When analysing the operation of electrical and electronic circuits, or trying to understand why a circuit does not work as expected, eventually you will need to use a Voltmeter to measure the various voltage levels. Voltmeters, which are used to measure voltage, can be either digital or analogue or even a part of a digital multimeter, which is now more frequently used. They come in a variety of sizes and shapes.

Voltmeters can also be used to measure Both sinusoidal AC and DC voltages can be introduced into a circuit, but the steady state operation of the circuit can be hampered by the addition of a voltmeter as a measuring instrument.

How Do You Measure Voltage?

The potential or voltage difference between the positive and negative terminals that are placed in contact with specific locations in a conducting electrical circuit is measured in many applications by the use of a voltmeter, a hand-held device. Since voltage is characterized as the potential difference between two points in a conducting circuit, many circuits also include a built-in voltage measuring capability. Any electrical device must have voltage because it is the electrical difference between two points in a circuit that carries a current of one amp and a watt of work energy.

When measuring voltage, if the readout shows a positive value, the voltage measurement device is showing the true nature of current flow within the circuit, from the positive lead through the voltage measurement device to the negative lead and back into the circuit. The positive and negative leads are inverted and current is actually flowing in the opposite direction if the voltmeter reads a negative value. Voltage measurement is strictly speaking a variable measurement and not an accurate representation of actual current flow, regardless of how the leads of a voltmeter are positioned at various points in a circuit. Voltage measurement only indicates a potential difference between two points in a circuit.

In a circuit, current moves from a positive potential to a negative potential. Voltmeter readouts show this as a positive value. As a result of the counterbalancing effect of current flow, it is also true that the actual physical flow of electrons in circuitry is from negative to positive. Current flow is frequently confused with voltage measurement, but current flow is actually measured in amperes, whereas voltage can be thought of as a moment in time when there is an electrical potential difference between two points in a circuit.

In order to measure the voltage through the device, a voltage measuring device must also temporarily reroute some of the current in a circuit. While the voltmeter is attached, this changes the circuit’s actual voltage. However, the majority of voltmeters are designed to only slightly affect the performance of the circuit during such measurements.

Oscilloscopes and computer analog-to-digital converter (A/D) cards are examples of additional external and internal circuit devices that measure voltage. Multimeters are devices that measure more than just voltage, including combined voltage and current measurements, resistance of a circuit, and other measurements. In the case of multimeters, many contemporary versions of voltage-measuring instruments are digital, offer a single discrete readout value, and are referred to informally as digital multimeters (DMMs).

Early voltmeters had graduated gauges with needles that would move up and down in response to changes in voltage. These early voltmeters were constructed on the analog principle of continuous measurement of a voltage value. They resemble the readout of an oscilloscope, which creates a graphical display of the continuous measurement of voltage over a predetermined time period, in this regard to some extent. Using A/D converters, computers can measure voltage and convert the result to a binary signal that can be processed by the computer. These devices also convert the actual input voltage from an analog signal into a digital value that can be analyzed by computer software.

How To Make A Dc Voltage Measurement

Although a lot of sensors produce DC voltages that can be measured with a multimeter or data acquisition tool, the main focus of this white paper is to look at general DC measurements that don’t require an intermediary sensor setup.

Voltage Measurement Fundamentals

Understanding the context of how the measurement is made is crucial to learning how to measure voltages. Voltage, in an electrical circuit, is essentially the difference in electrical potential between two important points. But how the measurement reference point is established is frequently a source of confusion. The voltage level that the measurement is referenced to is known as the measurement reference point.

Reference Point Methods

There are essentially two ways to measure voltages: differential and ground referenced.

Ground Referenced Voltage Measurement

One method is to measure voltage with respect to a common, or a “ground” point. Oftentimes, these “grounds” are stable and unchanging and are most commonly around 0 V. Historically, the term ground originated from the usual application of ensuring the voltage potential is at 0 V by connecting the signal directly to the earth.Ground referenced input connections are particularly good for a channel that meets the following conditions:

• The input signal is high-level (greater than 1 V)
• The leads connecting the signal to the device are less than 10 ft (3 m)
• The input signal can share a common reference point with other signals

Either the measuring device or the external signal being measured provides the ground reference. This configuration is referred to as ground referenced single-ended mode (RSE) when the device supplies the ground, and non-referenced single-ended mode (NRSE) when the signal does so.

Similar pin configurations are available for analog input measurements on the majority of instruments. The example that follows uses a CompactDAQ chassis and an NI 9205 analog input module to demonstrate this kind of measurement (see Figure 1).

An NI cDAQ-9178 chassis with an NI 9205 is used to measure RSE voltage, and the pinouts for the module are shown in Figure 2. In Figure 2, Pin 1 corresponds to the “Analog Input 0” channel and The common ground is represented by pin 17.

The connection scheme for NRSE voltage measurements with a cDAQ-9178 and an NI 9205 is shown in Figure 3. In the figure, Pin 1 corresponds to the “Analog Input 0” channel and Pin 35 corresponds to the “Analog Input Sense” channel. The ground voltage supplied by the signal can be detected by this channel, specifically for NRSE measurements.

Differential Voltage Measurement

Another way to measure voltage is to determine the “differential” voltage between two separate points in an electrical circuit. For instance, you would measure the voltage at the resistor’s two ends in order to determine the voltage across a single resistor. The resistor’s voltage across is the difference between the voltages. Typically, differential voltage measurements are helpful for figuring out the voltage across specific circuit components or whether noisy signal sources are present.

Differential input connections are particularly well-suited for a channel that meets any of the following conditions:

• The input signal is low-level (less than 1 V)
• The leads connecting the signal to the device are greater than 3 m (10 ft)
• The input signal requires a separate ground reference point or return signal
• The signal leads travel through noisy environments

The connection diagram for differential voltage measurements using a cDAQ- 9178 and an NI 9205 is shown in Figure 4. In the figure, Pin 1 corresponds to the “Analog Input 0” channel and Pin 19 corresponds to the “Analog Input 8” channel.

When operating in differential mode, the analog pin facing the analog channel carrying the positive signal is wired with the negative signal. For example, “Analog Input 0” would be connected to positive, and “Analog Input 8” would be connected to the negative signals, and “Analog Input 1” for positive and “Analog Input 9” for negative and so on. The drawback of differential mode is that it effectively cuts the number of measurement channels for analog input in half.

Permanent Magnet Moving Coil Meter Construction

The amount by which the electromagnetic coil moves, called “deflection”, is proportional to the strength of current flowing through the coil needed to produce the magnetic field required to deflect the needle.

Typically, a pointer, or needle, is connected to the coil so that as the coil moves, it deflects the pointer over a linear scale to show the value being measured, with the deflection angle being proportional to the input current. As a result, a galvanometer’s pointer moves in response to current.

When no current is flowing through the coil, thin helical watch movement type damping springs are frequently used to control the angle of deflection and prevent oscillations or rapid movements that could harm the pointer. They also maintain the coil’s movement at rest.

In most cases, the pointer moves between zero on the left and full-scale deflection (FSD) at the extreme right of the scale. There are some meter movements that allow for pointer movement in both directions and have a spring-centered pointer with the zero rest position in the middle of the scale. For measuring voltage of either polarity, this is useful.

Although the moving coil of this PMMC meter responds linearly to the flow of current, it can be modified to measure voltage by connecting a resistance in series with the coil’s motion. Once calibrated, a DC voltmeter made up of a series resistance and a moving-coil meter movement can provide precise readings.

Measurement Of Voltage

The voltage between any two points of a circuit when electrical charges are in equilibrium is zero, as we have seen in these tutorials. However, if a current—the flow of charge—flows through the circuit, a voltage will exist between two or more different points.

The voltage difference between two points can also be measured with a galvanometer because, in accordance with Ohm’s law, these two quantities are proportional to one another. Thus, the potential difference between any two points of a circuit can be determined using a graduated voltmeter.

But how do we change a meter from one that measures current to one that can measure voltage? As we stated earlier, the strength of the current flowing through the moving coil of the permanent magnet moving-coil meter determines how far it will deflect.

The moving magnet moving-coil meter can be made to read voltage instead of current by multiplying its full-scale deflection (FSD) by the internal resistance of the moving coils. This turns the meter into a DC voltmeter.

However due to the design of the coil movement, most PMMC meters are very sensitive devices which can have full-scale deflection current, I_G ratings as low as 100µA (or less). If, for example, the moving coils resistive value R_G is 500&In that case, 50mV would be the maximum full-scale voltage that could be measured (V = I*R = 100µA x 500Ω).

Therefore, we must find a way to lower the voltage being measured to a value the meter can handle in order for the sensitive coil movement of a PMMC voltmeter to measure higher voltage values. This is accomplished by connecting a resistor, known as a multiplier, in series with the meter’s internal coil resistance.

Assume for the moment that we want to use the 100uA, 500&Omega galvanometer described above to measure circuit voltages up to 1.0 volt. The meter’s maximum voltage range, as we have seen, is 50 millivolts (50mV), so it is obvious that we cannot connect it directly to measure 1 volt. But by using Ohm’s Law we can calculate the value of series resistor, R_S required which will produce a full-scale meter movement when used to measure a potential difference of one volt.

Thus if the current for which the galvanometer gives full scale deflection is 100uA, then the series resistance R_S required is calculated as 9.5k&Therefore, by simply joining a significant enough resistance in series with a galvanometer as shown, a galvanometer can be transformed into a voltmeter.

Voltmeter Series Resistance

Note that this series resistance, R_S will always be higher than the coil’s internal resistance, R_G to limit the strength of the current through the coil’s windings. A basic analogue voltmeter is then built on the movement of the meter in conjunction with this external series resistance.

Voltmeter Example No1

A PMMC galvanometer has an internal coil resistance of 100Ω and generates a full-scale deflection of 200 mV. To get a full deflection from the meter when measuring a DC voltage of 5 volts, determine the multiplier resistance needed.

Therefore the series resistance required has a value of 2.4kΩ

We can use this method to measure any voltage value by changing the value of the multiplier resistors as required providing we know the the current or voltage full-scale deflection (FSD) values (I_FSD or V_FSD) of the galvanometer. After that, all we have to do is re-label the scale to read from zero to the new voltage value that was measured.

This simple series-connected voltage divider circuit can be expanded further to have a range of different “multiplier” resistors within it design thereby allowing the voltmeter to be used to measure a range of different voltage levels at the flick of a switch.

Multi-range Voltmeter Design

By using a number of series resistances, each one sized for a specific voltage range, which can be selected one at a time by a single multi-pole switch, our basic DC voltmeter from above can be further extended to measure a wider range of voltage levels with a single movement.

The ranges chosen for this type of voltmeter configuration are known as multirange voltmeters and depend on how many positions the switch has, for example, 4-position, 5-position, etc.

Direct Multi-range Voltmeter Configuration

In this voltmeter configuration each multiplier resistor, R_S of the multirange voltmeter is connected in series with the meter as before to give the desired voltage range. So if we assume our 50mV FSD meter from above is required to measure the following voltage ranges of 10V, 50V, 100V, 250V, and 500V, then the required series resistors are calculated the same as before as:

While this direct voltmeter configuration reads our range of voltages very well, the multiplier resistor values needed to get the right FSD of the meter for the calculated ranges can give resistive values that are not standard preferred values or necessitate soldering resistors together to produce the exact value.

We need to find a different version of the voltmeter design that would use resistor values that are more widely available because the calculated values of 99.5kΩ through 4.9995MΩ are not common.

Indirect Multi-range Voltmeter Configuration

The indirect voltmeter configuration, in which one or more series resistances are linked in a series chain with the meter to provide the desired voltage range, is a more useful design. Here, we have the benefit of using multiplier resistors with standard preferred values.

If we assume again our 50mV FSD meter and the voltage ranges of 10V, 50V, 100V, 250V, and 500V, then the required series multiplier resistors are calculated

The higher the voltage to be measured, the more multiplier resistors the switch selects, as can be seen with this indirect 5-range voltmeter configuration. The total resistance connected in series with the PMMC meter will be the sum of the resistances, as R_TOTAL = R_S1 + R_S2 + R_S3 … etc.

In spite of the fact that both the direct and indirect voltmeter configurations are capable of reading the same voltage levels, the indirect method is simpler and more affordable to build because it uses resistors with standard and preferred values such as 400k, 500k, 1M5, and 2M5.

It is obvious that the FSD of the galvanometer being used and the voltage levels that need to be measured will ultimately determine the resistor values. In either case, a switch and higher series multiplier resistors can be used to build a straightforward multi-range analogue DC voltmeter. Nowadays, auto-ranging digital multimeters are the norm.

One last thing to keep in mind when creating a DC voltmeter is that an ideal voltmeter will not affect the component or area of the circuit being measured because it will have an infinite equivalent resistance.

However, in practice, attaching a voltmeter to a circuit—especially a high-resistance circuit—can reduce the circuit’s effective resistance, which has the effect of lowering the voltage being measured between the two points.

In order to lessen this loading effect, a meter with a high sensitivity should be used, meaning that its full-scale deflection can be attained with a lower deflecting current. This will allow the voltmeter’s multiplier resistance to be as high as possible, reducing the current that flows through the PMMC meter. The sensitivity of a voltmeter is expressed in Ohms/Volt, or Ω/V.

Types Of Signal Sources

Identifying whether the signal sources are floating or ground referenced is necessary before configuring the input channels and creating signal connections.

Floating Signal Sources

Although it has an isolated ground reference point, a floating signal source is not connected to the building’s ground system. The outputs of transformers, thermocouples, battery-operated devices, optical isolators, and isolation amplifiers are a few instances of floating signal sources. A floating signal source is a piece of equipment or a device with an isolated output. To create a local or onboard reference for a floating signal, the ground reference of the signal must be connected to the device’s ground. As the source floats outside the common-mode input range, the measured input signal changes in the absence of this.

Ground Referenced Signal Sources

Assuming that the measurement device is plugged into the same power system as the source, a ground referenced signal source is connected to the building system ground, making it already connected to a common ground point with respect to the device. This category includes all non-isolated outputs from tools and equipment that are plugged into a building’s electrical system. The difference in ground potential between two devices connected to the same building power system is typically between 1 and 100 mV, but it can be much higher if power distribution circuits are connected incorrectly. This difference may appear as measurement error if a grounded signal source is measured incorrectly. The ground potential difference can be removed from the measured signal by following the connection instructions for grounded signal sources.

The various signal source types are displayed in Figure 5, along with the best connection diagrams based on each measurement method. It should be noted that different voltage measurement techniques may yield better results than others depending on the type of signal being used.

High-voltage Measurements And Isolation

When measuring higher voltages, there are numerous factors to take into account. The first thing you should consider when specifying a data acquisition system is its safety. Making high-voltage measurements can be dangerous for your tools, the unit being tested, and even you and your coworkers. Provide an insulation barrier between the user and potentially dangerous voltages with isolated measurement devices to guarantee the safety of your system.

Electrical isolation and safety isolation are two subcategories of isolation, which is a technique for physically and electrically separating two components of a measurement device. Eliminating ground paths between two electrical systems is referred to as electrical isolation. By supplying electrical isolation, you can eliminate ground loops, expand the data acquisition system’s common-mode range, and level-shift the signal ground reference to a single system ground. Safety isolation refers to regulations that have particular demands for separating people from hazardous voltages. It describes an electrical system’s capacity to stop high-voltage and transient voltages from being transmitted over its boundary to other electrical systems that the user might come into contact with.

Isolation has three main purposes in a data acquisition system: it prevents ground loops, rejects common-mode voltage, and offers safety.

Increase your knowledge of High-Voltage Measurements and Isolation.

Ground Loops

In applications involving data acquisition, ground loops are the most frequent source of noise. They happen when the ground potentials of two connected terminals in a circuit are different, causing current to flow between the two points. Your system’s local ground may be several volts above or below the ground beneath the closest building, and lightning strikes close by may increase this difference to several hundreds or even thousands of volts. The additional voltage itself has the potential to significantly alter the measurement, but the current that generates it has the potential to couple voltages in nearby wires. Both transient and periodic signals may be present when these errors occur. For instance, the unwanted AC signal manifests itself as a periodic voltage error in the measurement if a ground loop is created with 60 Hz AC power lines.

When a ground loop exists, the measured voltage, Vm, is the sum of the signal voltage, Vs, and the potential difference, Between the ground of the measurement system and the signal source is a space called Vg (Figure 6). Since the potential in question is typically not a DC level, the measurement system is noisy and the readings frequently include components of power-line frequency (60 Hz).

Figure 6. A Grounded Signal Source Measured with a
Ground Referenced System Introduces Ground Loops

Use isolated measurement hardware or make sure there is only one ground reference in the measurement system to prevent ground loops. By removing the path between the measurement device’s ground and the ground of the signal source through the use of isolated hardware, any current that might otherwise flow between different ground points is stopped.

The NI 9229 analog input module offers 250 V channel-to-channel isolation in the previously mentioned CompactDAQ configuration.

Figure 7. NI 9229 Channel-to-Channel Isolated Analog Input Module

Common-mode Voltage

A perfect differential measurement system only reacts to the potential difference between its two inputs, (+) and (-), at its two terminals. Although the desired signal is the differential voltage across the circuit pair, there may also be a signal that is common to both sides of the circuit pair. The term “common-mode voltage” refers to this voltage. Instead of measuring the common-mode voltage, the ideal differential measurement system completely rejects it. However, practical devices have a number of restrictions that are outlined by terms like common-mode voltage range and common-mode rejection ratio (CMRR), which restrict this capability to reject the common-mode voltage.

The greatest permitted voltage swing on each input in relation to the measurement system ground is referred to as the common-mode voltage range. Failure to adhere to this restriction leads to measurement errors as well as the potential destruction of board-mounted components.

The term “common-mode rejection ratio” refers to a measurement system’s capacity to filter out common-mode voltages. Common-mode voltages can be rejected more successfully by amplifiers with higher common-mode rejection ratios.

A circuit still has an electrical path between the input and the output in a non-isolated differential measurement system. The common-mode signal level you can apply to the input is therefore constrained by the electrical properties of the amplifier. The conductive electrical path is removed when isolation amplifiers are used, and the common-mode rejection ratio is significantly raised.

Isolation Topologies

When setting up a measurement system, it’s critical to comprehend the isolation topology of a device. Numerous cost and speed factors are related to various topologies.

Channel-to-channel

Channel-to-channel isolation is the most reliable isolation topology. Each channel in this topology is discretely isolated from the other channels and from other, non-isolated system elements. Additionally, every channel has a separate, isolated power supply.

There are a variety of architectures to choose from in terms of speed. It is typically quicker to use an isolation amplifier with an analog-to-digital converter (ADC) for each channel because you can access every channel simultaneously. For the highest level of measurement accuracy, the NI 9229 and NI 9239 analog input modules offer channel-to-channel isolation.

Multiplexing every isolated input channel into a single ADC results in a slower, more cost-effective architecture.

Utilizing a common isolated power supply for all of the channels is another method of ensuring channel-to-channel isolation. This means that, barring the use of front-end attenuators, the common-mode range of the amplifiers is constrained to the supply rails of that power supply.

Bank

A different type of isolation topology involves banking, or grouping, a number of channels to use a single isolation amplifier. The common-mode voltage difference between channels in this topology is constrained, but the common-mode voltage between the bank of channels and the measurement system’s non-isolated portion can be substantial. Although banks of channels are isolated from one another and the ground, individual channels are not isolated from one another. Because this design uses a single isolation amplifier and power supply, this topology is a less expensive isolation solution.

The majority of NI C Series analog input modules, like the NI 9201 and NI 9221, are bank-isolated and can deliver precise analog voltage measurements for a lower price.

Getting To See Your Measurement: Ni Labview

Using LabVIEW graphical programming software, you can visualize and analyze data as needed once the sensor and measurement device are connected. See Figure 8, which is taken from the shipped LabVIEW Example: Voltage – Finite Input.vi and found here:

\National Instruments\<LabVIEW Version>\examples\DAQmx\Analog Input\Voltage – Finite Input.vi