Illustration of current measurement method

1. Direct measurement method with ammeter

The method of directly measuring current is usually to insert an ammeter of appropriate range in the path of the current being measured, so that all or part of the current being measured flows through the ammeter. Read the measured current value or measured current shunt value directly from the ammeter.

For the circuit shown in Figure 1(a), the actual value of the measured current is

(1)

In the formula, r0 and rl are the signal source internal resistance and load resistance respectively. r=r0+rl is the current loop resistance.

Figure 1 Measuring current with an ammeter

An ammeter with internal resistance r is connected in series to the circuit, as shown in Figure 1(b). Then the current flowing through the ammeter, that is, the ammeter reading value is

(2)

The relative measurement error is

(3)

It can be seen from the above formula that in order to make the ammeter reading value as close as possible to the actual value of the measured current lx, the internal resistance r of the ammeter is required to be as close to zero as possible. In other words, the smaller the internal resistance of the ammeter, the better.

When it is inconvenient to insert an ammeter in series or there is no ammeter with an appropriate range, the indirect measurement method can be adopted, that is, the current is converted into physical quantities such as voltage, frequency, magnetic field intensity, etc., and the converted quantity is measured directly according to the correspondence between the converted quantity and the measured current. The relationship is used to obtain the current value. Here are several conversion methods for indirectly measuring current.

2. Current-voltage conversion method

You can use a small standard resistor r (called a sampling resistor) to be inserted in series with the measured current loop to convert the measured current into the measured voltage ux.

ux= ·r (4)

When the condition r < r is satisfied, it can be obtained from equation (2) and equation (4)

(5)

If the measured current ix is ​​very large, you can directly use a high-impedance voltmeter to measure the voltage ux across the standard resistor; if the measured current ix is ​​small, ux should be measured. After amplifying to an appropriate value close to the voltmeter range, it is measured by the voltmeter. In order to reduce the measurement error of ux, the amplifier circuit is required to have extremely high input impedance and extremely low output impedance. For this reason, voltage is generally used. Series negative feedback amplifier circuit.

3． Current-magnetic field conversion method

Whether you use an ammeter to measure current directly or use the above conversion method to measure current indirectly, you need to cut off the circuit and connect to the measuring device. When the circuit is not allowed to be cut off or the measured current is too large, the value of the current can be measured indirectly by measuring the magnetic field generated by the current.

Figure 2 is a schematic structural diagram of a clamp-type ammeter using Hall. The function of the cold-rolled silicon steel sheet ring is to concentrate the magnetic field generated by the measured current ix onto the Hall element to improve the sensitivity. The magnetic induction intensity b acting on the Hall piece is

(6)

In the formula, kb is the electromagnetic conversion sensitivity.

The output voltage u0 of the linear integrated Hall chip is

(7)

In the formula, i is the Hall chip control current; kh is the Hall chip sensitivity; k is the ammeter sensitivity, k=khkbi.

If ix is ​​DC, then u0 is also DC; if ix is ​​AC, then u0 is also AC. The maximum current that can be measured by the Hall-type clamp ammeter is more than 100ka. It can be used to measure the current on transmission lines, and can also be used to measure beams, ion beams and other currents that cannot be measured directly with ordinary ammeters. In Figure 2, if the measured current conductor is wound around the silicon steel ring several times, the sensitivity of the ammeter will be reduced several times. In this way, the sensitivity and range of the Hall-type clamp ammeter can be adjusted.

4． galvanic method

In addition to the above methods, the current in the circuit can also be measured without cutting off the circuit. The structure of the current transformer is shown in Figure 3. It is composed of some coils wound on a magnetic ring (or iron core). Assume that the measured current (primary side current) is i1, the number of primary winding turns is n1, and the secondary winding The number of winding turns is n2, then the secondary side current is

Figure 2 Hall-type clamp-type ammeter steel ring;

1-cold rolled silicon; 2-measured current pin wire; 3-Hall element; 4-Hall element

Figure 3 Current transformer

It can be seen that as long as the secondary side current i2 is measured, the size of the measured current (primary side current) can be known.

Since the number of turns of the secondary winding of the current transformer is much larger than the number of turns of the primary winding, the secondary side must not be open-circuited during use. Otherwise, the primary side current will completely turn into the excitation current, and the iron core will reach a highly saturated state, causing the iron core to become seriously heated and cause serious damage to the core. A very high voltage is generated on the secondary side, causing thermal damage and electrical breakdown of the transformer, causing personal injury. In addition, for personal safety, one end of the secondary winding of the transformer must be reliably grounded (safety grounded).

The output of the current transformer is current. When measuring, the secondary winding of the transformer is connected to a resistor r, and the voltage is obtained from r and connected to an amplifier or AC-DC converter. The size of r is determined by the capacity of the transformer (generally commonly used current transformers The device is 10v·a or 5v·a), the output voltage u0 on r is

(8)