DC booster circuit suitable for use with multimeters

The multimeter is an essential tool for electricians and electronic technicians. Its high resistance usually uses a 9V, 15A or 22.5V laminated battery. This kind of battery is not only more expensive, but also has a short lifespan, making frequent replacement uneconomical. Here are several small DC booster circuits suitable for use with multimeters. These circuits have simple structures and few components. After modification, the circuit board can be placed directly in the position of the stacked battery in the multimeter to be used instead. As shown in the figure, it is a DC booster circuit with an output voltage up to 22.5 V, which can be used to replace the 22.5 V laminated battery. It is powered by a 1.5V battery in the multimeter, with a working current of 25mA and an output current of about 0.5mA, which is sufficient for the high resistance of the multimeter. In the circuit, VT1 and VT2 form a complementary multivibrator, and its oscillation frequency is about 2kHz. T is a step-up transformer, the primary is the load of the complementary multivibrator, and the secondary is the step-up winding, which outputs a higher pulse voltage. This voltage becomes a DC high voltage after being rectified and filtered by diode VD1 and capacitor C2, and then stabilized by resistor R3 and voltage regulator tube VD2 to output a relatively stable high voltage. The transformer T in the circuit can be a 502-type audio output transformer used in transistor radios. The secondary is used as the primary of the step-up transformer. The tap in the middle of the primary is not used, and the taps at both ends are used as the secondary of the step-up transformer. If you can't find a suitable transformer, you can also make your own from the silicon steel sheet of the radio input and output transformer. The primary is wound with 110 turns of high-strength enameled wire with a diameter of 0.25mm, and the secondary is wound with 520 turns of high-strength enameled wire with a diameter of 0.21mm. . Add a layer of insulating paper between the primary and secondary coils, and pay attention to the same ends of the primary and secondary coils. As shown in the picture, it is a small DC booster with a very simple structure, which can be used to replace the 15V laminated battery. The core component of the circuit, the transformer T, uses a dedicated transformer for a pocket currency validator. The circuit consumes about 40mA and the output voltage is 15V. If the positive pole of the multimeter's 15V battery is connected to the negative pole of the 1.5V battery, you only need to adjust the polarity of VD1 (C1, VDZ) in Figure 7-70, so that a -15V voltage will be output. As shown in the figure is a Stabilized DC boost circuit. This circuit can boost a 1.5V battery to 9V and is used to replace the 9V laminated battery. The no-load input current of the circuit is less than 1.2mA, and the conversion efficiency is as high as 60%. . The circuit consists of an oscillation circuit and a voltage stabilizing circuit, in which VT1, VT2 and C2 form an oscillator, the color code inductor L is the energy storage inductor, VD2 is the rectifier diode, C3 is the output filter capacitor, VT3, VD1, VD3 and R2 are A voltage stabilizing circuit that stabilizes the output voltage. The output voltage is approximately equal to the voltage stabilization value of VD3. As shown in the figure, it is an inverter power supply circuit that uses a 1.2V, 500mAh nickel-cadmium battery as the power source. The output DC voltage is 9V. A digital multimeter is used. The transformer T in Figure 7-72 ​​is made of a 15mm magnetic ring (the name of the power switch of the digital multimeter is marked in the component parameter diagram. As shown in the figure is a self-controlled digital meter inverter Power circuit. It does not require a separate power switch or modification of the switch in the meter. This circuit has the characteristics of low power consumption, stability and reliability, and does not affect the accuracy of the meter. The transformer T in the circuit uses E3 type ferrite core. Cut off a corner of each and process it into a square shape, with L2 inside and L1 outside. When the entire inverter power supply is working, the battery operating current is about 70mA.