Current sensor on the Hall element. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The functional diagram of the current sensor of the compensation type is shown in fig. 1. The Hall element sensitive to the magnetic field is located in the gap of the annular magnetic circuit.

Fig. 1

The measured current Imeas, flowing through the winding I. creates a magnetic flux in the magnetic circuit, inducing an EMF proportional to this current in the Hall sensing element. The signal taken from the element after amplification enters the compensation winding II. The current Ik flowing through it creates a magnetic flux in the opposite direction in the magnetic circuit. The magnetic system, Hall element and amplifier form a negative feedback loop that maintains equality

,

where W1 and W|| - the number of turns of windings I and II. Resistor R1, connected in series with winding II, converts the compensating current into the output voltage of the sensor. If we choose the resistance of this resistor in ohms numerically equal to the ratio of the number of turns of winding II to the number of turns of winding I, then the output voltage in volts will become numerically equal to the measured current in amperes.

Fig. 2

The overall drawing of the Hall element DKhK-0.5A used in the sensor is shown in fig. 2 The Hall voltage, proportional to the control current and magnetic field induction, is measured between the +U and -U terminals. The sensitivity of the element at a nominal value of the control current of 3 mA (flowing into the +I output and flowing out of the -I output) is 280 mV / T. The specified voltage polarity and current direction correspond to the magnetic induction vector B, directed as shown in fig. 2 arrow. Residual output voltage (in the absence of a magnetic field) does not exceed 7 mV Input resistance (between terminals I) - 1,8 ... 3 kOhm, output resistance (between terminals U) - no more than 3 kOhm.

Fig. 3

If there is a Hall element of unknown sensitivity, it can be determined experimentally by placing the element in an air gap of length d of any magnetic circuit on which a known number of turns W of any wire is wound. A control current source is connected to the "current" terminals of the element, and a millivoltmeter is connected to the other two. A direct current I is passed through the winding. Sensitivity (mV / T) is the quotient of dividing the millivoltmeter readings by the magnetic induction, calculated by the formula

The current sensor circuit is shown in fig. 3 The magnetic system is shown on it as a transformer T1, in the gap of the magnetic circuit of which the Hall element B1 is inserted. The amplifier is assembled on the op-amp DA1 and transistors VT2, VT3. The current stabilizer on the transistor VT1 sets the control current flowing through the Hall element.

To power the sensor, a bipolar DC voltage source +/-15 V is required. The main consumer of its energy is the winding II of the T1 transformer. In the described design, the windings are wound on a ferrite ring from a computer power supply. Winding II - 1000 turns of PEV-2 wire with a diameter of 0.15 mm. A winding of 1 - 10 turns of an insulated mounting wire with a cross section of 0,35 mm2 is wound on top of it. An air gap 2 mm long is made in the ring - it is equal to the thickness of the Hall element glued into the gap.

Fig. 4

It should be noted that the magnetic circuit does not have to be ferrite, it can be made of any ferromagnetic material. The optimal cross-sectional area of ​​the magnetic circuit is 10...12 mm2. One should not strive to increase the cross section. This will lead to an increase in the length of the turns of the compensation winding and, consequently, its resistance For the same reason, a wire of the largest possible diameter should be chosen for the compensating winding.

Fig. 5

The fabricated sensor is shown in Fig. 4, and its transfer characteristic - in fig. 5 It was taken when measuring a sinusoidal current with a frequency of 50 Hz. The effective values ​​of current and voltage are plotted along the axes of the graph. There was no resistor R4 in the device. which provided a current-to-voltage conversion ratio of 1 V/A, constant in the range of values ​​of the measured current 0,25 ... 6 A.

The violation of the linearity of the characteristics at low current is explained by the fact that the power amplifier on transistors VT2 and VT3 operates in class B without initial bias. The reason for the non-linearity at high current values ​​is the signal limitation in the K140UD7 op-amp, as a result of which the shape of the compensating current no longer matches the shape of the measured one and there is no full compensation of magnetic fluxes in the magnetic circuit.

By installing the same resistor R3 in parallel with resistor R4, it was possible to make the characteristic linear when measuring current up to 10 A. However, the conversion coefficient decreased to 0,5 V / A.

Author: N. Salimov. Revda city, Sverdlovsk region; Publication: radioradar.net

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