Voltage and current measurements
Figure 1: Voltage measurement
Voltage and current measurements
When measuring a voltage or a current, the quantity to be measured is converted into a ratio of the respective unit of measurement. The voltage is thus displayed as a multiple (or even fractions of) one volt. Current is measured in amperes.
In a voltage measurement, the measuring device is connected in parallel to a load (for example, a resistor) or a voltage source. The internal resistance of the measuring device should be as high as possible so that the measuring device does not falsify this measurement unnecessarily. This is because the measuring device is now parallel to the resistor and also draws some current. For the voltage source, this means that it is loaded with the total current (resistance and measuring device) and thus the voltage value changes, even if sometimes only slightly.
In practice, the smaller the voltage to be measured, the greater the distorting influence of the measuring device.
For certain voltage values that are measured, however, the measuring accuracy is of secondary importance. This usually applies to operate voltages: for example, the electronics do not care whether the operating voltage is only 4.95 or 5.05 volts instead of 5 volts. However, there are also voltages that must be adjusted as precisely as possible. This is especially true for reference voltages, for example for analog/digital converters, which should be adjusted to an accuracy of a thousandth of a volt.
With older, analog measuring instruments, it must be ensured that the measuring range is correctly selected for the expected result. If the voltages are unknown, start with the largest measuring range and reduce it until the analog display shows up to about 2/3 of full scale. Only now is the measurement result accurate!
Figure 2: The R&S® Scope Rider is a handy oscilloscope with direct voltage measurement capability.
(Courtesy of Rohde & Schwarz)
Modern measuring instruments usually have a digital display of the measurement result. They also automatically switch the measuring range if the voltage to be measured requires it. Although this is convenient, it also has a disadvantage: even if there is no voltage at all at the measuring point, the meter will display something: usually a voltage in the lower millivolt range caused by the smallest creeping currents. Unfortunately, often the symbol indicating the measuring range (mV) is also very small. So you should take a closer look at what the meter shows.
Voltage measurement with an oscilloscope
Each oscilloscope is also suitable for measuring DC or AC voltages directly on the screen. The oscilloscope must be switched to the mode in which DC voltages are also displayed. For this purpose, a suitable scale is selected in which this DC voltage draws a horizontal line on the screen. The scale (for example 1 Volt per scale line) is then the measure for the magnitude of the voltage: 5 scale lines are then exactly 5 Volts. When measuring the AC voltage, however, this is how the peak voltage US is measured. The effective voltage Ueff would then have to be calculated. For a sinusoidal voltage is considered an approximation:
|Ueff ≈ 0,707· US||(1)|
There are also multifunctional measuring devices which, for example, can also use the analog/digital converter, which is required in the device anyway, for direct voltage measurement (see Figure 2). This is very practical because if you measure with the instrument, for example, on the turntable of a radar antenna on a high tower, you might save yourself the trouble of climbing down to get another instrument.
Figure 3: Current measurement
When measuring current, the measuring instrument is usually connected in series in the circuit. This means that the circuit is opened and the measuring device is inserted at this point. Here the measuring device should have the lowest possible internal resistance to influence the total resistance as little as possible, i.e. not to falsify the measurement result.
For this reason, an extremely small serial measuring resistor is usually used in the measuring device. The measuring device than measures only the voltage drop across this small resistor and converts the result into a current according to Ohm's law.
For very large currents another measuring principle can also be used: the so-called current clamp. It also measures the current only indirectly, because it measures the magnetic field created by the current flow in the conductor and converts this into a current intensity. However, it can only be used if a single conductor can be covered with the clamp. A multicore cable causes an incorrect measurement.