Electric field indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Field strength detectors
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A simple but sensitive device reacts to the electric field of wires and cables for various purposes, including high-voltage power lines, and signals the field strength above a certain level. The device can be used to warn people working near electrical installations of the danger of electric shock. In everyday life, it is useful as a phase indicator and for detecting hidden wiring.
The proposed indicator of the electric field (IEF) is easy to use, equipped with adjustments for sensitivity and threshold. The scheme of the IEP is shown in fig. 1.
(click to enlarge)
The sensing element is a field-effect transistor VT1, a signal proportional to the field strength is induced on the connected gate and housing of which. The high-resistance resistor R1 prevents the accumulation of static charges. After passing through the emitter follower transistor VT2, the induced signal is allocated to the resistor R3. Capacitor C3 suppresses high frequency pulses and noise. Through resistor R4 and capacitor C4, the signal is fed to the non-inverting input of the op-amp DA1. Resistors R5, R6 create an artificial zero point, and R7 and R11 are used to set the required operating mode of the op-amp. The variable resistor R10 regulates the gain of the cascade. Capacitors C6 and C7 are elements for correcting the frequency response of the op-amp.
Op-amp DA2 compares the voltage supplied through the resistor R15 from the output DA1, with a reference. The latter is regulated by a variable resistor R13. The purpose of the resistors R17 and R21 is similar to R7 and R11. Due to the positive feedback through the resistor R18, the comparator on the op-amp DA2 has a small hysteresis, which increases the clarity of its operation. The output signal of the comparator through the switch on the transistor VT3 controls the LED HL1.
The printed circuit board of the IEP (Fig. 2) made of one-sided foil fiberglass is placed in a plastic case with dimensions of 90x60x22 mm.
The cutout of the board is designed to place a 1 V GB9 battery next to it ("Krona", "Korund"). The controls (R10, R13, SA1) and the HL1 LED are displayed on one of the side walls of the case. The operability of the IED is maintained when the supply voltage drops to 3 V. In standby mode, the current consumed does not exceed 0,2 mA, increasing to 5 mA when the LED is activated and lit. When establishing an IED, it may be necessary to select resistors R9 and R18.
Transistor VT1 can be replaced by any of the 303 series, and VT2 and VT3 can be replaced with low-power silicon bipolar transistors of the appropriate structure. Variable resistors R10 and R13 - SPZ-4. Switch SA1 - P1T-1-1V.
If the slider of the variable resistor R10 is set to the left, and R13 is set to the upper position according to the diagram, the amplification of the signal induced on the IED sensor is minimal, and the response threshold is maximum. In this state, the device allows the ignition of the HL1 LED to distinguish the "phase" wire of the household electrical network from the "zero". It is enough to alternately bring the sensor - the body of the transistor VT1 - to the insulation of each of these wires. It is unacceptable to touch the sensor to bare wires and metal objects, for example, equipment cases.
In order to use the IED to find the wiring hidden in the wall, it is necessary to increase the gain and lower the threshold. The proximity of the sensor to the route of live wires is indicated by the ignition of the HL1 LED. With a further increase in gain and a decrease in the response threshold, it is possible to detect current-carrying wires at a considerable distance. For example, an appropriately configured IED gave a signal at a distance of 3 m from the busbars energized with 10 kV.
Author: B.Sokolov, Protvino, Moscow region
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