Transistor drive load skills and selection considerations

In electronic production, it is often necessary to control components such as buzzers, relays, and motors. It is found that different connection methods of transistor loads have very different effects. Some connection methods may even cause the circuit to work unreliably. The following will introduce common load drive circuits, drive circuits, and the selection and use of components for discussion.

Transistors (also known as triodes) can be divided into NPN type and PNP type. Currently commonly used NPN triodes include 8050, 9013, 2N5551, etc., and PNP triodes include 8550, 9012, 2N5401, etc.

1. Common load driving circuit

Figure 1 is a typical circuit for NPN and PNP transistors to drive various loads. The requirement is to maximize the power on the load and minimize the power consumed by the transistor.

Figure 1 Typical circuits of NPN and PNP transistors driving various loads

2. Driving circuit and component selection

The use of transistors to drive loads mainly utilizes the switching characteristics of transistors, that is, by controlling the transistor to switch between the saturation region and the cutoff region to control the on and off of the load. Then some electronics enthusiasts will ask: When to choose an NPN transistor drive circuit? When to choose a PNP transistor to drive the load? For transistors of the same type, how to choose the specific model of the transistor? How should the base resistor be selected? The following is a brief explanation of these issues.
2.1. Selection of load drive circuit

Whether to use NPN or PNP transistors to drive the load depends on the designer's requirements. Figures 1 (a), (c), and (e) are circuits using NPN transistors to drive the load. A high level "1" can control the transistor to turn on (power on the load), and a low level "0" can turn off the transistor (power off the load). Figures 1 (b), (d), and (f) are circuits using PNP transistors to drive the load, and the conduction conditions are just the opposite.

2.2. Selection of transistor model

The maximum collector currents of NPN transistors 8050, 9013, and 2N5551 are 1500mA, 500mA, and 600mA respectively, and the maximum collector currents of PNP transistors 8550, 9012, and 2N5401 are 1500mA, 500mA, and 500mA respectively. To drive loads such as buzzers, relays, and motors, it mainly depends on whether the collector current of the transistor can meet the load requirements.

2.3. Selection of base resistor

As shown in Figure 2, it is the characteristic curve of NPN transistor 2N5551. The bottom selected curve corresponds to Ib=1mA, and the top curve corresponds to Ib=5mA.

Figure 2 Characteristic curve of NPN transistor 2N5551

As can be seen from Figure 2, the larger the base current Ib, the larger the corresponding collector current Ic. If we want to get a large current on the load, then the base current needs to be as large as possible. In the various drive circuits shown in Figure 1, we often take the base resistance of about 1k, the corresponding base current is about 4~5mA, and the collector current can meet the load requirements.
3. Multisim simulation

When beginners use transistors to drive loads, they sometimes mistakenly connect the load to the emitter end of the transistor, as shown in Figure 3 (b). This will result in less power being applied to the load. We can analyze this through simulation.

Figure 3

In order to illustrate the influence of different connection methods of transistor load on the driving circuit, we use Multisim 10 simulation software to analyze the various parameters of the circuit in Figure 3 (a) and (b) when the load resistance is 100, 1k, and 10k. The simulation results are shown in Table 1 and Table 2.

It can be seen from Table 1 and Table 2 that, in the driving circuit with the load connected to the collector of the transistor as shown in Figure 3 (a), the voltage Ur obtained on the load is close to the power supply voltage and the power Pr obtained is large. In the driving circuit with the load connected to the emitter of the transistor as shown in Figure 3 (b), since the transistor conduction needs to meet the condition of Ube>0.7V, the voltage obtained on the load is 0.7V lower than the bias voltage, and the maximum is about 4.3V, resulting in a small power obtained on the load.

Similarly, for PNP transistors, the power obtained by the load is also larger when the load is connected to the collector end of the transistor.
4. Hardware Experiment

We made a test circuit board for transistors and conducted physical tests on different connection methods of the same load in the NPN transistor 2N5551 and PNP transistor 2N5401 drive circuits. Figure 4 (a) is the schematic diagram of the transistor test circuit board. By short-circuiting the 1 and 2 pins of J1 on the left side of Figure 4 (a), the NPN transistor load drive circuit can be tested; by short-circuiting the 2 and 3 pins of J1, the PNP transistor load drive circuit can be tested.

Figure 4 (a) is the schematic diagram of the transistor test circuit board, and Figure 4 (b) is the physical connection diagram of the transistor test circuit. The ammeter on the left shows the base current Ib of the circuit, the ammeter in the middle shows the collector current Ic or emitter current Ie, and the voltmeter on the right shows the voltage Uce between the collector and emitter of the transistor. The experimentally measured data are shown in Table 3.

Table 3 (Test condition Vcc=5.04V)

It can be seen from Table 3 that under the same load conditions, no matter whether an NPN transistor or a PNP transistor is selected, when the load is connected to the collector end of the transistor, the voltage obtained on the load is larger, the power obtained is also more, and the transistor voltage drop is smaller.

5. Conclusion

Through Multsim software simulation and hardware testing, we have come to the following conclusion: When using a transistor as a switch to drive a load, in order to obtain greater power on the load, the load should be connected to the collector of the transistor, and the current passing through the base should be 0.1 times the load current or greater.