Typically, the multimeter is powered by a nine-volt battery type "Krona". It is inexpensive, but it ends abruptly. Moreover, now almost everyone has a device powered by a USB port - laptops, tablets, smartphones, and the corresponding chargers. Here is a description of the voltage converter unit to power the multimeter from any USB port or from a charger with a USB connector. The task of this unit is to increase the voltage 5V, available on the USB-connector, to a voltage of 9V, necessary to power the multimeter.The figure shows a diagram of a simple unit that allows you to get a stable DC voltage of 9V from the USB port, provided that the load current does not exceed 150 mA. Using such an adapter, it is possible to power not only a multimeter from a USB port, but also other circuits and devices powered by Krona.
The circuit is based on a DC-DC converter on the LM3578AM chip. Since the converter is pulsed, an inductance L1 is installed at the input, which prevents the penetration of pulsed noise into the circuit of the device, the USB port of which serves as a power source. As part of the A1 chips, there is a pulse generator with PWM, and an output key. The key goes to conclusions 6 and 5. Pin 7 is the input of the control protection circuit, which determines the current through the key by the value of the voltage drop across the resistance of the resistors R2-R4. And when it exceeds the permissible value, protection is triggered.
Voltage is pumped at inductance L2. Then, this alternating voltage is rectified by the Schottky diode VD1 and smoothed by capacitors C6 and C7. The output voltage is stabilized by comparing the voltage removed from the R5-R6 divider with the reference voltage generated by the stabilizer, which is part of the microcircuit. The control input is terminal 1. Resistors R5 and R6 form a voltage divider, with which the output voltage is divided so that with the required output voltage of 9V, terminal 1 A1 has a voltage of 1V.
The output voltage is greater, the greater the division ratio of the divider R5-R6. In principle, the maximum output voltage can be made much higher (but not more than 30V). You can increase the output voltage by decreasing the resistance R6 or increasing the resistance R5.
Mounting the converter is most conveniently done on a small printed breadboard.
Chokes L1 and L2 - finished, industrial production.
Conventional resistors have a significant variation in resistance, so if the output voltage is less than 8V or more than 9.5V, you need to choose R5 or R6.
Author: Zhurbin A