How to add an inductor short circuit tester to a multimeter

The commonly used 555 integrated circuit is connected into a direct feedback type astable circuit and installed in the space inside the multimeter. It is very convenient to use it to determine whether the inductor (such as the high-voltage package of a TV) is short-circuited.

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The circuit principle of the tester is shown in the figure. To save space, small or micro components are used as much as possible. Two holes are opened at the appropriate position of the case to install the double-pole double-throw switch K and the potentiometer W. The circuit of the multimeter itself is not changed. The author soldered the entire circuit on a 1.5×3cm insulating board. It was installed on one side of the head of the MF-47 multimeter and fixed with hot melt adhesive. There is more than enough space inside the meter.

How to use: Usually, put switch K in the "normal" position, this circuit does not work, and has no effect on the normal use of the multimeter. When it is necessary to measure the inductance. Set the multimeter to AC10V and turn K to the "measure L" position. At this time, adjust the potentiometer W, and the multimeter can have a maximum voltage indication of 4-5V, which can be called "no-load voltage". Connect the test leads to both ends of the inductor to be measured, and adjust the potentiometer to make the displayed voltage value the largest. Due to the inductance, this maximum value may be greater than or less than the no-load voltage. The measured primary of a 25-inch color TV with a good high-voltage package can reach a maximum value of 40V during resonance, which can be slightly adjusted to a measurement range of 10V. The measured primary of a 17-inch black machine with a good high-voltage package is always lower than the no-load voltage. During the test, use another wire to pass through the magnetic core of the inductor and make a circle. When this circle of wire is short-circuited, the displayed voltage should drop significantly. If the drop is small or even does not drop, it can be judged that there is a short circuit inside the measured inductor.