A pressure transmitter normally provides a stable 4…20 mA signal, yet the corresponding analog value in the PLC fluctuates. A second sensor also shows sporadic jumps and, when a solenoid valve is switched, communication errors even occur for a short period of time. Because several components are affected at the same time, it is reasonable to suspect that the individual sensors themselves are not necessarily the cause.
As a first step, the common 24 V supply is therefore checked with a multimeter. The instrument shows, for example, 24.1 V DC. This value appears completely normal and lies exactly within the range expected from an industrial 24 V power supply. Nevertheless, the power supply can still be part of the problem.
The reason lies in the type of measurement. A digital multimeter is very useful for checking the DC voltage level and slow changes. However, short voltage dips lasting only a few milliseconds, high-frequency switching spikes or the actual waveform of a ripple component may remain largely hidden depending on the instrument, measuring range and bandwidth. An additional AC measurement can provide an important indication, but it still does not show when the disturbance occurs or what its waveform looks like over time.
For systematic troubleshooting, it is therefore necessary to distinguish whether the 24 V DC voltage is slowly fluctuating, whether periodic ripple is present, whether high-frequency interference pulses occur or whether the supply only dips briefly when specific loads are switched on. The 0 V side of the supply is equally important. If load currents from solenoid valves, relays or other devices flow through the same return conductor as sensitive sensor signals, the local reference potential can shift even though a perfect 24 V is still measured directly at the power supply.
Possible EMC coupling and errors caused by the measurement setup itself must also be considered. Especially when measuring very small ripple voltages, a long oscilloscope probe ground lead can pick up interference and generate a signal on the display that is not actually present on the supply in that form.
A stable reading of 24.0 V or 24.1 V on a multimeter therefore does not prove that the supply is sufficiently free of interference for sensitive sensors, analog signals and PLC modules. What matters is how the voltage behaves directly at the affected load over time and, in particular, at the exact moment when the system disturbance occurs.
What Does Ripple Mean on a 24 V Power Supply?
An industrial power supply is intended to convert its input voltage into a DC output voltage that is as constant as possible. In an ideal case, the output would remain permanently at exactly 24.000 V DC. A completely constant DC voltage does not exist in a real power supply, however. Small time-dependent voltage components are always superimposed on the actual DC voltage.
In simplified form, the output voltage can be described as u(t) = UDC + uAC(t). Here, UDC is the desired DC component, while uAC(t) represents the time-dependent AC and interference components.
A periodic or approximately periodic AC component is often referred to as residual ripple or simply ripple. In a switching power supply, its characteristic frequency depends, among other things, on the circuit topology, switching frequency and control behavior of the power supply. In addition to the actual ripple, much faster switching spikes, ringing or pulse-like disturbances may also occur.
This distinction is important for troubleshooting. A power supply may have very low periodic ripple and still dip significantly for a short period when a load is switched on. Conversely, the DC level may remain completely stable while high-frequency pulses couple into sensitive sensor or communication circuits.
Distinguishing Ripple, Noise and Voltage Dips
The terms ripple and noise are not used in exactly the same way in every manufacturer’s data sheet. For practical troubleshooting, it is therefore useful to distinguish the different types of disturbance according to their behavior over time.
| Disturbance Type | Typical Behavior | Possible Cause | Possible Effect |
|---|---|---|---|
| Residual ripple / ripple | periodic or approximately periodic fluctuation | rectification, switching frequency, control behavior of the power supply | AC component superimposed on 24 V DC |
| High-frequency noise | short high-frequency spikes or ringing | switching processes, power semiconductors, inductive loads, EMC coupling | disturbance of sensors, electronics or communication |
| Dynamic voltage dip | brief drop of the overall DC level | high inrush current, power-supply limiting, wiring losses | reset, communication error or incorrect analog value |
| 0 V shift | reference potential changes with load | shared return conductor and its resistance | measurement offset despite stable power-supply voltage |
For comparison with a data-sheet specification, it must also be checked what the manufacturer specifically means by terms such as Ripple, Ripple & Noise or Residual Ripple. In addition to the numerical value, the specified frequency bandwidth and prescribed measurement setup are particularly relevant.
What Can a Multimeter Detect?
A multimeter remains a very useful first step in troubleshooting. First, the DC voltage is checked directly at the power-supply output. A value such as 24.08 V DC confirms that the static output level is basically plausible. The same measurement should then be repeated as close as possible to the most critical load.
This allows an incorrectly adjusted output voltage, large static wiring losses, a permanently reduced voltage due to overload or problematic terminal connections to be identified relatively quickly. If the power supply shows 24.1 V while only 21.8 V reaches the sensor, there is no need to begin by searching for a high-frequency ripple problem. A significant static voltage drop is already present.
Many multimeters also allow an AC voltage measurement to be made on the 24 V rail. A displayed value of, for example, 35 mV AC may provide an initial indication of an AC component. However, this value must not automatically be compared with a data-sheet specification of, for example, 35 mV peak-to-peak. Depending on the instrument, an RMS value is determined within a certain frequency bandwidth, while the power-supply manufacturer may specify a peak-to-peak value under completely different measurement conditions.
| Measurement Task | Digital Multimeter | Oscilloscope |
|---|---|---|
| Static 24 V level | very suitable | also possible |
| Slow voltage drop | easy to detect | very clearly displayed |
| AC RMS value | possible depending on the instrument | can be determined from the waveform |
| Peak-to-peak value | normally not directly | directly visible |
| Dip lasting only a few milliseconds | may remain hidden | easy to capture with a suitable trigger |
| High-frequency switching spike | often not visible | waveform visible over time |
| Correlation with sensor signal | only limited | direct comparison possible with multiple channels |
An AC value of 30 mV RMS and an interference voltage of 30 mV peak-to-peak therefore do not describe the same signal level.
Why an Oscilloscope Is Crucial for Troubleshooting
The key advantage of an oscilloscope is that it displays not only a numerical value but the actual voltage waveform over time. This makes it possible to determine whether a disturbance occurs regularly, sporadically, as a pulse, under load or directly in connection with a specific machine event.
A two-channel measurement is particularly informative. The 24 V sensor supply can be monitored on the first channel while the second channel simultaneously measures an analog sensor signal. If a voltage dip on the supply occurs at exactly the same moment as a jump in the sensor signal, this provides a much stronger indication of a causal relationship than two separate multimeter measurements taken at different times.
Alternatively, two different supply points can be compared. For example, the voltage directly at the power supply and directly at the sensor can be monitored simultaneously during the same switching event.
Using AC Coupling Correctly
During a ripple measurement, a very small interference signal often lies on top of a comparatively large DC voltage. A supply may consist, for example, of 24 V DC + 80 mV ripple. With normal DC coupling, the oscilloscope must display the complete 24 V level while also resolving the AC component of only a few millivolts. This may result in an unfavorable vertical scale for the ripple signal.
With AC coupling, the DC component is blocked at the oscilloscope input. The display then essentially shows only the time-dependent AC component. This allows, for example, a vertical sensitivity of 20 or 50 mV per division to be used without the 24 V DC component dominating the display.
AC coupling is not the correct setting for every type of disturbance, however. It has its own lower cutoff frequency and can distort or largely suppress slow changes. If, for example, a voltage dip lasting several tens or hundreds of milliseconds is to be investigated, an additional measurement using DC coupling may be necessary.
For complete troubleshooting, both views are therefore often useful: AC coupling for detailed analysis of small superimposed interference components and DC coupling for observing the actual supply-voltage behavior.
Connecting the Probe and Ground Correctly
When measuring small ripple and noise voltages, the measurement setup itself has a major influence on the result. A conventional oscilloscope probe with a long ground lead forms a relatively large conductor loop together with the probe tip. This loop can pick up electromagnetic interference from the surrounding environment.
The oscilloscope may then display not only the actual output disturbance of the power supply but also interference picked up by the measurement setup. In a control cabinet containing frequency converters, contactors, motor cables and fast-switching power semiconductors, this difference can be significant.
For a reliable ripple measurement, the measurement and return leads should therefore be kept as short as possible and connected directly at the same supply point. Large loops should be avoided. With suitable probes, a short ground spring can significantly reduce coupling compared with a long ground lead. For differential leads, routing the conductors close together or twisted is also advantageous.
It is equally important to define the measurement location clearly. A measurement directly at the output terminals of the power supply describes only the voltage present at that point. At the end of a long sensor cable, a significantly different waveform may be present because of wiring losses, return currents and coupled interference.
Comparing Measurement Bandwidth with the Data Sheet
The measured interference voltage depends directly on the frequency bandwidth being observed. The greater the measurement bandwidth, the more high-frequency signal components can contribute to the result. A measurement with a 20 MHz bandwidth can therefore produce a significantly lower peak-to-peak value than the same measurement with a 100 MHz bandwidth.
Many manufacturers therefore specify ripple and noise values only under defined conditions. If, for example, a measurement bandwidth of 20 MHz is specified, the same bandwidth should be used whenever possible for a data-sheet comparison. If the oscilloscope has a corresponding 20 MHz bandwidth limit, this function may be useful for that purpose.
For general troubleshooting in a control cabinet, however, a larger bandwidth may deliberately be selected. This may reveal very fast interference spikes that are not part of the standardized power-supply specification but can nevertheless affect sensors, digital communication or other electronic components.
Measuring Ripple Under Real Load
A power supply should not be evaluated exclusively under no-load conditions. Output voltage, control behavior and interference components may change with the actual load. The operating condition in which the system disturbance actually occurs is therefore of particular interest.
A switching power supply may, for example, provide a completely unremarkable output voltage while the machine is idle. When several valves, contactors, HMI components or distributed I/O modules are then activated at the same time, the current demand can rise abruptly. Only under these conditions may a short voltage dip become visible.
For meaningful troubleshooting, measurements should therefore not be limited to minimum load. Comparative measurements during normal machine operation, at high continuous load and especially during the maximum dynamic load step provide a much better indication of whether the power supply is adequately sized and whether the downstream wiring can carry the current without problematic voltage drops.
Detecting Voltage Dips During Load Steps
Not every disturbance on a 24 V rail is classic residual ripple. In machines and control cabinets, short dynamic voltage dips are often more relevant. The supply may remain continuously at 24.2 V and then fall to 18.5 V for only 3 ms when several loads are switched on. It then immediately returns to 24.2 V.
A conventional multimeter may completely miss such an event. Its display can remain apparently unchanged at around 24 V. With an oscilloscope, however, it is possible to trigger specifically on the voltage dip or on the associated switching signal.
Possible causes include an undersized power supply, high inrush currents, activation of current limiting, long or undersized supply cables or increased contact resistance at terminals. A common supply shared by highly dynamic loads and sensitive sensor or control electronics can also be critical.
Measuring at the Power Supply and at the Load
One of the most important comparative measurements is to examine the supply directly at the power-supply output and then directly at the affected load. This makes it relatively easy to determine whether the disturbance is already generated by the source or only develops along the path to the sensor or PLC module.
| Measurement Observation | Likely Area to Investigate | Typical Further Check |
|---|---|---|
| Disturbance already present at the power-supply output | power supply, input voltage or load | check load condition, sizing and thermal conditions |
| Power supply clean, sensor supply disturbed | 24 V distribution and wiring | check cables, terminals, fuses and connectors |
| +24 V apparently stable, analog signal still jumps | 0 V return path or reference potential | measure potential difference between sensor 0 V and PLC reference |
| Disturbance only when certain loads switch | load step or EMC coupling | trigger on the switching event |
| Disturbance only at the remote load | wiring loss or shared return conductor | compare voltage simultaneously at source and load |
The statement “the power supply provides a clean 24 V” is therefore only truly meaningful if it is also known what voltage actually reaches the sensitive load.
Why the 0 V Line Itself Can Cause Interference
During troubleshooting of a 24 V supply, attention is often focused mainly on the positive conductor. For sensor and analog signals, however, the 0 V return conductor is equally important. Every conductor has a finite resistance. If additional load current flows through a shared return conductor, an additional voltage drop is produced according to U = R × I.
If a shared conductor section including its terminals has, for example, a resistance of 0.08 Ω and the current increases by 2 A when a solenoid valve is switched on, the local potential changes by 0.08 Ω × 2 A = 0.16 V. Completely stable 24 V may still be present directly at the power supply during this event.
For a sensor or PLC analog input, however, a 160 mV shift in potential is not necessarily insignificant. Particularly with voltage signals, it can directly result in a measurement error. Depending on the context, this effect is referred to as ground shift, common impedance coupling or a shift in the reference potential.
For unexplained analog-value fluctuations, it is therefore useful not only to measure the voltage between +24 V and 0 V. A measurement between the local 0 V point of the sensor and the reference potential of the PLC input can be equally informative.
Systematically Isolating Fluctuating PLC Analog Values
If an analog value in the PLC jumps sporadically, the connected sensor should not automatically be replaced first. It is more effective to examine the measurement chain step by step from the supply all the way to the PLC input.
First, it is checked whether the supply is stable directly at the power supply. The same measurement is then performed directly at the sensor. Next, it is checked whether a relevant potential difference occurs between sensor 0 V and the PLC reference potential. Only after that should the actual analog signal be compared directly at the sensor and then at the PLC input.
Time correlation is particularly valuable. If the analog value jump always occurs at exactly the same moment as a solenoid valve, contactor or other load is switched, that event should be used deliberately as the trigger for the oscilloscope measurement.
With two suitable measurement channels, the sensor supply and analog signal can be monitored simultaneously, for example. Alternatively, one channel can measure the supply while the second channel measures a relevant potential difference within the 0 V structure, provided that the input concept of the measuring instrument allows such a measurement to be performed safely.
Comparing 4…20 mA and 0…10 V Signals
Current and voltage signals respond differently to disturbances in the supply and wiring. A 4…20 mA transmitter transfers the measurement information as loop current and is therefore more robust against certain voltage drops than a directly transmitted voltage signal. However, a current signal is not completely independent of the supply either.
The transmitter requires a minimum operating voltage. In addition, voltage drops across cables, measuring resistors, isolators and PLC inputs must be taken into account. In simplified form, the condition Usupply ≥ Utransmitter,min + I × Rloop must remain fulfilled. If the supply drops far enough during a load step, even a 4…20 mA transmitter can leave its permissible operating range.
With a 0…10 V signal, the measurement information is transmitted directly as a potential difference. An interference voltage of only 100 mV already corresponds to 1% of the complete 10 V measuring span. Potential shifts between the sensor and PLC can therefore appear directly as a measurement error.
| Characteristic | 4…20 mA | 0…10 V |
|---|---|---|
| Transmitted measurement variable | current | voltage |
| Sensitivity to wiring voltage drop | comparatively robust as long as sufficient loop voltage is available | voltage drops or potential differences act directly |
| Effect of a 100 mV ground shift | depends on loop and input configuration | already 1% of the span for a 0…10 V signal |
| Supply voltage dip | critical if minimum loop voltage is undershot | can directly affect supply and reference potential |
| Troubleshooting | check supply, loop current and voltage reserve | check signal voltage and 0 V potentials separately |
Solenoid Valves, Relays and Contactors as Interference Sources
Solenoid valves, relays, contactors and other electromagnetic actuators are inductive loads. Especially when switched off, the energy stored in their inductance can generate high voltage spikes. Without suitable suppression circuitry, these disturbances can enter sensitive sensor and PLC circuits through the supply, common return conductors or capacitive and inductive coupling.
On the oscilloscope, such a disturbance often appears as a short high-frequency pulse or damped ringing. This also helps determine whether apparent “power-supply ripple” is actually generated internally by the power supply or is being fed back into the 24 V rail by a connected load.
In addition, the inrush current of an actuator can already cause a brief voltage dip when it is switched on. During troubleshooting, the switching-on and switching-off processes should therefore be considered separately. The appropriate suppression method depends on the load, switching method, operating voltage and requirements of the application.
Distinguishing Ground-Referenced and Floating Measurements
When using a conventional bench oscilloscope, the potential structure of the installation must be known. On many mains-powered instruments, the ground of the BNC inputs or standard probes is connected to protective earth. The probe ground clip must therefore not be connected arbitrarily to a presumed reference point in the system.
If this point is at a different potential relative to protective earth, the measurement connection can create a short circuit. At the same time, the system itself can be altered by the additional earth connection. In the worst case, both the measuring instrument and the electronics under test can be damaged.
For measurements on points that are not clearly earth-referenced or are floating, suitable galvanically isolated inputs, differential probes or other measurement methods explicitly approved for the relevant potential difference must therefore be used depending on the task.
The protective earth conductor of a bench oscilloscope must never be removed in an attempt to create an apparently floating input.
Practical Example: Analog Value Fluctuates When a Valve Is Switched
A PLC measures a pressure transmitter via a 4…20 mA signal. During normal machine operation, the displayed pressure remains stable. Whenever a certain 24 V solenoid valve is switched on, however, the PLC value jumps briefly.
The first test is performed with a multimeter directly at the power supply. A stable 24.2 V DC is measured there. No obvious fault is initially visible. The 24 V rail is then examined with an oscilloscope. Under static conditions, only a small and unremarkable amount of ripple is present.
The measurement is then synchronized with the switching event of the solenoid valve. It becomes visible that the supply briefly drops when the valve is energized. At the same time, a short high-frequency disturbance appears. A second measurement directly at the supply terminals of the pressure transmitter shows that the dip is more pronounced there than directly at the power supply.
This makes it clear that the control behavior of the power supply alone cannot be responsible. The wiring is investigated more closely. The sensor and solenoid valve partly share the same 0 V return conductor. The additional valve current produces a brief voltage drop in this conductor section and therefore shifts the reference potential.
The sensor supply is then routed separately in a suitable manner to the central 24 V / 0 V distribution point. In addition, the inductive load is equipped with the suppression circuitry intended for the application. During the repeated measurement, the voltage dip directly at the sensor is significantly reduced and the PLC analog value remains stable.
The example clearly demonstrates why a fluctuating analog value does not automatically mean that the sensor is defective. Likewise, a stable reading of 24.2 V directly at the power supply does not prove that the supply at the sensor is free of interference at the critical moment.
Systematic Test Procedure
- Measure the DC output voltage first directly at the power supply and then directly at the affected sensor or PLC module.
- Repeat the measurement under exactly the operating condition in which the disturbance occurs.
- Use a suitable multimeter to obtain an initial indication of the AC component, then use an oscilloscope to investigate waveform, spikes and short voltage dips.
- For small ripple signals, use AC coupling if appropriate, but investigate slower voltage dips additionally with DC coupling.
- Keep measurement and ground leads as short as possible and adapt the measurement bandwidth to the test task or the data-sheet conditions.
- Compare the power-supply output and the voltage directly at the load simultaneously or under identical conditions.
- Check common 0 V lines for dynamic voltage drops or potential shifts.
- Compare the sensor or PLC signal over time with the supply-voltage waveform and any unusual machine events.
- Check inductive loads, their suppression circuitry and the physical routing of signal, motor and power cables.
- For measurement points that are not clearly earth-referenced, use only suitable isolated or differential measurement equipment.
Common Measurement Errors
- Looking only at the DC reading on the multimeter: Short voltage dips and high-frequency disturbances can remain hidden.
- Comparing an AC multimeter value directly with an mVpp data-sheet specification: RMS and peak-to-peak values are different quantities.
- Measuring only under no-load conditions: The disturbance may only occur under high or dynamic load.
- Measuring only directly at the power supply: Wiring, terminal and return-path problems between the source and the load remain undetected.
- Using a long oscilloscope ground lead: The resulting measurement loop can pick up additional electromagnetic interference.
- Always using the maximum oscilloscope bandwidth: The manufacturer’s defined measurement conditions must be considered when comparing with a data-sheet specification.
- Considering only the +24 V conductor: A dynamic voltage drop on the 0 V return can produce equally significant measurement errors.
- Attributing every disturbance to the power supply: Relays, solenoid valves, motor controllers and frequency converters can feed interference back into the common supply.
- Using AC coupling for every disturbance: Slow voltage dips can be attenuated or suppressed.
- Connecting oscilloscope ground without checking: Ground-referenced inputs can create an unintended earth connection or short circuit.
- Replacing the sensor immediately: Supply, 0 V reference, analog signal and PLC input should first be checked separately.
- Treating shield, 0 V and PE as the same conductor: These conductors serve different functions and must be connected according to the system design.
Suitable Measuring Instruments and Power Supplies
For analyzing ripple, fast voltage dips and pulse-like disturbances, the CA 942 HANDSCOPE, for example, is suitable. The portable instrument combines a 40 MHz oscilloscope with two isolated measurement channels, a sampling rate of up to 2 GS/s and an integrated multimeter function.
The two measurement channels are particularly useful for the troubleshooting described here. They allow, for example, the supply voltage and sensor signal to be observed simultaneously or two different points in the 24 V distribution system to be compared over time. Its portable design also makes it suitable for service and maintenance work directly on machines and control cabinets.
Further instruments for time-dependent signal analysis can be found under Oscilloscopes at ICS Schneider. For initial voltage, current and resistance measurements, digital multimeters are also available.
Suitable industrial power supplies, DC/DC converters and other supply components can be found under Power Supplies at ICS Schneider. Further test and diagnostic instruments for commissioning, control cabinet construction and troubleshooting can be found under Measuring Instruments for Control Cabinet Construction.
Conclusion
A measured DC voltage of approximately 24 V is not sufficient to fully assess the quality of an industrial power supply. Periodic ripple, high-frequency switching spikes and short dynamic voltage dips may be superimposed on the DC voltage and may not be visible, or may only be partly visible, with a normal DC multimeter measurement.
The multimeter is still the correct starting point. It shows whether the basic DC level is correct and whether static voltage drops are already present. An oscilloscope is then required for a detailed investigation of the waveform over time. This makes it possible in particular to determine whether a disturbance occurs at exactly the same moment as a sensor-value jump, a solenoid-valve switching event or a communication error.
The quality of the measurement depends strongly on the measurement setup. Short probe connections, suitable bandwidth and the correct choice between AC and DC coupling help prevent misinterpretation. The measurement location is equally important. A clean voltage directly at the power supply says little about the voltage actually present at the sensor after several meters of wiring and shared return conductors.
When PLC analog values fluctuate, it is therefore not sufficient to investigate only the power supply and sensor. Load steps, contact resistance, shared 0 V paths, inductive loads and EMC coupling can be equally relevant. Particularly with 0…10 V signals, even a small potential shift can become clearly visible in the measured value. A 4…20 mA signal is more robust against certain influences, but it still requires sufficient loop voltage.
The decisive diagnostic question is therefore not: “Are 24 V present?”, but rather: “How do the supply voltage and reference potential behave directly at the affected load over time and at the exact moment when the system disturbance occurs?”
FAQ: Measuring Ripple on a 24 V Power Supply
What is ripple on a 24 V power supply?
Ripple is a small time-dependent voltage component superimposed on the actual 24 V DC voltage. In switching power supplies, its characteristic waveform depends, among other things, on the switching and control principle. High-frequency interference spikes can also occur and should be distinguished from the actual periodic ripple.
Can I measure ripple with a multimeter?
A suitable multimeter can provide an initial indication of an AC component when used in the AC measuring range. However, it does not show the actual waveform and may miss short voltage spikes or dips. An oscilloscope is therefore much more informative for detailed analysis.
Why does my multimeter show 24 V even though the PLC has disturbances?
A multimeter measures the voltage over a certain measurement time and bandwidth. A dip lasting only a few milliseconds may therefore not be visible on the display. In addition, the supply may be stable directly at the power supply while a different voltage waveform is present at the sensor or PLC module because of wiring losses or return currents.
How do I measure ripple with an oscilloscope?
The measurement is made between +24 V and 0 V directly at the relevant measurement point. For small AC components, AC coupling can be useful because it suppresses the large DC component. The measurement and return leads should be kept as short as possible and the bandwidth used must match the measurement task or the data-sheet specification.
What does mVpp mean?
mVpp, or mV peak-to-peak, describes the difference between the highest and lowest instantaneous value of a voltage waveform. A peak-to-peak value must not be compared directly with an RMS value from a multimeter.
Why should ripple be measured under load?
The control behavior and output voltage of a power supply can change with load. In addition, loads can produce short dynamic load steps when switched on or off. The most informative measurement is therefore one taken under exactly the machine condition in which the disturbance actually occurs.
Can a solenoid valve disturb the 24 V rail?
Yes. When switched on, the additional current demand can cause a short voltage dip. When an inductive load is switched off, high voltage spikes can also occur. Suitable suppression circuitry and clean wiring are therefore important parts of the system design.
Can a poor 0 V connection affect the PLC analog value?
Yes. If sensor and load currents flow through a shared return conductor, its electrical resistance creates a load-dependent voltage drop. This can shift the local reference potentials of the sensor and PLC relative to each other even though the power supply itself continues to provide a stable 24 V.
Are 4…20 mA signals insensitive to supply fluctuations?
They are more robust against certain wiring and potential effects than directly transmitted voltage signals. However, the transmitter still requires sufficient supply voltage. If the available loop voltage falls below the required level, the 4…20 mA signal can also be disturbed.
Why can a 0…10 V signal be particularly sensitive?
With a voltage signal, the measurement information is transmitted directly as a potential difference. An additional shift of only 100 mV already corresponds to 1% of the full measuring span of a 0…10 V signal. Shared return conductors and potential differences are therefore particularly critical.
When should I use AC coupling and when should I use DC coupling?
AC coupling is useful for making small high-frequency or periodic interference components on a large DC voltage visible. For slower voltage dips, startup events and the actual change in the DC level, the signal should additionally be measured using DC coupling.
Can I simply connect the ground clip of a bench oscilloscope to 0 V?
Only if it has been clearly established that this is compatible with the grounding and potential structure of the installation and the input design of the oscilloscope. On many bench oscilloscopes, the probe ground is connected to protective earth. Suitable isolated inputs or differential probes are required for floating or differential measurements.
What should I check first if a PLC analog value fluctuates?
First check the supply directly at the power supply and then immediately at the sensor or PLC module. Next investigate the 0 V reference potentials, the actual analog signal and the timing relationship with switching loads. This makes it much easier to determine whether the actual cause is the sensor, power supply, wiring or EMC interference.