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1、双极性三极管开关特性晶体三极管工作于截止区时，内阻很大，相当于开关断开状态；工作于饱和区时，内阻很低，相当于开关接通状态。三极管开关电路如图 2.2.2(a)示。 输入控制信号 为矩形电压脉冲，电源电压 ,输出信号为 ,三极管开关电路输入输出波形如图 2.2.2(b)。下实例中为 12V 的开关控制信号，为单片机可接收的 TTL 信号，为了与输入控制信号一致，加入反相器 74LS14。TransistorsA Transistor is a solid-state device designed to control DC current. Transistors are most commo
2、nly found in low DC powered sensors as the output switch. There are two types of transistors - NPN and PNP. The figure below shows a NPN (Current Sink) Open Collector Transistor Figure 1: Sensor NPN OutputOutput StyleDepending on model, incremental encoders are available with several different elect
3、rical output styles. Choice of signal depends on receiving instrument and cable distance. Line driver outputs with complimentary outputs can be used with longer cables as noise spikes can be cancelled.NPN Uses an NPN type transistor and an internal resistor pulling up to the power supply rail. The o
4、utput is an active voltage.NPN Open Collector Uses an NPN type transistor but without an internal pull up resistor to the supply rail. The output is passive so a separate power supply can be used.PNP Uses a PNP type transistor and an internal resistor pulling down to zero volts.PNP Open Collector Us
5、es a PNP type transistor but without an internal pull down resistor to zero volts.Push Pull A problem with NPN and PNP type outputs is the high output impedance. This can be solved by a complementary output allowing better switching to zero and positive supply rails.Line Driver This output style has
6、 two complimentary outputs per channel allowing better transmission in noisy environments and long cable lengths. The receiver can process the signal, eliminating noise spikes.PTC protection A positive temperature coefficient resistor can be added to the output of a NPN or PNP encoder, protecting it
7、 from output short circuits.Sinking/SourcingSinking sensors allow current to flow into the sensor to the voltage common, while sourcing sensors allow current to flow out of the sensor from a positive source. For both of these methods the emphasis is on current flow, not voltage. By using current flo
8、w, instead of voltage, many of the electrical noise problems are reduced. When discussing sourcing and sinking we are referring to the output of the sensor that is acting like a switch. In fact the output of the sensor is normally a transistor, that will act like a switch (with some voltage loss). A
9、 PNP transistor is used for the sourcing output, and an NPN transistor is used for the sinking input. When discussing these sensors the term sourcing is often interchanged with PNP, and sinking with NPN. A simplified example of a sinking output sensor is shown in See A Simplified NPN/Sinking Sensor.
10、 The sensor will have some part that deals with detection, this is on the left. The sensor needs a voltage supply to operate, so a voltage supply is needed for the sensor. If the sensor has detected some phenomenon then it will trigger the active line. The active line is directly connected to an NPN
11、 transistor. (Note: for an NPN transistor the arrow always points away from the center.) If the voltage to the transistor on the active line is 0V, then the transistor will not allow current to flow into the sensor. If the voltage on the active line becomes larger (say 12V) then the transistor will
12、switch on and allow current to flow into the sensor to the common. A Simplified NPN/Sinking Sensor Sourcing sensors are the complement to sinking sensors. The sourcing sensors use a PNP transistor, as shown in See A Simplified Sourcing/PNP Sensor. (Note: PNP transistors are always drawn with the arr
13、ow pointing to the center.) When the sensor is inactive the active line stays at the V+ value, and the transistor stays switched off. When the sensor becomes active the active line will be made 0V, and the transistor will allow current to flow out of the sensor.A Simplified Sourcing/PNP Sensor Most
14、NPN/PNP sensors are capable of handling currents up to a few amps, and they can be used to switch loads directly. (Note: always check the documentation for rated voltages and currents.) An example using sourcing and sinking sensors to control lights is shown in See Direct Control Using NPN/PNP Senso
15、rs. (Note: This example could be for a motion detector that turns on lights in dark hallways.)Direct Control Using NPN/PNP SensorsIn the sinking system in See Direct Control Using NPN/PNP Sensors the light has V+ applied to one side. The other side is connected to the NPN output of the sensor. When
16、the sensor turns on the current will be able to flow through the light, into the output to V- common. (Note: Yes, the current will be allowed to flow into the output for an NPN sensor.) In the sourcing arrangement the light will turn on when the output becomes active, allowing current to flow from the V+, thought the sensor, the light and to V- (the common). At this point it is worth stating the obvious - The output of a sensor will be an
