قياس التيار المتردد - المقاومات - القدرة - القياسات في الدارات الالكترونية

AC measurement - Resistors - Power -
Measurements in electronic circuits .. Electronics

AC measurement

The moving coil cannot respond to AC unless we add a current rectifier (254) to the measuring device that we saw in Figure (250). This results in the AC being converted into a pulsed DC current. The device gives the "root mean square value" or the effective value of the current, and this is what we need to know: the effective value is the value of the current that gives the same thermal effect or power that we get by direct current.

Also resistors

We may need to measure the resistors used in the circuit.

For this, a battery, a variable resistor mm, a fixed resistor m and a measuring device (255) are used in series. Then the ends of the meter are connected together and the variable resistance is adjusted until the device pointer deviates to the end of the scale (full reading). This represents zero resistance. If we open the circuit, i.e. separate the ends of the meter from each other, we notice that the current has disappeared and the reading is equal to zero, which corresponds to infinite resistance.

When we connect the measuring ends to a specific resistance, or to a circuit that has been tested, the indicator deviates to a position that indicates the value of the current passing through the resistance, and we can, based on Ohm's law, make the device reading in resistance units directly.

254 - Using the rectifier in measuring devices.

255 - Circuit for measuring resistance.

"Bridges" are used to measure precision resistances (256). It is known that the state of equilibrium is characterized by the following equation:

M A / M B = M S / M G, and in this case the device reading is yellow as M represents the resistance to be measured and M represents a variable standard resistance or a resistance box in which the required resistance is selected using switches for this purpose. As for the two resistors M A and M B, they are resistors with a known value, and thus the value of M can be found with the utmost accuracy when we choose the M value that makes the device reading zero.

Power is also measured in watts and is equal to the product of the square of the current intensity by the resistance. It is possible to measure the power in a resistor connected in series with a measuring device (257). If the resistance value is
M = 50 ohms and the current intensity is T = 2 amperes, the power becomes 2 × 2 × 50 = 200 watts. Sometimes, it is easy to measure the voltage drop across the resistor and then calculate the lost power. If the resistance value is M = 2 ohms, which represents the value of the receiver loudspeaker resistance and the voltage drop value across it is G = 3 volts, the power becomes: G / M = 2/3 - 2/9 watts.

That is, the power of the audio device is 4.5 watts.

DC current measuring devices are used in the case of circuits that operate on direct current. However, in radio frequency, audio, or alternating current circuits, the appropriate measuring device must be used each time. There are commonly used measuring devices that have more than one standard scale and the ability to measure direct current and alternating current and also to measure resistances, by using special keys to select the type of measurement required (258).

256 - Bridge measurement circuit.

257 - Power measurement in watts.

258 - Multi-range meter.