1.5V boost converter for digital multimeter

Most of the 1.5V to 9V circuits introduced in newspapers and magazines have high power consumption, and some have working currents of more than 50mA. The working current of the boost circuit introduced in this article (as shown in Figure 3) is relatively small, only about 6mA when unloaded. When connected to a digital multimeter with a working current of 1mA~3mA, the battery current consumed is between 15mA-30mA. In Figure 3, 8050 and 8550 form a self-excited oscillator, and the back electromotive force of the inductor connected to the collector of Q2 is rectified to form a high voltage. The voltage level is determined by the voltage regulator D1. When the output voltage exceeds the breakdown voltage of the voltage regulator, the voltage regulator is turned on, the base voltage of Q1 rises, the current of Q2 decreases, and the output voltage stabilizes at 9v. Replacing the voltage regulator can change the output voltage to meet the needs of other occasions. According to the author's actual experience, under no-load conditions, the maximum output voltage can reach 50V. The inductor L is wound with a 7mmX7mm mid-circumference magnetic cap core, and Φ0.1mm enameled wire is wound all over the I-shaped magnetic core. Then it is wrapped with a layer of tape and stuffed into the magnetic cap. It is sealed on the circuit board with glue and both ends are connected to the circuit.