How to distinguish between NPN and PNP? How to connect them to PLC?

How to distinguish between NPN and PNP? How to connect them to PLC?

NPN and PNP Devices and Siemens PLC Connection and Selection

Sensors can be classified into two major types based on their output types: NPN (also known as source - type sensors) and PNP (also known as sink - type sensors). Both types of sensors have three leads, which are connected to 24V, 0V, and Out (signal output). So, what types of sensors are supported by the Siemens S7 series PLCs?

Types of Sensors Supported by Siemens PLCs and Modules

Sensors Supported by Siemens S7 - 200 : The input terminals of the S7 - 200 series support both source - type and sink - type sensors, meaning they can be connected to either NPN or PNP sensors.

Sensors Supported by Siemens S7 - 200smart : Similar to the S7 - 200, the S7 - 200smart's input terminals support both sink - type and source - type sensors, allowing connection to both NPN and PNP sensors.

Sensors Supported by Siemens S7 - 1200 : The S7 - 1200's input terminals support both sink - type and source - type sensors, enabling connection to either NPN or PNP sensors.

Sensors Supported by Siemens S7 - 300 : The S7 - 300 has a variety of DI modules, and the type of sensor they support needs to be checked in the hardware configuration based on the parameters.

Most DI modules of S7 - 300 are sink - type (PNP sensors should be selected) : In the hardware configuration, there is no prompt for source or sink type, which means they are default to sink - type. For example, the 321 - 1BL00 module.

Source - type DI modules of S7 - 300 (NPN sensors should be selected) : For example, the 6ES7 321 - 1BH50 - 0AA0 module.

Universal DI modules that support both source and sink types (both NPN and PNP sensors can be used) : For example, the 6ES7 321 - 1BP00 - 0AA0 module.

Sensors Supported by Siemens S7 - 1500 : The DI modules currently available in the S7 - 1500 series include sink - type modules that support PNP sensors and source - type modules that support NPN sensors.

Sink - type DI modules (PNP sensors) : For example, the 6ES7 523 - 1BL00 - 0AA0 module.

Source - type DI modules (NPN sensors) : For example, the 6ES7 131 - 6BF60 - 0AA0 module.

Wiring Methods for NPN and PNP Sensors

Understanding the Input Terminal Types of PLCs : The definitions of source - type and sink - type for Siemens PLC input terminals are based on the direction of current flow at the I - point of the PLC terminal. (Note: This is opposite to the definition used by Mitsubishi PLCs, which is based on the direction of current flow at the COM terminal.)

Source - type : When current flows out of the I - point, it is considered the source of the current.

Sink - type : When current flows into the I - point, it is considered the destination of the current.

Connecting NPN Sensors to PLC (Source - type Input for Siemens PLC) : Current flow: 24V + → COM terminal → I0.0 (current flows out of the I - point) → sensor out terminal → sensor 0V terminal → 0V.

Connecting PNP Sensors to PLC (Sink - type Input for Siemens PLC) : Current flow: 24V + → sensor 24V → sensor out terminal → I0.0 (current flows into the I - point) → COM → 0V.

Classification of Sensors

NPN (Source - type) : When activated, it outputs a low - level signal.

NPN - NO (Normally Open) : No output when not activated; outputs a low - level signal when activated.

NPN - NC (Normally Closed) : Outputs a low - level signal when not activated; no output when activated.

NPN - NC + NO (Combined Normally Open and Normally Closed) : Two output terminals (one open, one closed).

PNP (Sink - type) : When activated, it outputs a high - level signal.

PNP - NO (Normally Open) : No output when not activated; outputs a high - level signal when activated.

PNP - NC (Normally Closed) : Outputs a high - level signal when not activated; no output when activated.

PNP - NC + NO (Combined Normally Open and Normally Closed) : Two output terminals (one open, one closed).

Fundamental Differences Between PNP and NPN Sensors

PNP and NPN sensors are both switch - type sensors that use the saturation and cutoff of transistors to output two states. However, the output signals are opposite: NPN outputs a low - level signal (0V), while PNP outputs a high - level signal (24V).

PNP and NPN sensors are categorized into six types:

NPN - NO (Normally Open)

NPN - NC (Normally Closed)

NPN - NC + NO (Combined Normally Open and Normally Closed)

PNP - NO (Normally Open)

PNP - NC (Normally Closed)

PNP - NC + NO (Combined Normally Open and Normally Closed)

PNP and NPN sensors typically have three leads: the power supply line (VCC), the 0V line, and the out signal output line.

PNP Type : When triggered, the out line connects to the power supply line (VCC), outputting a high - level signal.

PNP - NO : The out line is open (disconnected from VCC) when not triggered. When triggered, it outputs the same voltage as the VCC power supply line (high - level signal).

PNP - NC : The out line is connected to the VCC power supply line (outputting a high - level signal) when not triggered. When triggered, the out line is open (disconnected from VCC).

PNP - NC + NO : It has an additional out terminal, which can be used as needed.

NPN Type : When triggered, the out line connects to the 0V line, outputting a low - level signal.

NPN - NO : The out line is open (disconnected from 0V) when not triggered. When triggered, it outputs the same voltage as the 0V line (low - level signal).

NPN - NC : The out line is connected to the 0V line (outputting a low - level signal) when not triggered. When triggered, the out line is open (disconnected from 0V).

NPN - NC + NO : Similar to NPN - NC + NO, it has an additional out terminal, which can be used as needed.

NPN sensors output a low - level signal (0V), while PNP sensors output a high - level signal (24V).

For example, if the sensor's power supply is 24V, an NPN sensor will output 0V, while a PNP sensor will output 24V.

When connecting to the PLC input, if it is an NPN output, the PLC input's COM terminal should be connected to 24V. Conversely, if it is a PNP output, the PLC input terminal should be connected to 0V.

The key is to understand what PNP and NPN represent. P stands for positive, and N stands for negative. PNP indicates a high - level signal under normal conditions, and the signal becomes negative when triggered. NPN indicates a low - level signal under normal conditions, and the signal becomes high - level when triggered. The output of proximity switches and photoelectric switches is the same, but their detection circuits differ. As for PLC wiring, NPN types are generally used more often. Most Japanese PLCs in China are world - type or universal - type, which can directly use NPN sensors. The positive terminal of the proximity switch and photoelectric switch power supply is connected to the positive power supply, the negative terminal is connected to the common terminal, and the output is connected to the PLC input terminal.

In summary, for the switch - quantity input circuit of PLCs, Japanese Mitsubishi PLCs seem to be more practical and reliable than some well - known PLCs like Siemens. The main reason is that Japanese PLCs, including Mitsubishi, have optimized the design they learned