Analysis on the circuit design of pulse type underground metal detector

- Jan 01, 2020-

In order to study the influence of VFTO on electrical insulating materials, a fast pulse metal detector is designed to simulate VFTO. Its output waveform requires: rise time < 50ns, peak voltage up to 1000kV. The generator consists of Marx generator, load test chamber and intermediate connection section. Marx uses positive and negative charging circuit, including 10 0.015 μ f / 100kV molded case capacitors and 6 spark gap discharge switches. A 500 Ω resistor is connected in parallel at the load end to speed up the oscillation attenuation speed of the output high-voltage pulse and reduce the insulation requirements of the equipment, so as to make the Marx structure compact and reduce the inductance of the discharge circuit. At the same time, the capacitor and spark switch in Marx generator adopt special turn back wiring form to further reduce the inductance of discharge circuit. The design of the fast pulse generator is also given. The simulation results of PSPICE show that the rise time varies from 23 ns to 28 ns when the output voltage is 1000 kV, which meets the requirements.

There are a lot of circuits that appear on the Internet as pulse induction metal detectors. Although they process signals in different ways and generate magnetic field pulses, these electronic devices are basically the same. Its main part is the coil that produces the magnetic pulse.

The size of the coil mainly depends on the required detection depth and the minimum size of the detected object. Generally speaking, it can be said that the theoretical maximum detection depth is 5 times the diameter of the coil, and 5% of the diameter of the smallest size of the object detected by the coil. This is the most valuable and heavily dependent case. It's obvious that you can't detect a 5cm object at a depth of 5m with a one meter coil. However, what kind of coil do you need? This is a specific problem. Many people use metal detectors to search for coins and jewelry. For these cases, a 25 cm or 40 cm coil is enough. In my usage, I need to locate a 20cm iron cover or porcelain filled with metal at a depth of 2m. That's why I'm making a 1-meter coil. Although the physical size and shape of the coil may change (square or oval coil is used for the same work in a specific case, but it is better to be round), only slightly different physical designs between the inductive coils are available. The most commonly used range of coil inductance for pulse induction metal detectors is 300 to 500 μ H. In this design, I will assume that the coil used is 400 μ H. For smaller coils, it means more turns.

A new type of ns level high voltage narrow pulse metal detector is developed based on the principle of oscillation circuit. It can output adjustable ns level pulse voltage with pulse width of 20-100 ns and amplitude of 500-1000 v. theoretical and simulation analysis of the circuit parameters show that the obtained ns level high voltage pulse waveform is smooth and symmetrical. UWB is a kind of nanosecond level very narrow pulse The technology of transmitting information. A kind of UWB very narrow pulse generator with cascade avalanche transistor structure is mainly discussed. Its circuit and the working principle of avalanche transistor are analyzed in detail. The output pulse width of UWB is about 1.22ns and the rising time is about 863ps

The coil design of the pulse gold detector; there are many circuits, the pulse induction metal detector appearing on the Internet; the coil size mainly depends on the required detection depth and the detected object; the coil is powered by a common battery; when the voltage passes through a high-speed bipolar transistor or MOSFET, the total resistance value from MOSFET to negative power supply is determined; now, I We have defined the inductance and resistance of the coil, but the coil is in; coil size; size; 120 mm; 150 mm; 175 mm; 200 mm.