When assembling, adjusting and testing devices, it is required to measure the voltage, magnitude and frequency of the current in the electrical circuits of the device, as well as resistance, capacitance, etc. These measurements are made using so-called electrical measuring instruments - ammeters, voltmeters, ohmmeters, frequency meters, etc. To measure voltage and magnitude of a direct current are used by some devices, to measure voltage and magnitude of alternating current - others. There are devices suitable for measuring voltage and magnitude of both direct and alternating current.
Any measuring device, including an electrical measuring device, no matter how perfect it is, can not absolutely accurately show the value of the measured quantity. The difference between the reading of the device and the actual value of the measured value is called the absolute error, or measurement error. The error in the instrument readings obtained under so-called normal conditions (temperature 20 ° ± 5 ° C, pressure 760 mm Hg, etc.) and when tapping on the instrument (to eliminate the effect of friction on the reading of the instrument) is called the main error device. The basic error, expressed as a percentage of the instrument's scale range, is called the instrument's accuracy class. Electrical measuring devices are manufactured in the following accuracy classes: 0.2; 0.5; one; 1.5; 2.5, etc. The accuracy class is indicated on the instrument dial. If, for example, a voltmeter of accuracy class 0.5 has a scale from 0 to 50 V, then its main error is not
exceeds
To measure the magnitude of the current, ammeters, milliammeters and microammeters are used, depending on how large the measured current is. One milliampere (abbreviated as mA or mA) is equal to one thousandth, and one microampere (abbreviated to μA or μA) is one millionth of an ampere.
When measuring currents of greater magnitude than those for which the ammeter is designed, shunts are used. Shun t is a resistance that is connected in series in the circuit of the measured current. Parallel to the resistance of the shunt connect the clamps of the ammeter A (Fig. 1); so that a smaller part of the measured current passes through the ammeter, the resistance of the shunt must be less than the resistance of the ammeter.
Figure: 1. Scheme of switching on an ammeter (without and with a shunt) and a voltmeter (without additional resistance and with additional resistance).
1 - shunt; 2 - additional resistance; I - measured current; Ish is the current flowing through the shunt; Ia - current flowing through the ammeter
The shunt resistance is chosen depending on how much of the current they want to pass through the ammeter.If it is necessary that a current pass through the ammeter that is n times less than the measured current, then the shunt resistance
where RА is the resistance of the ammeter.
When using a shunt, the measured current (see Fig. 1)
I = nIA
where IА is the ammeter reading (the amount of current flowing through the ammeter).
The voltage, depending on its magnitude, is measured with voltmeters or millivoltmeters. One millivolt (abbreviated as mV or mV) equals one thousandth of a volt. To measure AC voltage, a so-called lamp voltmeter is often used, which has several measurement ranges and has a large internal resistance. The internal resistance of the voltmeter must be much greater than the resistance of the circuit section where the voltage is measured so that the voltmeter does not affect the distribution of currents and voltages in the electrical circuit.
If this voltmeter needs to measure a voltage that exceeds the highest voltage that can be measured by it, that is, exceeds the upper measurement limit of the device, then a so-called additional resistance is connected in series with the voltmeter. In this case, a voltmeter V with an additional resistance is connected in parallel to those points of the circuit, the voltage between which must be measured (see Fig. 1). Additional resistance is necessary so that the current passing through the device does not exceed the permissible value. The value of the additional resistance is determined by the formula
RД = RB (m-1),
where RД is the value of additional resistance;
RВ - resistance of the voltmeter;
m is a number that shows how many times the measured voltage exceeds the highest voltage that can be measured with this voltmeter without additional resistance.
When using an additional resistance, the measured voltage
U = mUB
where UB is the reading of the voltmeter.
Often, to measure voltage and current, as well as the magnitude of resistance, combined electrical measuring devices, called testers, are used.
"Basics of assembling devices" N. Safonov
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