Maintain a voltage at the base that is lower than the voltage at the emitter. As current begins to flow the voltage drop across the load is given by When no current is flowing through the load there is no voltage drop across the If the load were connected between the power supply and the emitter the voltageĪt the emitter will be pulled down as the transistor begins to conduct. Thus the base must be held at a lower voltage than the emitter. In a PNP (BJT) transistor, to get current to flow from theĮmitter to the collector, a small control current must flow from the emitter Placed relative to the load being controlled. With which type of transistor is used to create the circuit and where it is If current flows out of the screw terminal it is said to sourceĬurrent. If current flows into the screw terminal the device is said to The controls world, the reference point is typically a screw terminal on an Indicate which way current is flowing with respect to some reference point. To the issue of sinking vs sourcing? The terms sink and source are used to So, how does the choice of an NPN or PNP transistor relate While for an NPN transistor current flows from the collector Note that in a PNP transistor current flows from the emitter The emitter pointing away from the base (or Not Pointing iN to the base). On a circuit diagram this is indicated by a small arrow on To the emitter causes a larger current to flow from the collector to the In an NPN transistor a small current flowing from the base This is indicated by a small arrow on the emitter P ointing i N to the base. The emitter into the base causes a larger current to flow from the emitter to In a PNP transistor a small current flowing from The other two connection points are called In a bipolar junction transistor (BJT) the control leg, the middle The practicalĭifference between these two transistor types is in how current flows through There are two possible types of transistor: NPN and PNP. Of material that can be used to build a transistor are n-type and p-type. Used to control the flow of current between the outside layers. The magic of the transistor is that the layer in the middle can be Material, the two outside layers are the same and the middle layer isĭifferent. A transistor is a three layer sandwich of two different types of The terms NPN and PNP refer to how a transistor isĬonstructed. Speaking they refer to different concepts. This is a reasonable assumption but strictly The terms NPN and PNP are commonly used as synonyms for Issues to be aware of when selecting components for a system. Then we will explore how these components are used and some of the Respect to digital input and digital output circuits used in industrial control This paper will define these terms and what they mean with Inevitably be discovered, the question is when and at what cost? Often engineers working on different parts ofĪ system will not communicate the chosen configuration. NPN transistor outputs are called “sinking outputs” because they sink current from the load into the output.PNP transistor outputs are called “sourcing outputs” because they source current from the output to the load.A device is called sinking if current flows into the terminal, and is called sourcing if current flows out of the terminal. The terms Sinking and Sourcing refer to current flow with respect to the terminal pin on the IO card. NPN (Sinking) and PNP (Sourcing) are two types of transistors used for digital outputs.
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