The main parameters characterizing the mode of the electrical circuit are current and voltage.
Current is measured by the amount of electricity (the amount of electrical charges) passing through the cross-section of the conductor in 1 s.
A current of 1 A will flow if 1 Kl of electricity flows through the cross-section of the conductor for 1 s.
In electronics, as a rule, they deal with relatively small currents, therefore, derived units are used for them: milliampere 1 mA = O, 001 A and microampere 1 μA = 0.000001 A.
To measure the current with an ammeter, it is sequentially included in the circuit under study (Fig. 1, a).
Fig 1. Measurement of current with an ammeter (a) and voltage with a voltmeter (b)
Thus, the main current path is broken and an ammeter is included in this gap.
With the same number of electrons passing through the cross-section of the conductor, the work they do can be different, since the energy of the electrons is determined by the electric field, which creates the directional movement of the electrons, and an electric current flows.
The intensity of electric fields is determined by voltage.
There is a voltage of 1 V between two points if 1 J work is done to transfer 1 C of electricity.
A voltmeter is used to measure the voltage. To do this, it is connected in parallel to two points of the electrical circuit without breaking the main circuit (Fig. 1, b).
All conductors have a certain resistance to the passage of electric current through them.
A resistance of 1 Ohm has such a conductor through which a current of 1 A passes if a voltage of 1 V is applied to its ends.
The longer and thinner the conductor, the greater its resistance. In addition, the resistance depends on the material. For example, the resistance of a copper wire with a cross section of 1 mm 'and a length of 57 m is 1 Ohm; copper wire with a diameter of 0.1 mm and a length of 10 m - about 20 ohms; incandescent filament of an electric lamp with a power of 40 W - 1000 Ohm.
In direct current circuits, to measure voltage and current, the dial gauges of the magnetoelectric system are used.
In such devices, the measured current flows through a light coil placed in the field of a permanent magnet and capable of turning under the action of the passing current. The current is supplied through spiral springs that create a counter torque.
The main advantage of the measuring instruments of the magnetoelectric system is the linear scale and high measurement accuracy. However, devices of the magnetoelectric system can only be used for measuring in DC circuits.
Since the ammeter is connected in series to the circuit, in order not to distort the measurement results, its own resistance must be small. The voltmeter, on the contrary, is switched on in parallel, so that a significant current does not pass through it, its own resistance must be large.
The internal resistance of the voltmeter of the magnetoelectric system is 1000 Ohm per 1 V, and of special voltmeters - up to 20 000 Ohm nc 1 V scale.
Therefore, the internal resistance of 20,000 Ohm per 1 V corresponds to its own consumption current equal to 0.00005 A, or 50 μA. A measuring device designed for such a current can be universal: to measure high currents, shunts are used, which are connected in parallel with the device, and various additional resistors are used to measure voltages in series with the measuring head. Thus, you can create a universal combined device suitable for measuring both currents and voltages, and by adding a rectifier to it, it can be used for measurements in AC circuits.
Low frequency self-excitation amplifier
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