AC detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Indicators, detectors
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The device is designed to control a conductor with an alternating current flowing through it. The sensitivity of the device is such that it allows non-contact control of conductors with a current of 250 mA or more.
On fig. 1 shows the circuit diagram of the device.
(click to enlarge)
The sensor for alternating electric current with a frequency of a household network (50 Hz) is an inductor L1. L1 is made in the form of a U-shaped core with a diameter of 2,5 cm, on which 800 turns of wire made of magnetic material with a diameter of 0,15 ... 0,25 mm are wound (Fig. 2).
The core of the coil can be taken from the central part of interstage or low-frequency matching transformers, or small-sized electromagnetic bells. The main requirement for the core is that when the winding L1 is wound, a controlled conductor must be freely threaded through the center of the coil (its diameter can be several units, or even tens of millimeters). It should be noted that only one of the investigated wires (phase or neutral) should be passed through the sensor, since if there are two conductors inside the sensor, magnetic field compensation may occur and the device will not respond properly to the current flowing in the conductor. When experimenting with the device, a double network cable was taken, in which a longitudinal cut of the insulation was made, thus forming two separate conductors, one of which was placed in a U-shaped grip.
A voltage of approximately 4 mV is induced in the magnetic gripping winding (U-shaped probe) when examining a mains wire with a current of 250 mA (corresponding to the power consumed by a load of 55 W at a mains voltage of 220 V). The signal from the magnetic sensor is amplified by a factor of 200 by the DA1.1 operational amplifier, then it is detected by the peak detector VD1, C2 and fed to the non-inverting input of the second DA1.2 operational amplifier, which is connected according to the comparator circuit. The voltage at the DA1.2 input, at which the comparator switches to a high level state at its output, is 800 mV, which corresponds to the above sensitivity of the device. Elements R6, R7, VD2 are necessary to prevent false triggering of the detector due to the presence of a small positive voltage at the output of the comparator in a low state.
As the terminal device of the current sensor, an electromagnetic relay K1 is used, which is triggered when a high level is received from the output of the comparator to the base of the transistor switch VT1. Relay contacts can be used to control various kinds of devices, devices, etc. that monitor the presence of current in the controlled conductor. To increase the sensitivity of the sensor, it is necessary to increase the number of turns of the coil L1.
Author: G. Andreev, Samara
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