The WIKA pressure switch PSD-30 is available either with PNP or NPN switching output. With a zero signal applied to the Base of the transistor it turns OFF acting like an open switch and zero collector current flows. Then the transistor operates as a single-pole single-throw (SPST) solid state switch. In contrast, in North America, the country where the transistor was invented, relay outputs with potential-free contacts for AC 110 V loads are most widely used. For a PNP transistor, the Emitter potential must be positive with respect to the Base. In my experience, PNP switching outputs are used mainly in Europe, and NPN outputs almost exclusively in Asia. When a change in signal takes place by reaching a pressure value, the supply voltage (U+) is “switched on”, allowing current to flow from U+ through the load via the transistor and to GND. The PNP transistor thus provides a sourcing current and not a sinking current as the NPN transistor would. STs power bipolar NPN-PNP transistors with a voltage rating up to 500 V feature high switching speed and are available with both NPN and PNP polarity. The PNP transistor behaves like two PN junctions diodes connected back to back. A PNP transistor has three terminals a Collector (C), Emitter (E) and Base (B). PNP is a Bipolar Junction Transistor or BJT in which holes are the major current carriers, while electrons are minor. The load is connected to the switching output with U+ as the reference point. A PNP transistor is a bipolar junction transistor constructed by sandwiching an N-type semiconductor between two P-type semiconductors. When a change in signal takes place by reaching a pressure value, the supply voltage (U+) is “switched through”, allowing current to flow from U+ through the transistor and through the load to GND. The load is connected to the switching output with GND as the reference point. A bipolar transistor allows a small current injected at one of its terminals to control a much larger. In contrast, a unipolar transistor, such as a field-effect transistor (FET), uses only one kind of charge carrier. This is why the load must be connected differently to the transistor outputs. A bipolar junction transistor ( BJT) is a type of transistor that uses both electrons and electron holes as charge carriers.
In principle, both are bipolar transistors in which only the internal arrangement of the pn transitions differs. Here is a brief explanation how the two different outputs should be connected. Manufacturers of electronic pressure switches often offer both PNP and NPN switching outputs.
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