Chinese LCD probe. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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In stores and kiosks, you can find several types of probes (phase indicators) of South Asian origin with a liquid crystal indicator at a ridiculous price. The author of this note was interested in the interior of the probe, or rather its alleged ability to determine several levels of direct or alternating voltage.
Rice. 1. Appearance of the probe
The appearance of the probe is shown in fig. 1, board - in fig. 2, and the schematic diagram - in fig. 3.
Rice. 2. Fee
The resistor values are measured with an accuracy of 0,2 MΩ. Thick film resistors, the resistive layer is applied directly to the printed circuit board between the foil tracks. A liquid crystal indicator of a conventional design with a single substrate, the voltage at which the contrast becomes normal is about 3,5 ... 1 V. The segments of the indicator in the diagram are shown as capacitors. Together with the resistors, they form a frequency-dependent divider. When controlling the AC voltage, you need to touch the touch contact E1.1. In this case, the current of all enabled segments also passes through the first segment of HG7. The area (capacitance) of the first segment is smaller than the other segments, the voltage drop is larger, it manifests itself at any voltage. This segment is not shunted by a resistor, it goes out for a long time, the voltage on it can reach the breakdown voltage of the liquid crystal. The white stripe on the resistor R2 (Fig. XNUMX) is such a "laser fit" in Chinese. Humor is humor, but, frankly speaking, it was not possible to understand the method of setting up such a board.
Rice. 3 Schematic diagram
Here I would like to note that the breakdown voltage of such homegrown resistors is unknown. Moreover, the safety of the user directly depends on the conscientiousness of Asian manufacturers (we are talking about the thoroughness of flushing the board). No one guessed to varnish or seal the board, which is intended to be plugged into a socket. The only thing that soothes is that the touch contacts are made of high-resistance conductive rubber. The author failed to press the tester probes against this rubber band to achieve resistance below 150 kOhm. Without pressure, the resistance of the touch contacts El, E2 has a value of 0,5 ... 1 MΩ.
Author: Oleg Fedorov, Moscow; Publication: radioradar.net
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