A switching transistor has the same external shape as a regular transistor. It operates in the cutoff and saturation regions, effectively switching the circuit between open and closed states. Because of its ability to break and connect circuits, it is widely used in various switching circuits, such as commonly used switching power supply circuits, driver circuits, high-frequency oscillation circuits, analog-to-digital converter circuits, pulse circuits, and output circuits.
The load resistor is directly connected between the collector of the transistor and the power supply, and is located in the main current loop of the transistor. The input voltage Vin controls the opening and closing of the transistor switch. When the transistor is in the open state, the load current is blocked. Conversely, when the transistor is in the closed state, the current can flow.
In detail, when Vin is at a low voltage, there is no current at the base, and therefore no current at the collector. Consequently, the load connected to the collector terminal also has no current, which is equivalent to the switch being on (off). At this time, the transistor is operating in the cutoff region.
Similarly, when Vin is at a high voltage, due to the base current flowing, a larger amplified current flows through the collector, thus the load circuit is turned on, which is equivalent to the switch being closed (connected state). At this time, the transistor is working in the saturation region.
Working principle of switching transistor
Cutoff status
When the voltage applied to the emitter junction of a transistor is less than the forward voltage of the PN junction, the base current is zero, and both the collector and emitter currents are zero. The transistor then loses its current amplification function, and the collector and emitter are essentially in an open state, which is the transistor's cutoff state. The characteristic of a switching transistor in the cutoff state is that both the emitter and collector junctions are reverse biased.
On state
When the voltage applied to the emitter junction of a transistor exceeds the forward voltage of the PN junction, and when the base current increases to a certain level, the collector current no longer increases with the base current but remains relatively constant around a certain value. At this point, the transistor loses its current amplification function, and the voltage between the collector and emitter is very small. This is equivalent to the transistor being in a switched-mode state, also known as the on-state. A switching transistor in saturation conduction is characterized by both the emitter and collector junctions being forward biased. Conversely, a transistor in amplification state is characterized by the emitter junction being forward biased and the collector junction being reverse biased. This is the principle behind using a voltmeter to test the voltage values at the emitter and collector junctions to determine the transistor's operating condition. Switching transistors operate based on the switching characteristics of transistors.
Work mode
There are many types of transistors, and different models have different applications. Most transistors are packaged in plastic or metal. A common transistor appearance shows an arrow pointing to the emitter; an arrow pointing outwards indicates an NPN transistor, while an arrow pointing inwards indicates a PNP transistor. In fact, the direction the arrow points indicates the direction of current flow.
Bipolar junction transistors (BJTs) come in two types: NPN and PNP.
The NPN type contains two n-type regions and a p-type region separating them; the PNP type contains two p-type regions and an n-type region separating them.
Switching function of a transistor
Simple Transistor Switch: The circuit is shown in Figure 5. Resistor RC is used for LED current limiting to prevent excessive voltage from burning out the LED (light-emitting diode). Adjust the input signal VIN from 0 to its maximum (divided into approximately 20 intervals), and observe and record the corresponding VOUT and LED brightness. When the transistor switch is open, VOUT = VCC = 12V, and the LED is not lit. When the transistor switch is closed, VOUT = 0.2V, and the LED lights up. Improved Transistor Switch: Because the transition of a transistor from the cutoff region to the saturation region requires passing through the linear region, the switching effect does not have a clear boundary. To make the effect of the transistor switch clear, two transistors can be connected in series, as shown in Figure 6. Similarly, adjust the input signal VIN from 0 to its maximum (divided into approximately 20 intervals), and observe and record the corresponding VOUT and LED brightness.
Comparison of transistor switches and mechanical switches
Up to this point, we have assumed that when a transistor is switched on, its base and emitter are completely short-circuited. This is not the case. No transistor can be completely short-circuited to make VCE = 0. Most small-signal silicon transistors have a VCE (saturation) value of approximately 0.2 volts when saturated. Even switching transistors designed specifically for switching applications have a VCE (saturation) value that is at most around 0.1 volts. Moreover, the VCE (saturation) value will increase slightly with higher load current. Although the VCE (saturation) value can be disregarded for most analytical calculations, it is crucial to understand that the VCE (saturation) value is not truly 0 when testing switching circuits.
Although the voltage of VCE (saturation) is very small and negligible, the total voltage drop effect of several transistor switches connected in series is considerable. Unfortunately, mechanical switches often operate in series, as shown in Figure 3(a). Transistor switches cannot simulate the equivalent circuit of mechanical switches (as shown in Figure 3(b)), which is a major drawback of transistor switches.
(1) Transistor switches do not have moving contact parts, so there is no concern about wear and tear. They can be used an unlimited number of times. Ordinary mechanical switches can only be used a few million times at most due to contact wear. Moreover, their contacts are easily contaminated and affect their operation. Therefore, they cannot operate in dirty environments. Transistor switches have no contacts and are sealed, so there is no such concern.
(2) Transistor switches operate faster than ordinary switches. The opening and closing time of ordinary switches is calculated in milliseconds (ms), while that of transistor switches is measured in microseconds (μs).
(3) The transistor switch does not exhibit a bounce phenomenon. A typical mechanical switch will have a rapid and continuous opening and closing action at the moment of conduction, and then gradually reach a stable state.
(4) When a transistor switch is used to drive an inductive load, no sparks will be generated at the moment the switch is turned on. On the contrary, when a mechanical switch is turned on, the current on the inductive load is cut off instantly, and the instantaneous induced voltage of the inductor will cause an arc at the contact. This arc will not only erode the surface of the contact, but may also cause interference or damage.
L7912ACV Negative 12V Three-Terminal Voltage Regulator Specifications Manufacturer: STMicroelectronics Product Type: Linear Regulator - RoHS Standard: Yes Polarity: Negative Number of Output Terminals: 1 Output Type: Fixed Output Voltage: -12V Output Current: 500mA Line Regulation: 240mV Load Regulation: 240mV Feedback Voltage (Maximum): 1.1V Maximum Input Voltage: -19V Maximum Operating Temperature: +125°C Minimum Operating Temperature: 0°C Package:
Voltage regulator L7912ACV-DG: http://www.dzsc.com/ic-detail/9_2821.html
Product Category: Integrated Circuits (ICs) >> PMIC - Voltage Regulators - Linear Standard Package: 50 Regulator Topology: Negative, Fixed Input Voltage: Adjustable down to -35V
Number of voltage regulators: 1
Current Limit (Minimum): -
Mounting type: Through hole
Supplier equipment packaging: TO-220
Output voltage: -12V
Voltage drop (standard): 1.1V@1A
Operating temperature: 0°C~125°C
Packaging: Pipe Fittings
Current output: 1.5A
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