1. Hall device
Hall device is a kind of magnetoelectric converter made of semiconductor materials. If the control current IC is connected to the input end, when a magnetic field B passes through the magnetic sensing surface of the device, Hall potential VH appears at the output end. As shown in Figure 1-1.
The magnitude of Hall potential VH is proportional to the product of control current IC and magnetic flux density B, that is, VH = khicbsin Θ
The Hall current sensor is made according to the principle of Ampere’s law, that is, a magnetic field proportional to the current is generated around the current carrying conductor, and the hall device is used to measure this magnetic field. Therefore, the non-contact measurement of current is possible.
Indirectly measure the current of current carrying conductor by measuring the Hall potential. Therefore, the current sensor has undergone electrical magnetic electrical isolation conversion.
2. Hall DC detection principle
As shown in Figure 1-2. Because the magnetic circuit has a good linear relationship with the output of the hall device, the voltage signal U0 output by the hall device can indirectly reflect the size of the measured current I1, that is, I1 ∝ B1 ∝ U0
We calibrate U0 to be equal to 50mV or 100mV when the measured current I1 is the rated value. This makes hall direct detection (no amplification) current sensor.
3. Hall magnetic compensation principle
The primary main circuit has a measured current I1, which will generate magnetic flux Φ 1. Magnetic flux generated by current I2 passed by secondary compensation coil Φ 2 maintain magnetic balance after compensation, and the hall device is always in the role of detecting zero magnetic flux. So it is called Hall magnetic compensation current sensor. This advanced principle mode is superior to the direct detection principle mode. Its outstanding advantages are fast response time and high measurement accuracy, which is especially suitable for the detection of weak and small current. The principle of Hall magnetic compensation is shown in Figure 1-3.
Figure 1-3 shows: Φ 1＝ Φ two
I1N1＝I2N2
I2＝NI/N2·I1
When the compensation current I2 flows through the measuring resistance RM, it is converted into voltage at both ends of RM. As a sensor, measure the voltage U0, that is, U0 = i2rm
According to the principle of Hall magnetic compensation, a current sensor with rated input from to series specifications is made.
Because the magnetic compensation current sensor must be wound with thousands of turns of compensation coil on the magnetic ring, the cost increases; Secondly, the working current consumption also increases correspondingly; However, it has the advantages of higher accuracy and fast response than direct inspection.
4. Magnetic compensation voltage sensor
In order to measure the small current of Ma level, according to Φ 1 = i1n1, increasing the number of turns of N1 can also obtain high magnetic flux Φ 1。 The small current sensor made by this method can measure not only Ma level current, but also voltage.
Different from the current sensor, when measuring voltage, the multi turn winding on the primary side of the voltage sensor is connected in series with a current limiting resistor R1, and then connected in parallel to the measured voltage U1 to obtain the current I1 proportional to the measured voltage U1, as shown in Figure 1-4.
The principle of the secondary side is the same as that of the current sensor. When the compensation current I2 flows through the measuring resistance RM, it is converted into voltage at both ends of RM as the measuring voltage U0 of the sensor, that is, U0 = i2rm
5. Output of current sensor
The direct detection (non amplification) current sensor has a high impedance output voltage. In application, the load impedance should be greater than 10k Ω. Usually, its ± 50mV or ± 100mV suspended output voltage is amplified to ± 4V or ± 5V with a differential input proportional amplifier. Figure 5-1 shows two practical circuits for reference.
(a) The figure can meet the general accuracy requirements; (b) The graph has good performance and is suitable for occasions with high accuracy requirements.
The direct detection amplified current sensor has a high impedance output voltage. In application, the load impedance should be greater than 2K Ω.
Magnetic compensation current, voltage magnetic compensation current and voltage sensors are current output type. It can be seen from figure 1-3 that the “m” end is connected to the power supply “O”
The terminal is the path of current I2. Therefore, the signal output from the “m” end of the sensor is a current signal. The current signal can be transmitted remotely in a certain range and the accuracy can be guaranteed. In use, the measuring resistance RM only needs to be designed on the secondary instrument input or terminal control panel interface.
In order to ensure high-precision measurement, attention should be paid to: ① the accuracy of measurement resistance is generally selected as metal film resistance, with an accuracy of ≤± 0.5%. See Table 1-1 for details. ② the circuit input impedance of secondary instrument or terminal control board should be more than 100 times greater than the measurement resistance.
6. Calculation of sampling voltage and measuring resistance
From the previous formula
U0＝I2RM
RM＝U0/I2
Where: U0 – measured voltage, also known as sampling voltage (V).
I2 – secondary coil compensation current (a).
RM – measure resistance (Ω).
When calculating I2, the output current (rated effective value) I2 corresponding to the measured current (rated effective value) I1 can be found out from the technical parameter table of the magnetic compensation current sensor. If I2 is to be converted into U0 = 5V, see Table 1-1 for RM selection.
7. Calculation of saturation point and * large measured current
It can be seen from figure 1-3 that the circuit of output current I2 is: v+ → Collector Emitter of final power amplifier → N2 → RM → 0. The equivalent resistance of the circuit is shown in Figure 1-6. (the circuit of v- ~ 0 is the same, and the current is opposite)
When the output current i2* is large, the current value will no longer increase with the increase of I1, which is called the saturation point of the sensor.
Calculate according to the following formula
I2max＝V+-VCES/RN2+RM
Where: V + – positive power supply (V).
Vces – Collector Saturation Voltage of power tube, (V) is generally 0.5V.
RN2 – DC internal resistance of secondary coil (Ω), see table 1-2 for details.
RM – measure resistance (Ω).
It can be seen from the calculation that the saturation point changes with the change of the measured resistance RM. When the measured resistance RM is determined, there is a definite saturation point. Calculate * large measured current i1max according to the following formula: i1max = i1/i2 · i2max
When measuring AC or pulse, when RM is determined, calculate * large measured current i1max. If i1max value is lower than the peak value of AC current or lower than the pulse amplitude, it will cause output waveform clipping or amplitude limiting. In this case, choose a smaller RM to solve.
8. Calculation example:
Example 1
Take the current sensor lt100-p as an example:
(1) Measurement required
Rated current: DC
*High current: DC (overload time ≤ 1 minute / hour)
(2) Look up the table and know
Working voltage: stabilized voltage ± 15V, coil internal resistance 20 Ω (see table 1-2 for details)
Output current: (rated value)
(3) Required sampling voltage: 5V
Calculate whether the measured current and sampling voltage are appropriate
RM＝U0/I2＝5/0.1＝50（Ω）
I2max＝V+－VCES/RN2＋RM＝15－0.5/20＋50＝0.207（A）
I1max＝I1/I2·I2max＝100/0.1 × 0.207＝207（A）
It is known from the above calculation results that the requirements of (1) and (3) are met.
9. Description and example of magnetic compensation voltage sensor
Lv50-p voltage sensor has the primary and secondary electrical resistance ≥ 4000vrms (50hz.1min), which is used to measure DC, AC and pulse voltages. When measuring the voltage, according to the voltage rating, a current limiting resistor is connected in series at the primary side + HT terminal, that is, the measured voltage gets the primary side current through the resistor
U1/r1 = I1, R1 = u1/10ma (K Ω), the power of the resistance should be 2 ~ 4 times greater than the calculated value, and the accuracy of the resistance should be ≤± 0.5%. R1 precision wire wound power resistor can be ordered by the manufacturer.
10. Wiring method of current sensor
(1) The wiring diagram of direct inspection (no amplification) current sensor is shown in Figure 1-7.
(a) The figure shows p-type (printed board pin type) connection, (b) the figure shows C-type (socket plug type) connection, vn VN represents Hall output voltage.
(2) The wiring diagram of direct inspection amplified current sensor is shown in Figure 1-8.
(a) The figure is p-type connection, (b) the figure is C-type connection, in which U0 represents the output voltage and RL represents the load resistance.
(3) The wiring diagram of magnetic compensation current sensor is shown in Figure 1-9.
(a) The figure shows p-type connection, (b) the figure shows C-type connection (note that the third pin of the four pin socket is an empty pin)
The printed board pin connection method of the above three sensors is consistent with the arrangement method of the real object, and the socket plug connection method is also consistent with the arrangement method of the real object, so as to avoid wiring errors.
On the above wiring diagram, the measured current I1 of the main circuit has an arrow in the hole to show the positive direction of the current, and the positive direction of the current is also marked on the physical shell. This is because the current sensor stipulates that the positive direction of the measured current I1 is of the same polarity as the output current I2. This is important in three-phase AC or multi-channel DC detection.
11. Working power supply of current and voltage sensor
The current sensor is an active module, such as hall devices, operational amplifiers and final power tubes, which all need working power supply and power consumption. Figure 1-10 is a practical schematic diagram of a typical working power supply.
(1) The output ground terminal is centrally connected to the large electrolysis for noise reduction.
(2) Capacitance bit UF, diode 1N4004.
(3) The transformer depends on the power consumption of the sensor.
(4) The working current of the sensor.
Direct inspection (no amplification) power consumption: * 5mA; Direct detection amplification power consumption: * large ± 20mA; Magnetic compensation power consumption: 20 + output current* Large consumption of working current 20 + twice the output current. The power consumption can be calculated according to the consumed working current.
12. Precautions for the use of current and voltage sensors
(1) The current sensor must properly select products of different specifications according to the rated effective value of the measured current. If the measured current exceeds the limit for a long time, it will damage the end pole power amplifier tube (referring to the magnetic compensation type). Generally, the duration of twice the overload current shall not exceed 1 minute.
(2) The voltage sensor must be connected with a current limiting resistor R1 in series on the primary side according to the product instructions, so that the primary side can get the rated current. In general, the duration of double overvoltage shall not exceed 1 minute.
(3) The good accuracy of the current and voltage sensor is obtained under the condition of the primary side rating, so when the measured current is higher than the rated value of the current sensor, the corresponding large sensor should be selected; When the measured voltage is higher than the rated value of the voltage sensor, the current limiting resistance should be readjusted. When the measured current is less than 1/2 of the rated value, in order to obtain good accuracy, the method of multiple turns can be used.
(4) Sensors with 3KV insulation and withstand voltage can work normally in AC systems of 1kV and below and DC systems of 1.5kV and below for a long time. 6kV sensors can work normally in AC systems of 2KV and below and DC systems of 2.5KV and below for a long time. Be careful not to use them under overpressure.
(5) When used on devices requiring good dynamic characteristics, * it is easy to use a single copper aluminum busbar and coincide with the aperture. Replacing small or more turns with large ones will affect the dynamic characteristics.
(6) When used in high current DC system, if the working power supply is open circuit or faulty for some reason, the iron core will produce large remanence, which is worthy of attention. Remanence affects accuracy. The method of demagnetization is to turn on an AC at the primary side without adding a working power supply and gradually reduce its value.
(7) The anti external magnetic field ability of the sensor is: a current 5 ~ 10cm away from the sensor, which is more than twice the current value of the original side of the sensor, and the magnetic field interference generated can be resisted. When wiring three-phase high current, the distance between phases should be greater than 5 ~ 10cm.
(8) In order to make the sensor work in a good measurement state, a simple typical regulated power supply introduced in Figure 1-10 should be used.
(9) The magnetic saturation point and circuit saturation point of the sensor make it have a strong overload capacity, but the overload capacity is time limited. When testing the overload capacity, the overload current of more than 2 times shall not exceed 1 minute.
(10) The temperature of the primary current bus should not exceed 85 ℃, which is determined by the characteristics of ABS engineering plastics. Users have special requirements and can choose high-temperature plastics as the shell.
13. Advantages of current sensor in use
(1) Non contact detection. In the reconstruction of imported equipment and the technical transformation of old equipment, it shows the superiority of non-contact measurement; The current value can be measured without any change to the electrical wiring of the original equipment.
(2) The disadvantage of using the shunt is that it cannot be electrically isolated, and there is also insertion loss. The larger the current is, the greater the loss is, and the larger the volume is. People also found that the shunt has inevitable inductance when detecting high-frequency and high current, and it can not truly transmit the measured current waveform, let alone non sine wave type. The current sensor completely eliminates the above disadvantages of the shunt, and the accuracy and output voltage value can be the same as that of the shunt, such as accuracy level 0.5, 1.0, output voltage level 50, 75mV and 100mV.
(3) It is very convenient to use. Take an lt100-c current sensor, connect a 100mA analog meter or digital multimeter in series at the M end and the zero end of the power supply, connect the working power supply, and put the sensor on the wire circuit, so that the current value of the main circuit 0 ~ 100A can be accurately displayed.
(4) Although the traditional current and voltage transformer has many working current and voltage levels and has high accuracy under the specified sinusoidal working frequency, it can adapt to a very narrow frequency band and cannot transmit DC. In addition, there is exciting current during operation, so this is an inductive device, so its response time can only be tens of milliseconds. As we all know, once the secondary side of current transformer is open circuit, it will produce high voltage hazards. In the use of microcomputer detection, multi-channel signal acquisition is required. People are looking for a way to isolate and collect signals
Post time: Jul-06-2022