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Brief theoretical information

Instruments program Multisim require no configuration as it automatically change the measuring ranges. However, real voltmeters and ammeters usually have one limited range. Therefore there is a need to expand the range of their work, creating, thus, multirange electrical measuring instruments

In the same model circuit program Multisim is possible to use several similar devices to control current and voltage simultaneously.

Voltmeters used to measure AC and DC voltages. They are included in parallel circuit in which you want to measure the voltage. The polarity of the DC voltmeter indicated on their housing. If it does not match the actual polarity of the voltage on the controlled section of the schema, in the window of the voltmeter to the left of the digital value of the voltage appears a minus sign.

If you double click LMB on the image of the voltmeter, a dialog box appears which displays the settings for this device:

1) the value of the input resistance;

2) type of measured voltage (voltmeter mode).

The parameters in the dialog box, the device can be changed. The digital value of the input resistance of the voltmeter can be changed using the keypad by clicking the LMB derived voltage values from μOhm to TOhm. Change mode using LMB, using the drop-down list of modes of the voltmeter.

When measuring AC voltage sine wave (AC) the voltmeter will show the current value of the voltage U which can be defined by the formula

U = Um / ,

where Um is the amplitude of the voltage.

The input impedance of the voltmeter, the default value is 10 MOhm, and in most cases can not be ignored in the analysis of the simulated schemes. Due to the fact that the resistance of real voltmeters often is of far less importance, there is the necessity of considering the input resistance of such devices.

To extend the measuring range of voltmeters use an incremental resistance which decreases the part of the measured voltage that allows to achieve the desired objective.

Ammeters in the program Multisim can be used to measure AC and DC. To do this, they are involved in a chain sequentially. The polarity of the DC ammeters indicated on their housing. In case of a mismatch it with the real direction of current in the controlled circuit by the circuit model in the window of the ammeter to the left of the digital value of the current appears a minus sign.

If you double click LMB on the image of the ammeter, a dialog box appears which shows the options of this device:

1) the internal resistance values;

2) the kind of the measured current (ammeter mode).

The digital value of the internal resistance of the ammeter can be changed using the keypad by clicking the LMB derived units of measure current from pOhm to TOhm. Change mode using LMB, using the drop down list of modes of the ammeter.

When measuring current sinusoidal (AC) ammeter will show the effective value current I , which can also be defined by the formula

I = Im / ,

where Im is the amplitude value of the current.



The internal resistance of the ammeter, the default value is 1 nOhm and in most cases can not be ignored in the analysis of the simulated schemes. Due to the fact that the real resistance of ammeters is often a much larger value, there is a necessity of taking into account the internal resistance of such devices.

To expand the measurement range ammeters use a shunt, which runs part of the measured current, which gives the opportunity to expand the limits of measurement of these electrical appliances.

Depending on the method of obtaining the results of all measurements divided:

1) to direct, for example, the current measurement by the ammeter;

2) indirectly, for example by measuring the voltage drop across the resistor and determine the current through it using Ohm's law;

3) overall, involving the use of groups for direct and indirect measurements, e.g. determination of temperature coefficient of resistance material on the basis of measuring its resistance at different temperatures.

There are also zero and the differential methods. Zero methods of electric measurements are accurate overhead and compensation methods. An even higher measurement accuracy can be obtained using the differential method in which the measured value is balanced by a known not completely, and then measure the difference of these values.

Measurement errors are divided into absolute, relative and given.

Absolute error is the difference between the measured and actual values of the controlled quantities:

∆A=Ameas - A.

For example, the readings of the ammeter is 10 A, and the actual value of the current obtained using the exemplary devices, theoretically either the aggregate method, equal to 9.8 A. Then ∆A= 10-9,8=0.2 A.

 

Typically, the measurement accuracy estimate not an absolute but a relative error.

Relative error is the percentage ratio of absolute error to the actual value of the measured quantity:

δ0 = (∆A / A) ∙ 100 % = (0,2 / 9,8) ∙ 100 % = 2,041 %.

Because of the inconvenience of the use of relative error to assess the accuracy of actually showing the electrical appliances it is customary to use reduced error.

Given uncertainty is the ratio of absolute error to the nominal value corresponding to the maximum instrument reading:

δR = (∆A / Anom) ∙ 100 %.

If in the above example, the measurement limit of the ammeter equal to 10A, the error of the measurement

δR = (0,2 / 10) ∙ 100 % = 2 %.

Given the error introduced by the instrument itself, is called main. Additional error due to the influence of changes in ambient temperature relative to normal (20 ℃), changes in the magnitude, frequency and shape the voltage and current, exposure to electromagnetic fields, the device's orientation in space, etc.

The designation of the accuracy class of the instrument is acceptable basic error of the instrument belonging to this class: 0,05; 0,1; 0,2; 0,5; 1; 1,5; 2,5; 4.

The principle of the devices is divided into magnetoelectric, electromagnetic, electrodynamic, ferrodinamic, induction, electrostatic, thermal, electronic continuous and digital, etc.


Date: 2018-08-27; view: 171


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Examples of modeling schemes | Experiment 1. Measuring the resistance of resistors
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