A Design of Triode Switch Circuit

introduction

Transistor can not only be used as an AC signal amplifier, but also as a switch. Strictly speaking, the triode is not exactly the same as the general mechanical contact switch in action, but it has some characteristics that the mechanical switch does not have. Figure 1 shows the basic circuit diagram of a triode electronic switch. It can be seen from the figure that the load resistance is directly connected between the collector of the triode and the power supply, and is located on the circuit of the main current of the triode.



The input voltage Vin controls the open and closed actions of the transistor switch. When the transistor is in the open state, the load current is blocked. On the contrary, when the transistor is in the closed state, the current can flow. In detail, when Vin is low voltage, since there is no current at the base, there is no current at the collector. As a result, the load connected to the collector has no current, which is equivalent to the opening of the switch. At this time, the triode is better than Cut off area.

In the same way, when Vin is high voltage, because the base current flows, a larger amplified current flows through the collector, so the load loop is turned on, which is equivalent to the closure of the switch, and the triode is better at this time. In the saturation zone (saturation).

1 Analysis and design of triode switch circuit

Since the forward bias value of the base-emitter junction of a silicon triode is about 0.6 volts, to cut off the triode, Vin must be lower than 0.6 volts so that the base current of the triode is zero. Usually, in the design, in order to be more certain that the triode must be in the cut-off state, the Vin value is often lower than 0.3 volts. (838 Electronic Resources) Of course, the closer the input voltage is to zero volts, the more it can ensure that the transistor switch must be in the off state. To transmit current to the load, the collector and emitter of the triode 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 triode into the saturated working area. When the triode is in saturation, the collector current is quite large, so that almost the entire power supply voltage Vcc is across the load resistance. Then VcE is close to 0, and the collector and emitter of the transistor are almost short-circuited. Under ideal conditions, when the transistor is saturated according to Ohm's law, its collector current should be:



Therefore, the base current should be less:



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 make the switch closed, its Vin value must be high enough to send out more than or equal to the base current value required by (Equation 1). Since the base loop is just a series circuit of resistor and base-emitter junction, Vin can be solved by the following formula:



Once the base voltage exceeds or equals the value obtained by (Equation 2), the transistor will be turned on, so that all the supply voltage is across the load resistance, and the closing action of the switch is completed.

All in all, after the triode is connected to the circuit of Figure 1, its function is the same as a mechanical switch connected in series with the load, and its opening and closing switch can be directly controlled by the input voltage without using Control methods such as mechanical actuator, solenoid plunger, or relay armature commonly used in mechanical switches.

In order 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 they are relatively uncommon.

Try to explain the input voltage required to make the switch closed (transistor saturation) in the switch circuit of Figure 2﹖ And explain the solution of the load current and base current value at this time: It can be seen from the formula 2 that in the saturation state, all the supply voltages are completely dropped across the load resistance, so from the equation (1):



Therefore, the input voltage can be obtained by the following formula:



Use a triode as a light bulb switch

Figure 2 Using a triode as a light bulb switch

If you want to use a triode switch to control the opening and closing action of a load current as large as 1.5A, you only need to use a very small control voltage and current. In addition, although the transistor flows through a large current, it is not necessary to install a heat sink, because when the load current flows, the transistor is in a saturated state, and its VCE approaches zero, so the power multiplied by the current and voltage is very small. No need for heat sink.

2 Comparison of triode switch and mechanical switch

Up to now, we have assumed that when the transistor switch is turned on, there is a complete short-circuit between its base and emitter. This is not the case. No transistor can be completely short-circuited to make VCE=0. When most small-signal silicon transistors are saturated, the VCE (saturation) value is about 0.2 volts, even though they are switching transistors designed for switching applications. Its VCE (saturation) value can only be as low as about 0.1 volts at most, and when the load current is high, the VCE (saturation) value will rise slightly, although for most analysis calculations, VCE (saturation) The value can be ignored, but when testing the switching circuit, it must be understood that the VCE (saturation) value is not really 0.

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

Triode switch and mechanical switch circuit

Figure 3 Triode switch and mechanical switch circuit

Fortunately, although the triode switch is not suitable for series connection, it can be perfectly suitable for parallel connection, as shown in Figure 4 as an example. Compared with traditional mechanical switches, triode switches have the following four advantages:

(1) The triode switch does not have a movable contact part, so there is no fear of wear. It can be used an unlimited number of times. A general mechanical switch can only be used for millions of times at most due to contact wear, and its contacts are easy to use. The work is affected by contamination, so it cannot operate in a messy environment. The triode switch has no contacts and is sealed, so there is no such concern.

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

(3) There is no bounce phenomenon in the triode switch. The general mechanical switch will have a rapid and continuous opening and closing action at the moment of conduction, and then it can gradually reach a stable state.

(4) When using a triode switch to drive an inductive load, no sparks will be generated when the switch is turned on. Conversely, when the mechanical switch is turned on, the current on the inductive load is instantly cut off. Therefore, the instantaneous induced voltage of the inductance 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.

Parallel connection of triode switch

Figure 4 Parallel connection of triode switch

3 Triode switch test

The triode switch is not like a mechanical switch, which can judge its current open and closed state by the naked eye, so it must be tested with an electric meter. In the standard triode switch circuit shown in Figure 5, when the switch is turned on, the reading value of VEC should be 0, and when the switch is turned off, VCE should be for VCC.

When the triode switch is cut off, since there is no current flowing through the load, there is no voltage drop, so all the supply voltage is dropped across the two ends of the switch, so its VCE value should be equal to VCC, which is the same as a mechanical switch Exactly the same. If the switch itself should be conducting but not conducting, then the size of Vin has to be tested. To ensure that the transistor is turned on, the voltage value of Vin at its base must be high enough. If the value of Vin is too low, the problem comes from the signal source rather than the transistor itself. If the level of Vin is high enough, there is no problem with driving the transistor to turn on, but the load is still not turned on, then it is necessary to test whether the power supply voltage is normal.

In the on state, the VBE value of the silicon transistor is about 0.6 volts. If the Vin value is high enough, the VBE value is higher and lower than 0.6 volts. For example, VBE is 1.5 volts or 0.2 volts, which means that the base emitter is connected. The surface may be damaged and the transistor must be replaced. Of course, this criterion may not be 100% correct. Many high-current rated power transistors often have a VBE value exceeding 1 volt. Therefore, even if the VBE reading reaches 1.5 volts, it may not be certain that the junction of the transistor is damaged. Check the transistor specification table before making an assertion.

Once the VBE is normal and the base current is flowing, the VCE value must be tested. If VCE approaches VCC, it means that the base junction of the triode is damaged and the triode must be replaced. If VCE approaches zero volts and the load is still not turned on, it may be that the load itself has an open circuit phenomenon, so the load must be checked and replaced.

Triode switch circuit, the voltage diagram of each main test circuit

Figure 5 Triode switch circuit, the voltage diagram of each main test circuit

When Vin drops to a low voltage level, the three-pole management should be cut off and the load is cut off. If the load is still not cut off, it may be that the collector base and collector emitter of the triode are short-circuited and must be replaced.

3.1 Improved circuit of basic triode switch

Sometimes, the low voltage level that we set may not necessarily turn off the transistor switch, especially when the input level is close to 0.6 volts. To overcome this critical condition, corrective steps must be taken to ensure that the triode must be cut off. Figure 6 shows two common improved circuits designed for this situation.

To ensure that the transistor is switched on and off, the correct two improved circuits

Figure 6 Two improved circuits to ensure the correct switching of the triode

In the circuit of Figure 6(a), a diode is connected in series between the base and the emitter, so that the input voltage value that can make the base current conduction is increased by 0.6 volts, so even if the Vin value is close due to the malfunction of the signal source At 0.6 volts, the transistor will not be turned on, so the switch can still be in an off state.

The circuit in Figure 6(b) adds an auxiliary-off (hold-off) resistor R2. The appropriate values of R1, R2 and Vin can be designed to ensure that the switch is off when the critical input voltage is reached. Figure 6(b) shows that before the base-emitter junction is not turned on (IB0), R1 and R2 form a series voltage divider circuit, so R1 must cross a fixed (variable with Vin) partial voltage, so the base The voltage must be lower than the value of Vin, so even if Vin is close to the critical value (Vin=0.6 volts), the base voltage will still be pulled down by the auxiliary-off resistance connected to the negative power supply, making it lower than 0.6 volts. Due to the deliberate design of the R1, R2 and VBB values, as long as Vin is within a high value range, the base will still have enough voltage to make the transistor turn on without being affected by the auxiliary-off resistance.

3.1.1 Accelerating capacitor

In applications requiring fast switching action, the switching speed of the triode switch must be accelerated. Figure 7 shows a common method. This method only needs to connect an accelerating capacitor in parallel with the RB resistor, so when Vin rises from zero voltage and starts to send current to the base, the capacitor cannot be charged instantaneously, so it is like a short circuit. However, there is an instantaneous large current flowing from the capacitor to the base at this time, thus speeding up the turn-on of the switch. Later, after the charging is completed, the capacitor will be like an open circuit without affecting the normal operation of the triode.

Circuit with accelerating capacitor

Figure 7 Circuit with acceleration capacitor

Once the input voltage drops from the high level to the zero voltage level, the capacitor will turn the base-emitter junction into a reverse bias in a very short time, and the transistor switch will be cut off quickly. This is due to the original left end of the capacitor. It has been charged to a positive voltage, so at the moment when the input voltage drops, the voltage across the capacitor cannot be changed instantaneously and will remain at a constant value. Therefore, the drop in the input voltage immediately causes the base voltage to drop accordingly, so the base emitter is connected It becomes a reverse bias, and quickly turns off the triode. Proper selection of the accelerating capacitor value can reduce the switching time of the triode switch to less than a few tenths of a microsecond, and most accelerating capacitor values are about hundreds of picofarads (pF).

Sometimes the load of the triode switch is not directly added between the collector and the power supply, but connected as shown in Figure 8. This connection is very close to the circuit of the small-signal AC amplifier, except that one output coupling capacitor is missing. This kind of connection is exactly the opposite of the normal connection. When the transistor is cut off, the load is energized, and when the transistor is turned on, the load is cut off. These two circuit forms are common, so they must have a clear Resolving power.

Improved connection method for connecting load to triode switch circuit

Figure 8 Improved connection method for connecting the load to the triode switch circuit

3.1.2 Totem switch

If a capacitive load is added to the triode switch in Figure 8 (assuming it is connected in parallel with RLD), after the triode is turned off, since the load voltage must be established by slowly charging the capacitor through the RC resistor, the greater the capacitance or resistance value, The greater the time constant (RC), the slower the rate of rise of the load voltage. In some applications, this phenomenon is not allowed. Therefore, the improved circuit shown in Figure 9 must be used.

Totem Triode Switch

Figure 9 Totem-type triode switch

Totem circuit is constructed by directly superposing a triode on another triode, and it is named after it. To enable the load, the transistor Q1 must be turned on and the transistor Q2 must be cut off at the same time, so that the load can be connected to VCC via Q1. To disable the load, the transistor Q1 must be cut off and the transistor Q2 must be turned on at the same time. So the load will be grounded via Q2. Since the collector of Q1 has almost no resistance except for the extremely small contact resistance (as shown in Figure 9), the load is almost directly connected to the positive power supply, and therefore when Q1 is turned on, there is no more The phenomenon of slow charging of capacitors exists. Therefore, it can be said that Q1 "pulls up the load" and is called "pull up transistor", and Q2 is called "pull down transistor". The input control circuit in the left half of Figure 9 is responsible for the turn-on and cut-off control of the transistors Q1 and Q2, but it must be ensured that Q1 and Q2 are not turned on at the same time, otherwise it will be a short circuit between VCC and ground via Q1 and Q2 , If this is the case, the short-circuit high current will burn at least one triode. Therefore, the totem-type transistor switch cannot be used in parallel as shown in Figure 6-4. Otherwise, as long as any one of the transistors Q1 group on the top of the totem is turned on, and one of the Q2 groups below the totem is turned on, the power supply will be turned on. Q1 and Q2 that are turned on are short-circuited, causing serious consequences.

3.2 Application of triode switch

3.2.1 Drive indication

One of the common applications of transistor switches is to drive indicator lights. The indicator lights can indicate the operation status of a certain point in the circuit, and can also indicate whether the motor controller is activated, and also can indicate whether a certain limit switch is turned on. Or whether a certain digital circuit is in a high potential state.

For example, Figure 10(a) uses a transistor switch to indicate the output state of a digital flip-flop. If the output of the flip-flop is at a high level (usually 5 volts), the transistor switch is turned on and the indicator light is on. Therefore, the operator can know the current working status of the flip-flop by just looking at the indicator light. , Without the need to use an electric meter to detect.

Sometimes the current capacity of the output circuit of a signal source (such as a flip-flop) is too small to drive the transistor switch. At this time, in order to avoid malfunctions caused by the overload of the signal source, the improved circuit shown in Figure 10(b) must be used. When the output is at a high level, first drive the emitter follower transistor Q1 to amplify the current, and then turn on Q2 to drive the indicator light. Since the input impedance of the emitter follower stage is quite high, the flip-flop needs to be provided Satisfactory work can be obtained with a small amount of input current.

Digital display The circuit in Figure 10(a) is often used on digital displays.

(A) Basic circuit diagram (b) Improved circuit

Figure 10 (a) Basic circuit diagram (b) Improved circuit

3.2.2 Interface circuit of different voltage levels

In industrial equipment, solid-state logic circuits must often be used for control. The principles of digital logic circuits will be introduced in detail in the next chapter. For the purpose of explaining the interface circuits, the control circuits of industrial equipment are first divided into three Most part: (1) input part, (2) logic part, (3) output part.

In order to achieve reliable operation, the input and output parts of industrial equipment usually work at a higher voltage level, generally 220 volts. The logic part is operated at a low voltage level. In order for the system to work normally, it must be able to communicate between these two different voltage levels. This matching work between different voltages is called interface )problem. The circuit that performs the interface matching work is called the interface circuit. Triode switches are often used for this type of work.

Figure 11 uses a triode switch as an example of an interface circuit that controls low-voltage logic by high-voltage input. When the micro switch of the input part is closed, the step-down transformer is turned on, and the full-wave rectifier filter circuit sends out low-voltage DC control. Signal, this signal turns the transistor on, and the collector voltage drops to 0 (saturated) volts. This 0 volt signal can be sent to the logic circuit to indicate that the micro switch is in the closed state.

Conversely, if the micro switch is turned on, the transformer will not be energized, and the transistor will be turned off. At this time, the collector voltage will rise to the VCC value. This VCC signal can be sent to the logic circuit to indicate that the micro switch is in the on state . In Figure 11, the logic circuit is regarded as the load of the transistor, connected between the collector and the ground (Figure 11). Therefore, the R1, R2 and RC values of the transistor switch circuit must be carefully selected to ensure that the transistor only works In the cut-off zone and saturation zone, it will not work in the active (linear) zone.

The triode switch is used as the interface between the input part and the logic part

Figure 11 The triode switch is used as the interface between the input part and the logic part