor. Measuring resistance is part of the chip.
An inductance L1 is connected to the collector of the output transistor, on which the AC voltage is “pumped”. Which is then rectified by the diode VD1 and smoothed by the capacitor C4. The output voltage is stabilized by changing the latitude of the pulses arriving at the base of the output transistor. The sensor for measuring the output voltage is a comparator. Pin 2 should have a voltage of 1.25V, voltage is supplied to this pin from the output of the circuit through a divider on resistors. And the comparator adjusts the pulse width so that it is exactly 1.25V on pin 2. Thus, by adjusting this voltage divider, the output voltage can be adjusted. With the resistance of the resistors R3-R5 indicated in the diagram, the output voltage can be adjusted from 1.4 to 35V.
Since the block was supposed to be universal with the ability to quickly adjust the output voltage, three VD2-VD5 diodes are included in the circuit between pin 2 and the common minus. Their task is to limit the voltage at pin 2 so as not to damage the microcircuit during a sharp rotation of the shaft R4 in the direction of reducing the output voltage.
With an output current of up to 0.35A, a radiator is not required.
Author: Zhurbin A
