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1. Zero ohm resistor
One of the components often used in electronic circuit design is resistors. We all know that resistors play the role of voltage dividing and current limiting in circuits. However, in actual use, a special resistor is used: a zero-ohm resistor, hence the name. The resistance value of a zero-ohm resistor is zero. Beginners may have a question: Since the resistance is zero, what is the difference between it and a wire? Why not connect directly? In fact, there is a difference between zero-ohm resistance and direct wire connection, and zero-ohm resistance has many ingenious uses in circuit design.
(1) Debugging and compatibility
We need to consider compatibility issues as much as possible when designing PCB boards, because after a circuit board is made and physically printed and coated with copper, it is physically fixed. If compatibility issues are not fully considered during design, it will bring a lot of inconvenience to engineers during the circuit board debugging stage.
(2) Convenient wiring
During the PCB layout and wiring stage, sometimes the wiring always fails, especially when the circuit board area is small, there are many connections, and there are few layers. If you encounter a certain connection that needs to go around a large circle to connect, you can consider whether you can easily skip the wire in front of you by connecting a zero-ohm resistor instead of going around a large circle.
(3) Reserved resistor position
If in the circuit design stage, it is not sure what value of resistor needs to be connected to a certain position, then you can leave a soldering position for the resistor at that position and solder a zero-ohm resistor. During actual circuit debugging, resistors with different resistance values can be easily changed. After debugging, the resistance parameters are determined and then a suitable resistor is connected.
(4) Convenient for testing current
After designing the circuit system, it is usually necessary to test the power consumption of the entire circuit when it is running. The conventional method is to measure the current and then use the current to calculate the power consumption. The method of testing the current is usually to insert an ammeter into the circuit and measure it.
At this time, if you place a zero-ohm resistor where the current needs to be measured, remove the resistor and connect the ammeter when the current needs to be measured. During normal operation, just solder the zero-ohm resistor directly.
(5) Noise suppression
Due to the characteristics of the zero-ohm resistor itself, the loop current can be effectively suppressed, thereby suppressing the noise. In fact, a zero-ohm resistor does not really mean there is no impedance. Therefore, the zero-ohm resistor actually plays an attenuating role in all frequency bands.
(6) Security protection
It is often seen on many circuit boards that there are many pins that need to be terminated with jumper caps. Or use a dip switch to control whether the circuit is closed. Although these two methods are more convenient during the debugging stage, it is best to use them as little as possible when making products. Since in high-frequency circuits, empty pins act as antennas, it is easy for signals to be interfered with.
In addition, the DIP switches can easily be messed up by unsuspecting people, causing errors in the circuit system. Therefore, for safety reasons, it is best to use zero-ohm resistors instead of pins and DIP switches. It can avoid misoperation and reduce maintenance costs.
(7) Ground isolation
In embedded circuit design, the ground of the power supply or analog signal is called analog ground (AGND), and the ground of the microcontroller is called digital ground (GND). AGND and GND are usually separated on the circuit to prevent analog signals from interfering with the microcontroller, causing the microcontroller to restart or interfere. Therefore, when designing the circuit, let AGND and GND on the circuit pass through a zero-ohm resistor before connecting them together to achieve a certain isolation effect. This connection method is also called single-point grounding.
2. Pull-up and pull-down resistors
Pull-up is to clamp the uncertain signal at a high level through a resistor. The resistor also acts as a current limiter. The same is true for pull-down.
Pull-up is to inject current into the device, and pull-down is to output current.
The difference between strong and weak is just the resistance value of the pull-up resistor, there is no strict distinction.
For non-collector (or drain) open-circuit output circuits (such as ordinary gate circuits), the ability to increase current and voltage is limited. The function of the pull-up resistor is mainly to provide open-collector output. type circuit output current channel.
(1) Function
(a) Determine the level
In some applications where a high-impedance state is not desired, a pull-up resistor or pull-down resistor can be used to stabilize the state to avoid false triggering during use. , depending on the design requirements.
(b) The pull-up resistor is used to provide current when the bus drive capability is insufficient.
(c) Reduce the interference caused by external signals to the bus or control line.
(d) Level conversion, commonly used in TTL-CMOS simple and unidirectional signals.
(e) Provide current to the OC and OD gates, capable of outputting the level "1" state.
(2) Upper and lower resistor selection
(a) Balance between driving capability and power consumption.
(b) Drive requirements of lower-level circuits.
(c) High and low level settings.
(d) Frequency characteristics.
Generally, the minimum value of the resistor must meet the requirements of the output end, and the maximum value must meet the requirements of the input end.
(3) The concepts of sinking, sourcing and sinking current
Pulling current: Pulling means leaking, actively outputting current, which is the current output from the output port (pull-down resistor).
Sink current: Sink and charge, passive input current, which flows into and sinks current from the output port (pull-up resistor).
Current sink: It actively sinks current, which is the current flowing from the input port. Current sinking means that the output port originally outputs current, but the current is forcibly input by the outside world.
3. Current limiting resistor
The current limiting resistor literally limits the current. Of course, depending on the angle of view, it can also be called a voltage dividing resistor. In our actual circuit, R in the LED circuit as shown below is the current limiting resistor of the LED. Without this resistor, the LED requires 3V voltage. Then if VCC is 3V, then the LED light can work normally, then if VCC is At 5V, the LED light may be damaged. So if we want this 3V LED light to work at 5V, we need to introduce a current limiting resistor or voltage divider resistor. As for the value of this resistor R, some people who have studied electronics for several years may not know how to get this value, so they can only get the value by trying. In fact, this value is obtained based on Ohm's law. First of all, we know that the rated voltage of this LED light is 3V, but the LED light needs a certain amount of current to light up. This parameter can be obtained according to the LED datasheet. Here we assume that it needs If the current is 10mA, then we can find the value of the resistor R.
4. Feedback resistor
In the adjustable switching power supply, the output voltage is obtained by dividing the voltage by the feedback resistor. When designing the product circuit, it is recommended to use high-precision resistors, at least 1% accuracy, otherwise it will lead to mass production Product output voltage deviation is too large, resulting in poor control and other quality problems. But sometimes you will find that the calculated resistance value for the desired voltage is not available in all E series. In this case, you can only take a nearby value. But if we want to output this value with high precision, we can design it as shown below.
5. Voltage and current sampling
Generally, the resistors we choose to sample must be high-precision and have good thermal stability. If the current of the branch that needs to be sampled is very large, then its power must be considered, but the greater the power, the larger the volume will generally be. Therefore, when using in large current situations, we can choose multiple small power resistors in parallel.