How to add an inductance short circuit tester to a multimeter
It is very convenient to connect the commonly used 555 integrated circuit into a feedback-type unstable circuit, and install it in the space inside the multimeter to judge whether the inductor (such as the high-voltage package of the TV) is short-circuited.
The principle of the tester circuit is shown in the figure. To save space, try to use small or miniature components. Open two holes in the appropriate position of the case, and install the double pole double throw switch K and potentiometer W. The circuit of the multimeter itself does not make any changes. The author soldered the entire circuit on a 1.5×3cm insulating board. It is installed on one side of the inner head of the MF-47 Wangang watch and fixed with hot melt glue, and there is more than enough space in the watch.
How to use: Usually, put the switch K in the "normal" position, the circuit does not work, and has no effect on the normal use of the multimeter. When the inductance needs to be measured. Set the multimeter to AC10V gear, and switch K to the "L" position. At this time, adjust the potentiometer W, and the multimeter can have a maximum voltage indication of 4 to 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 maximize the displayed voltage value. The maximum value of the inductance may be greater than or less than the no-load voltage. The actual measurement of a 25-inch color TV with intact high-voltage package primary, the maximum value can reach 40V during resonance, and the measurement range can be adjusted to 10V. The actual measurement of a 17-inch black self-machine intact high-voltage package primary, the maximum voltage is always lower than the no-load voltage. During the test, use a wire to pass through the magnetic core of the inductor and make a turn. When the wire is short-circuited, the displayed voltage should drop significantly. If the drop is very small or even no drop, it can be judged that the measured inductance is inside. There is a short circuit.
