A scheme for realizing a triode switch circuit

In addition to being used as an AC signal amplifier, the triode can also be used as a switch. Strictly speaking, the triode is not exactly the same as a general mechanical contact switch in terms of action, but it has some characteristics that mechanical switches do not have. Figure 1 shows the basic circuit diagram of a triode electronic switch . As can be seen from the figure, the load resistor is directly connected between the collector of the triode and the power supply, and is located in the loop of the main current of the triode.

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 a low voltage, there is no current in the base, so there is no current in the collector, resulting in no current in the load connected to the collector end, which is equivalent to the opening of the switch. At this time, the transistor is in the cut-off region.

Similarly, when Vin is a high voltage, due to the flow of base current, a larger amplified current flows through the collector, so the load circuit is turned on, which is equivalent to the closing of the switch. At this time, the transistor operates in the saturation region.

1 Analysis and design of transistor switch circuit

Since the forward bias voltage of the base-emitter junction of a silicon transistor is about 0.6 volts, in order to cut off the transistor, Vin must be lower than 0.6 volts to make the base current of the transistor zero. Usually, in order to be more sure that the transistor is in the cut-off state, the Vin value is often set lower than 0.3 volts. (838 Electronic Resources) Of course, the closer the input voltage is to zero volts, the more guaranteed that the transistor switch is in the cut-off state. In order to transmit current to the load, the collector and emitter of the transistor must be short-circuited, just like the closing action of a mechanical switch. To do this, Vin must reach a high enough level to drive the transistor into the saturation working area. When the transistor is in saturation, the collector current is quite large, almost making the entire power supply voltage Vcc across the load resistor. In this way, VcE is close to 0, and the collector and emitter of the transistor are almost short-circuited. Under ideal conditions, according to Ohm's law, when the transistor is saturated, its collector current should be:

Therefore, the minimum base current should be:

The above formula shows the basic relationship between IC and IB. The β value in the formula represents the DC current gain of the transistor. For some transistors, there is a big difference between the AC β value and the DC β value. To close the switch, the Vin value must be high enough to send out a minimum base current value greater than or equal to the minimum base current value required by (Formula 1). Since the base loop is just a series circuit of a resistor and the base-emitter junction, Vin can be solved by the following formula:

Once the base voltage exceeds or is equal to the value obtained by (Equation 2), the transistor is turned on, so that the entire supply voltage is applied across the load resistor, thus completing the closing action of the switch.

In summary, after the transistor is connected to form the circuit of Figure 1, its function is the same as a mechanical switch connected in series with the load, and the way to open and close the switch can be conveniently controlled by directly utilizing the input voltage, without the need to use the control methods commonly used by mechanical switches such as mechanical actuators, solenoid plungers, or relay armatures.

To avoid confusion, the transistor switches introduced in this article all use NPN transistors. Of course, NPN transistors can also be used as switches, but it is less common.

Explain what the input voltage is required to close the switch (transistor saturation) in the switch circuit of Figure 2? And explain the load current and base current values ​​at this time. Solution: From equation 2, we can see that in the saturation state, all the supply voltages are completely dropped across the load resistance. Therefore, from equation (1), we can know that:

Therefore, the input voltage can be calculated as follows:

Figure 2 Using a transistor as a light bulb switch

To use a transistor switch to control the on and off action of a load current as large as 1.5A, only a very small control voltage and current are needed. In addition, although a large current flows through the transistor, it does not need to be equipped with a heat sink, because when the load current flows through it, the transistor is in a saturated state and its VCE approaches zero, so the power multiplied by the current and voltage is very small, and a heat sink is not needed at all.

2 Comparison between transistor switch and mechanical switch

So far, we have assumed that when the transistor switch is turned on, the 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 about 0.2 volts when saturated. Even for switching transistors designed for switching applications, their VCE (saturation) value can only be as low as 0.1 volt at most. Moreover, when the load current is high, the VCE (saturation) value will rise slightly. Although for most analysis and calculations, the VCE (saturation) value can be ignored, but when testing switching circuits, it must be understood that the VCE (saturation) value is not really 0.

Although the VCE (saturation) voltage is very small and insignificant in itself, if several transistor switches are connected in series, the total voltage drop effect will be considerable. Unfortunately, mechanical switches often work in series, as shown in Figure 3(a). The transistor switch cannot simulate the equivalent circuit of the mechanical switch (as shown in Figure 3(b)) to work, which is a major disadvantage of the transistor switch.

Figure 3 Transistor switch and mechanical switch circuit

Fortunately, although transistor switches are not suitable for series connection, they are perfectly suitable for parallel connection, as shown in Figure 4. Compared with traditional mechanical switches, transistor switches have the following four advantages:

(1) Transistor switches do not have moving contacts, so there is no concern about wear and tear, and they can be used an unlimited number of times. Conventional mechanical switches can only be used millions of times at most due to contact wear, and their contacts are easily contaminated and affect their operation, so they cannot operate in dirty environments. Transistor switches have no contacts and are sealed, so there is no such concern.

(2) The operating speed of a transistor switch is faster than that of a general switch. The opening and closing time of a general switch is measured in milliseconds (ms), while that of a transistor switch is measured in microseconds (μs).

(3) Transistor switches do not have a bounce phenomenon. A general 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 using a transistor switch to drive an inductive load, no spark 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 instantly cut off, so the instantaneous induced voltage of the inductor will cause an arc on the contact. This arc will not only erode the surface of the contact, but may also cause interference or harm.

Figure 4 Parallel connection of transistor switches