The device described in this paper can easily measure the back pressure of triode, diode, Zener, thyristor, etc., the withstand voltage of the capacitor and the resistance greater than 10MΩ. The output voltage can be continuously adjusted up to 1400V. Made of TPU material imported from South Korea, it can effectively protect the screen from unnecessary wear or scratches and reduce the impact of falling.
The ultra-transparent thickness of 0.14mm ensures the sensitivity of the original touch screen. When using Apple Pencil, it can still maintain a high touch sensitivity. 99% transparency provides an ideal natural viewing experience.
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Working principle measurement circuit shown in Figure 1. 
Transformer T (6V) is rectified and filtered by D5, C3, as the base voltage of the adjustment tube Q (adjust W can change the degree of conduction of Q). That is, control the voltage of collector A point (4V-1400V).
The 500V voltage output from the high voltage winding is rectified by the voltage doubler rectifier circuit composed of C1, D1-D4, and the C2 filter output is 1400V high voltage.
When the device under test is connected between A and B, the W resistance is reduced. At this time, the base potential of Q decreases, and the potential at point A rises until the mA index becomes the rated reverse current of the tube under test ( The measured small power tube <0.1mA, high power tube <1mA is appropriate). At this time, the voltmeter index is the back pressure under the reverse current.
When connecting a large resistance resistor between A and B, divide the reading of the voltmeter by the reading of the milliampere meter as the resistance value. The unit is Ω=V/A.
After the measurement is finished, set W to the maximum and return the output voltage to the low point of 4V.
Component selection adjustment tube Q requires when Ic=1A, β≥5, BVcbo≥1500V, reverse leakage current <0.01uA. It can be selected from high pressure tube such as C1942, C2027, BU208, and heat sink.
Transformer T is a tube radio power transformer, available in the flea market.
The internal resistance of the voltmeter is 10kΩ/V or more. It is best to have 1500V, 500V, 250V, 50V, 10V. The milliampere meter should have 0.5mA and 5mA. It can be replaced by a multimeter.
Pay attention to the insulation between the circuits when making.
The flexible material design allows the Screen Protector to adapt to the contours of any device, so it can be connected to curved displays and rounded edges. The Full-Cover Screen Protector can perfectly fit your screen and provide maximum protection.
The oleophobic coating technology protects the glass from fingerprints left by sweat and grease, and fully supports the 3D touch function on the iPad.
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