When selecting a pressure switch, attention is often focused first on the measuring range, process connection and medium. However, the electrical output is at least equally important. A pressure switch with the wrong output logic may be unable to operate with the intended PLC input even though its pressure range is suitable.
PNP and NPN transistor outputs are particularly frequently confused. There are also potential-free switching contacts, relay outputs, normally open and normally closed functions, as well as modern IO-Link versions. These terms describe different electrical characteristics and must not be treated as equivalent.
Before ordering, the supply voltage, PLC input circuit, required switching state, maximum load current and type of connected load must therefore be known. If the pressure switch is intended not only to control a digital input but also to switch a valve, contactor or warning lamp directly, additional load and protection requirements must be considered.
Table of Contents
- What is the purpose of the switching output?
- Direct comparison of PNP, NPN and relay outputs
- How does a PNP output work?
- How does an NPN output work?
- Connecting PNP and NPN pressure switches to a PLC
- Relay output and potential-free contact
- Distinguishing between normally open, normally closed and changeover contacts
- Load current, voltage drop and residual current
- Valves, contactors and other inductive loads
- Open collector, pull-up and pull-down
- M12 connector and pin assignment
- When is IO-Link useful?
- Typical wiring and ordering errors
- Troubleshooting when the PLC signal is missing
- Practical example on a hydraulic system
- Selecting the correct output signal
- Which measuring instruments / products are suitable?
- Conclusion
- Frequently asked questions
What is the purpose of the switching output?
An electronic pressure switch detects the applied pressure using a sensor and compares it with a configured switching point. When this limit is reached, the electrical output changes state.
The output can, for example:
- control a digital PLC input,
- trigger a warning message,
- enable or inhibit a pump,
- shut down a compressor,
- control a valve or interface relay,
- signal minimum or maximum pressure,
- monitor a pressure range using a window function.
The pressure switch does not necessarily provide its own load-capable supply for the connected device. A transistor output is primarily intended as an electronic control signal. Whether it can switch a load directly depends on the permissible output current, inrush current and type of load.
With a pressure switch featuring two switching outputs, a pre-alarm and shutdown function can be implemented, for example. Alternatively, one output can be configured as a switching signal and the second as an analogue output for the continuous pressure value.
Direct comparison of PNP, NPN and relay outputs
| Characteristic | PNP output | NPN output | Relay or switching contact |
|---|---|---|---|
| Operating principle | Switches the positive supply to the output | Switches the output to 0 V | Opens or closes an electrical contact |
| Alternative description | Positive-switching, sourcing, high-side | Negative-switching, sinking, low-side | Potential-free contact, if designed accordingly |
| Typical supply | DC voltage, often 24 V DC | DC voltage, often 24 V DC | AC or DC depending on the instrument; an electronic instrument may require its own supply |
| Typical application | PLC digital input referenced to 0 V | PLC digital input referenced to the positive supply | Galvanically isolated switching, different voltages or higher loads |
| Switching speed | High, without mechanical wear | High, without mechanical wear | Slower and subject to wear with electromechanical contacts |
| Electrical isolation | Normally not potential-free | Normally not potential-free | Usually potential-free with a genuine relay contact |
| Important limitation | Observe maximum output current and common ground | Observe maximum output current and common supply | Observe contact rating, inrush current and switching frequency |
PNP and NPN describe the electrical transistor logic. Normally open and normally closed, by contrast, describe the logical switching behaviour. A PNP output can therefore be configured as either normally open or normally closed, just like an NPN output.
A relay output can also be designed as normally open, normally closed or changeover. These terms must be considered separately when ordering.
How does a PNP output work?
A PNP output is positive-switching. When the pressure switch becomes active, its output stage connects the signal output to the positive supply voltage. With a 24 V DC supply, the active output therefore carries approximately the positive supply voltage.
In simplified form, the current path is:
+24 V → pressure switch → switching output → PLC input or load → 0 V
The connected load is therefore located between the switching output and 0 V. The PNP pressure switch supplies or “sources” current to the input.
PNP outputs are widely used in many European machinery and automation systems. However, this does not mean that every PLC input automatically matches every PNP sensor. The input voltage, switching thresholds, common reference potential and permissible residual current must be compared with the data sheet of the PLC module.
Typical PNP wiring
| Connection | Wiring |
|---|---|
| Supply L+ | +24 V DC |
| Supply L− | 0 V |
| Switching output Q | PLC digital input |
| PLC reference potential | 0 V or M |
When the pressure switch changes state, the PLC input is supplied with a positive voltage. A missing common 0 V connection is a frequent reason why the PLC fails to detect a signal even though the switching indicator is illuminated.
How does an NPN output work?
An NPN output is negative-switching. When active, the output stage connects the signal to 0 V. The load or PLC input must therefore be supplied with current from the positive supply.
In simplified form, the current path is:
+24 V → PLC input or load → pressure-switch output → 0 V
The NPN output absorbs the current and conducts it to ground. It is therefore also referred to as a “sinking” or low-side output.
Typical NPN wiring
| Connection | Wiring |
|---|---|
| Supply L+ | +24 V DC |
| Supply L− | 0 V |
| Switching output Q | PLC digital input |
| Common terminal of the input module | Typically positive supply |
An NPN pressure switch cannot be connected to a PLC input designed for PNP sensors without checking compatibility. In the worst case, the input remains permanently inactive, assumes an undefined state or reacts to residual and leakage currents.
Connecting PNP and NPN pressure switches to a PLC
The decisive factor is not only the output type of the pressure switch, but also how it interacts with the input circuit of the PLC. The digital-input data sheet should be checked before selecting the pressure switch.
The following information is important:
- rated voltage of the digital input,
- permissible voltage range for signal “0” and signal “1”,
- input current,
- common terminal of the input group,
- PNP or NPN compatibility,
- galvanic isolation of the input channels,
- filter or debounce time,
- diagnostics for wire break or short circuit.
With a PNP sensor, the PLC input is typically driven positively when active. With an NPN sensor, the input is pulled to 0 V. The input and output must therefore be electrically complementary.
Even if the pressure switch and PLC both operate at 24 V DC, their wiring may still be incompatible. The common rated voltage alone does not prove that the switching logic is compatible.
Relay output and potential-free contact
A relay output operates differently from a PNP or NPN transistor output. The relay actuates a mechanical contact that may be galvanically isolated from the sensor electronics. This allows an external circuit to be opened or closed without directly connecting its potential to the pressure-switch supply.
A potential-free contact is useful when:
- different supply voltages must remain isolated,
- an AC voltage must be switched,
- an existing controller expects a conventional contact,
- galvanic isolation is required,
- the pressure switch is intended to replace an existing mechanical switch.
However, a relay contact does not have unlimited load capacity. The permissible values differ for DC and AC voltage as well as for resistive, inductive and capacitive loads. A contact permitted to switch a certain resistive current may be unsuitable for a coil with a high inrush or switch-off impulse.
It must also be noted that not every output described as a “contact” contains an electromechanical relay. Mechanical pressure switches often actuate a microswitch directly. Electronic instruments may use relays, solid-state relays or transistor outputs. The circuit diagram in the data sheet is always decisive.
Distinguishing between normally open, normally closed and changeover contacts
| Switching function | State without actuation | State when the switching point is reached | Typical application |
|---|---|---|---|
| Normally open, NO | Contact open or output inactive | Contact closed or output active | Signal “minimum pressure reached” or enable signal |
| Normally closed, NC | Contact closed or output active | Contact open or output inactive | Shutdown or monitoring based on the closed-circuit principle |
| Changeover, SPDT | Common contact connected to one contact | Switches to the second contact | Simultaneous on and off logic |
With electronic pressure switches, the NO/NC function can often be configured. “Normally closed” does not then necessarily mean that a mechanical contact is present. It simply describes which logical output signal is provided below and above the switching point.
For safety-oriented shutdown logic, the closed-circuit principle is often preferred: a signal is present in the fault-free state. If a limit is exceeded, a cable is interrupted or the supply fails, the signal drops out. Whether this actually produces a safety-related function depends on the complete safety chain and the approvals of the components.
Load current, voltage drop and residual current
Transistor outputs have a maximum permissible switching current. This value must not be exceeded either continuously or during switch-on. The current requirement of a PLC digital input is usually low, while solenoid valves, lamps or relays can draw significantly higher currents.
The following must be checked during design:
- maximum continuous current of the switching output,
- short-circuit and overload protection,
- inrush current of the load,
- voltage drop across the active output,
- residual or leakage current in the switched-off state,
- permissible switching frequency,
- ambient temperature and possible current derating.
An active transistor output does not behave like an ideally closed contact. A small voltage drop remains across its output stage. With a low supply voltage, the connected load may therefore receive insufficient voltage.
A small residual current may also flow in the switched-off state. This is generally uncritical for normal PLC inputs, but it may affect sensitive electronic loads or high-impedance inputs.
Valves, contactors and other inductive loads
Coils in solenoid valves, contactors or interface relays store magnetic energy. When the current is switched off, a high reverse voltage can be generated. Without suitable suppression, this places stress on the transistor output or relay contact of the pressure switch.
For DC voltage, flyback diodes, transient-voltage suppressor diodes or suitable varistors are commonly used. For AC voltage, RC networks or varistors may be required. The protective circuit must be suitable for the voltage, polarity and required switch-off time.
A simple flyback diode effectively reduces the switch-off overvoltage, but it may delay the release of a solenoid valve or relay. A different protective circuit may therefore be more suitable where rapid switch-off is required.
If the permissible output current of the pressure switch is insufficient, an interface relay, suitable solid-state module or PLC output stage should be installed between the pressure switch and load. The pressure switch then controls only the small input current of the interface device.
Open collector, pull-up and pull-down
With an open-collector or open-drain output, the pressure switch does not itself provide a complete high and low signal in every state. The external load or input circuit must provide the necessary opposite current path.
With an NPN open-collector output, the output is switched to 0 V when active. A pull-up resistor or the PLC input circuit pulls the signal to the positive supply when inactive.
With a corresponding PNP output, the external circuit provides a path to 0 V. In industrial PLC applications, the digital input often simultaneously acts as the required load, meaning that no separate resistor is necessary.
However, an input with excessively high resistance may allow leakage currents to create an apparent switching state. A resistor that is too small, by contrast, places unnecessary load on the output. Pull-up and pull-down resistors must therefore not be added without calculation.
Modern push-pull outputs can both source and sink current. This allows some pressure switches to be configured for either PNP or NPN behaviour.
M12 connector and pin assignment
Many electronic pressure switches use a 4- or 5-pin M12 connector. A commonly used pin assignment is:
| Pin | Common function |
|---|---|
| Pin 1 | L+ or positive supply |
| Pin 3 | L− or 0 V |
| Pin 4 | Switching output Q1 or IO-Link C/Q |
| Pin 2 | Second switching output or analogue output |
| Pin 5 | Model-dependent additional function |
This pin assignment is widespread, but it is not mandatory for every instrument. Pins 2 and 5 in particular may have different functions depending on the version. Cable-core colours, pin assignment and output configuration must always be checked against the instrument-specific wiring diagram.
A matching M12 connector also does not guarantee electrical compatibility. Two devices may be mechanically connected even though their pins carry different functions or voltages.
When is IO-Link useful?
With a conventional PNP or NPN output, the PLC essentially receives the information “switching point reached” or “switching point not reached”. IO-Link additionally enables the digital exchange of process values, parameters, status and diagnostic information.
Depending on the pressure switch, the following values and settings can, for example, be transmitted or configured:
- current pressure value,
- switching and reset point,
- hysteresis or window function,
- switch-on and switch-off delay,
- unit and damping,
- minimum and maximum value,
- temperature or device status,
- overload and diagnostic messages.
A suitable IO-Link master is required for IO-Link operation. In standard I/O mode, the same connection can still be used as a conventional switching output on many instruments. The specifically available operating modes are, however, model-dependent.
IO-Link is particularly useful when many identical pressure switches are used, settings must be transferred automatically to replacement instruments or additional diagnostic information is to be evaluated by the PLC.
Typical wiring and ordering errors
A PNP pressure switch is connected to an NPN input
The output and input operate with the same switching polarity and do not form a correct current path. The PLC does not detect the switching state reliably.
The common reference potential is missing
The pressure switch and PLC are supplied by different power supplies whose 0 V potentials are not connected. The transistor output therefore has no defined reference to the PLC input.
PNP is confused with normally open
PNP describes the electrical polarity of the output. Normally open or normally closed describes its logical function. Both details are required for an unambiguous order.
A solenoid valve is switched directly
The rated current appears to be below the permissible output current, but the inrush current or inductive switch-off voltage is not considered.
The relay contact is selected according to the maximum resistive rating
An inductive load can place significantly greater stress on the contact. The permissible utilisation category and manufacturer’s specifications for the specific load type are decisive.
The pin assignment is assumed solely from the core colour
Pre-assembled cables often use standardised colours. Nevertheless, the assignment at the connector and pressure switch must be checked.
The analogue output and switching output are confused
A 4–20 mA or 0–10 V output provides a continuous measured value and must be connected to an analogue input. A PNP or NPN output provides a binary signal for a digital input.
Troubleshooting when the PLC signal is missing
If the pressure switch displays the correct pressure and an active switching state but the PLC does not detect a signal, the following should be checked systematically:
- Measure the supply: Is the correct voltage present directly at the pressure switch between L+ and L−?
- Check the output configuration: Is the output set to PNP, NPN, push-pull, IO-Link or analogue mode?
- Check the switching logic: Is the output configured as normally open, normally closed, hysteresis or window function?
- Measure the output voltage: For PNP, measure between the output and 0 V; for NPN, measure between the output and positive supply depending on the circuit.
- Check the PLC input: Is the module suitable for the output logic used and correctly supplied?
- Check the reference potentials: Are the required 0 V or supply potentials connected?
- Compare the pin assignment: Do the instrument connector, cable and PLC terminal correspond to the wiring diagram?
- Disconnect the load: Check whether an overload or short circuit is switching off the output.
- Simulate the switching point: Change the pressure in a controlled manner above and below the switching and reset points.
If the pressure switch also has a 4–20 mA output, this can be checked using a suitable multimeter or the Druck UPS4E current-loop calibrator. A plausible analogue value with a missing switching signal indicates a problem with the output configuration, wiring or digital-input module.
Practical example: Pressure enable signal on a hydraulic system
A hydraulic pump is intended to enable a machining process only once a system pressure of 120 bar has been reached. The controller has 24 V DC digital inputs for positive-switching sensors.
An electronic pressure switch with a PNP output is selected for the application. The switching point is set to 120 bar and the reset point to 110 bar. The hysteresis prevents the enable signal from switching continuously on and off in response to minor pressure fluctuations.
The pressure-switch supply is connected to +24 V and 0 V. The PNP output is connected to a PLC digital input. The common terminal of the input group is connected to 0 V.
During commissioning, the pressure switch indicates an active output at 125 bar, but the PLC remains at “0”. A measurement shows approximately 24 V between the output and 0 V.
Inspection of the control cabinet reveals that the common terminal of the PLC input group was accidentally connected to +24 V. The input module was therefore wired for an NPN configuration. After correcting the connection to 0 V, the PLC detects the PNP signal correctly.
The example shows that a functioning pressure switch alone is not sufficient. Output logic, supply, common potentials and PLC input must be considered as one complete circuit.
Selecting the correct output signal
At least the following information should be clarified for selection:
- supply voltage of the pressure switch,
- PNP, NPN or universal PLC input,
- normally open, normally closed or changeover function,
- number of required switching points,
- maximum load and inrush current,
- resistive, inductive or capacitive load,
- required galvanic isolation,
- DC or AC voltage in the load circuit,
- additional 4–20 mA or 0–10 V output,
- IO-Link connection and diagnostic requirements,
- connection type and pin assignment,
- required hysteresis, window function and delay.
| Requirement | Generally suitable output |
|---|---|
| Direct connection to a European 24 V PLC input | Often PNP, but the input data must still be checked |
| The controller expects a signal switching to 0 V | NPN |
| Switching AC voltage or a separate potential | Relay or suitable potential-free contact |
| Switching a high load or solenoid valve | Interface relay or separate output stage |
| Recording switching status and continuous pressure value | Switching output plus 4–20 mA or 0–10 V |
| Central parameterisation and diagnostics | IO-Link |
| Replacing an existing mechanical pressure switch | Normally open, normally closed or changeover contact with suitable contact rating |
Which measuring instruments / products are suitable?
Electronic pressure switches
The electronic pressure switches category includes various instruments with PNP, NPN, analogue and IO-Link outputs.
The available outputs differ depending on the model and version. Before ordering, not only the measuring range and process connection but also the number of outputs, switching logic, load current, connector pin assignment and supply must therefore be specified.
IDS350 with switchable PNP/NPN output and IO-Link
The IDS350 is an electronic pressure switch with a stainless-steel sensor and IO-Link interface.
Depending on its configuration, its signal output can operate as IO-Link, PNP, NPN, 0–10 V or 4–20 mA. This makes the instrument particularly suitable for machines and systems in which different control systems or output types must be supported.
Switching points and other parameters can be configured using the menu or IO-Link. Process, status and diagnostic information can also be transmitted to the higher-level controller.
WIKA PSD-4
The WIKA PSD-4 is an electronic pressure switch with a digital display for applications such as machine tools, hydraulics, pneumatics, pumps and compressors.
Depending on the version, freely configurable PNP/NPN switching outputs and 4–20 mA or 0–10 V analogue signals are available. The instrument can optionally be integrated into an automation system using IO-Link.
The PSD-4 is therefore suitable where a continuous pressure value or digital parameterisation is required in addition to the binary limit signal.
WIKA PSM02 with mechanical switching contact
The WIKA PSM02 is a compact mechanical pressure switch with an adjustable switching point and adjustable hysteresis.
It is available with normally open, normally closed or changeover contacts. This makes it suitable for applications requiring a conventional electrical contact instead of a PNP or NPN transistor output.
The permissible contact rating must be checked against the supply and load. For inductive loads or high switching frequencies, an additional interface relay or protective circuit may be required.
Pressure switches and differential pressure switches
The higher-level pressure switches / differential pressure switches category includes mechanical pressure switches, differential pressure switches, pressure monitors and versions for special applications in addition to electronic instruments.
Mechanical instruments are often useful where a simple potential-free contact, robust limit monitoring or operation without electronic signal processing is required. Electronic pressure switches, by contrast, provide greater flexibility for switching points, hysteresis, display, analogue output and communication.
Conclusion: Output logic and PLC input must be compatible
PNP, NPN and relay outputs perform different electrical functions. A PNP output switches the positive supply to the input, while an NPN output conducts the current to 0 V. A relay or mechanical contact, by contrast, opens or closes a separate circuit.
Normally open and normally closed are not alternatives to PNP and NPN. They describe the logical function of the respective output. An unambiguous order therefore requires specifications such as “PNP, normally open” or “potential-free changeover contact”.
Before connecting the pressure switch to a PLC, the input circuit, common potentials, switching thresholds and pin assignment must be checked. The same supply voltage alone does not guarantee compatibility.
Transistor outputs are primarily intended for PLC inputs and small control loads. Solenoid valves, contactors and other inductive loads may only be switched directly if the output current, inrush current and protective circuit are explicitly suitable.
IO-Link provides additional options for parameterisation, diagnostics and transmission of the continuous pressure value. For simple limit-monitoring tasks, however, a conventional PNP output or mechanical switching contact may be the more economical solution.
Frequently asked questions about PNP, NPN and relay outputs
What is the most important difference between PNP and NPN?
When active, a PNP output switches the positive supply to the output. An NPN output connects the output to 0 V when active.
Can I connect a PNP pressure switch to any 24 V PLC input?
No. The digital input must be designed for a positive-switching signal and connected to the correct reference potential. The data sheet of the PLC module is decisive.
Is PNP the same as normally open?
No. PNP describes the electrical output stage. Normally open describes the logical function. Depending on the instrument, a PNP output may be configured as normally open or normally closed.
When do I need a relay output?
A relay or potential-free contact is useful when galvanically isolated circuits, AC voltage or an existing contact-based control system must be switched.
Can a pressure switch control a solenoid valve directly?
Only if the continuous current, inrush current, operating voltage and inductive switch-off energy remain within the output specification. An interface relay or separate output stage is often the more robust solution.
Why is the switching LED illuminated while the PLC detects no signal?
Possible causes include an incorrect PNP/NPN configuration, a missing reference potential, incorrect pin assignment, an unsuitable PLC input or an overloaded output.
What does push-pull output mean?
A push-pull output can actively drive the signal towards both the positive supply and 0 V. Depending on the instrument, it may be configured as a PNP, NPN or automatically detected output.
Which information does ICS Schneider require for product selection?
The required information includes the pressure range, medium, temperature, process connection, supply voltage, PLC input type, PNP/NPN or contact requirement, normally open or normally closed function, load current, load type, pin assignment and requirements for the analogue output and IO-Link.