Introduction to DC Motors
A DC motor is an electric motor that can rotate when it is supplied with direct current. Since the coil windings of the motor will frequently change direction, there will be sparks at the contact between the reversing carbon brush or the copper sheet and the coil. This kind of motor will have large electromagnetic interference when it works, and it will also bring peak glitches to the power supply. .
Transistor drive circuit
Some people will say, why not use MOS tube or IC driver? This article is about the triode, of course the triode is used.
Motor parameters: 12V 0.1A
Transistor selection
background knowledge
Here are two background knowledge,
knowledge 1 : When the DC motor is working, the spike pulse will have a spike pulse of 3-5 times the power supply voltage. If the internal resistance of the power supply is relatively large, then this voltage will affect the power supply to a greater extent. powered by. Of course, we will have some ways to reduce this spike, but the withstand voltage of the triode must not be just slightly greater than 12V, but must be 2 to 3 times this value to be more reliable.
Knowledge 2 : When the motor is working, the current will be relatively large when it starts and stalls. The maximum current can reach 5 to 8 times the rated current, and it is best to select the model according to 10 times.
The final triode requirements: at least 25V, preferably 36V or more, at least 1A, preferably 1.5A or more. That is to say, if the cost is sensitive, choose 25V1A, and if you want high reliability, choose 36V1.5A.
Selection principle
Basically follow the following principles: In the case of meeting the parameter requirements, cheapness is the first, and then it is easy to buy, easy to replace, small in size, etc.
NPN pipes are cheaper than PNP pipes and are easy to buy.
Selection website
When I check component parameters, I usually go to this website. The components in it are relatively complete. Commonly used components are basically available. The biggest advantage is that the DATASHEET of components can be downloaded directly, and the prices of components are also available. You can also sort them. Some components will There is an introduction to the parameters, which is very convenient.
The price of components is more expensive than that of X Bao, but they must be genuine products, so you can rest assured to use them.
It should be noted that the PDF document is not up to date, and it has been stepped on. The key components still need to download the latest version from the official website, but generally speaking, it is not a big problem.
model selection
Enter the triode, and then sort by sales or price, just choose from top to bottom, I usually sort by sales
The first one is fine, and the current also has a margin of 1.5 times. It should be noted that the withstand voltage margin is not much, and you need to pay attention to the power supply glitch handling.
Basic information such as packaging, parameters, price, model, and manufacturer are available.
Next, click the data sheet pointed by the arrow,
and click here to download the PDF document.
Check the documentation to determine the parameters
The table below is the limit parameter, if it is exceeded, the components will be damaged.
The following table is the normal use parameters, just take a look.
The most important thing is the following figure. Here are a few key points:
1 When the base current is given, the corresponding relationship between VCE and IC
For a given base current of 50uA, corresponding to the bottom curve, the maximum IC current that can be driven at this time is 12mA. For a 0.1A motor, the base current must be at least 400uA to be driven. The larger the base current is, the larger the collector current can be, and the VCE will be smaller.
Here we want to make the tube work in the saturation region, at this time VCE is the smallest, the empirical value is about 0.3V, and the red line roughly corresponds to this region.
Then it can give a base current of more than 500uA, and at the same time cooperate with a collector current of no more than 100mA, it can be determined that the tube is working in a saturated state. The point pointed by the tip of the red arrow is the working state of the tube, and its VCE should be about 0.2V.
2 The relationship between magnification and collector current IC
The working point is at the position indicated by the red arrow, and the magnification is about 250 times at this time
3 The temperature drift of the corresponding relationship between VCE and IC
is very small and can be ignored
4 Correspondence Table of Maximum Power and Temperature
In this example, the operating current is 100mA, the tube works in the saturation region, the CE voltage drop is 0.2V, and the actual power consumption is about 50mW. The starting current of the point pointed by the green arrow in the figure is assumed to be 10 times, that
is 1A. At this time, the power consumption of the tube is 0.2W. The point pointed by the blue arrow in the figure is 5 times the rated current. The power consumption of the tube is 0.1W. The state of the three points
pointed by the red arrow in the figure is
In the figure,
the most important thing to consider is the locked-rotor working point indicated by the red arrow, because the locked-rotor may be for a long time, so it is necessary to ensure that the pipe cannot be burned at this time. The corresponding temperature is about 108 degrees, that is to say, The long-term stable working temperature is 108 degrees, which meets the requirements of industrial-grade products. If the temperature is too high, you can consider cooling measures.
circuit design
circuit diagram
This is relatively simple,
Determine the parameters of peripheral components
C1 and C2 are used to suppress the peak pulse when the motor commutates, one is connected to the outlet of the motor, and the other is connected to the circuit board.
In order to eliminate the reverse high voltage, D1 needs to use a fast recovery diode. 1N5819 is enough.
R1 is directly connected to the microcontroller without isolation. For large motors, it must be driven in isolation. The output of the microcontroller is 3.3V, and the VBE of the triode is as shown in the figure below. When the temperature is different, it is between 0.62 and 0.72V, and the base current is above 500uA, so the voltage on the resistor R1 is between 2.51 and 2.61V, and the corresponding resistance is about 5K , Taking 4.7K or 3.6K can ensure that the triode works in a saturated state.
Verify the working status of the tube again for the value of the peripheral components
The base current has increased, but the increase is not much, and the power consumption of the triode is basically not affected by the base current IB when it is working in the saturation region. The
voltage, temperature, maximum power consumption and temperature are all within the range.
Summarize
Many people know that injecting 1mA current into the base of the triode can saturate it, but the specific reason is that few can explain clearly, and there is no concept of how much current enters saturation. This example should clarify this.
The high voltage induced by the winding of the motor and the increase of the locked rotor current are the knowledge points that must be known during the design, otherwise it is easy to burn the components after the design is completed.
As a development engineer, you need to have a wide range of knowledge, and developing products requires a lot of background knowledge. Kung fu is beyond poetry, and we all work together!