Measuring Voltage

How do you actually measure voltage? There are different ways to do this, depending on the type of signal. You can use a multimeter or use an oscilloscope.

DC voltage

DC voltage is measured with a multimeter. You switch it to "DC", connect the black cable to the minus pole and the red cable to the positive pole. The multimeter now measures the DC voltage the value is shown on the display. It is important here to select a suitable voltage range on the multimeter. The rotary slide needs to be set to a value higher than the maximum voltage to be measured.

AC voltage (sinusoidal)

A sinusoidal, constant alternating voltage can also be measured with the multimeter. To do this, it must be set to "AC". The polarity of the black and red contacts does not matter in this case, since the current alternates, which means if flows "back and forth". The multimeter shows the so-called "RMS" value, the average value of the sine wave. The peak peak value (i.e. from minimum to maximum) can be calculated by multiplying it by 1.41 (square root of 2). Important for measurements with the multimeter is that the vibration is regular. Only in this case the multimeter displays meaningful values. If the waveform of the AC voltage is unknown or has a different shapes (e.g. triangle, rectangle, etc.) you have to use an oscilloscope.

Measuring voltage with an oscilloscope

Any waveform can be measured with the oscilloscope. One connects the signal to a measurement channel, selects the _Trigger , sets the time base and the voltage swing (see oscilloscope usage). Using an oscilloscope it is irrelevant whether there is a sine, triangle or otherwise formed signal.

How does an oscilloscope measure a voltage?

Internally, an oscilloscope switches a circuit via a resistor. This so-called input resistance is depending on the type of oscilloscope. It is at 50 Ohm (high frequency measurements) or a megohm (details see chapter active probe). The voltage drop accros a resistor is represented by the oscilloscope. It is the same mechanism applied by a multimeter. In general, this means that each measurement influences also the circuit under investigation. This becomes more important when the current load from the multimeter or oscilloscope is high. If necessary, active probes must be used to decrease the load.

Voltage measurement in circuit simulators

An oscilloscope probe is also used in circuit simulators for voltage measurement. The picture shows a simulation of a LED strip. In the circuit simulators, symbolic probes are used for sampling the simulated voltage.