1. zero ohm resistance

In the electronic circuit design is often used in a component is the resistor, we all know that the resistor in the circuit plays the role of voltage limiting. However, in actual use, a special resistor will be used: zero ohm resistor, hence the name. The resistance value of zero ohm resistor is zero. For beginners may have a question: since the resistance is zero, what is the difference with a wire? Why not directly connected? In fact, there is a difference between zero ohm resistance and direct wire connection, and there are many clever uses of zero ohm resistance in circuit design.

(1) Debugging and Compatibility

We need to consider compatibility issues as much as possible when designing PCB boards, because a circuit board is physically fixed after being physically printed and copper-clad. If the compatibility problem is not fully considered in the design, it will bring a lot of inconvenience to the engineer in the circuit board debugging stage.

(2) Convenient wiring

In the PCB layout and wiring stage, sometimes the wiring is always impassable, especially when the circuit board area is small, the number of connections is large, and the number of layers is small. If you encounter a connection that requires 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 winding a large circle.

(3) Reserved resistance position

If in the circuit design stage, a certain position is not sure how much resistance resistance needs to be connected, at this time, the welding position of the resistance can be set aside in this position and the resistance of zero ohm can be welded. In the actual circuit debugging, it is convenient to change the resistance of different resistance values, and then connect the appropriate resistance after the debugging is completed to determine the resistance parameters.

(4) Convenient test current

After designing the circuit system, it is usually necessary to test the power consumption of the entire circuit during operation. The conventional practice is to calculate the power consumption by testing the current and then using the current, while the method of testing the current is usually to measure the current meter in series into the circuit.

At this time, if a zero ohm resistor is placed where the current needs to be measured, remove the resistor and connect the ammeter when it needs to be measured. Normal work, direct welding on zero ohm resistance can be.

(5) Noise suppression

Due to the characteristics of the zero ohm resistor itself, the loop current can be effectively suppressed, so that the noise is suppressed. Actually zero ohm resistance is not really no impedance. Therefore, zero ohm resistance actually plays a role in attenuation in all frequency bands.

(6) Safety protection

Many pins can often be seen on many circuit boards, which need to be terminated with jumper caps. Or use the dial code switch to control whether the circuit is closed. Although these two methods will be more convenient in the debugging stage, it is best to use as little as possible when making products. Because in the high frequency circuit, the vacant pin is equivalent to the antenna, it is easy to make the signal interference.

In addition, the dip switch is very tolerant of being dialed by unsuspecting people, resulting in circuit system errors. Therefore, for safety reasons, it is best to use zero ohm resistance instead of pins and dip switches. It can not only avoid misoperation, but also reduce maintenance costs.

(7) Ground wire isolation

In embedded circuit design, the ground of power supply or analog signal is called analog ground (AGND), and the ground of single chip microcomputer is called digital ground (GND). AGND and GND are usually separated on the circuit to avoid analog signal interference to the microcontroller, causing the microcontroller to restart or interfere. Therefore, when designing the circuit, the AGND and GND on the circuit will pass through the zero ohm resistor and then connect them together to play a certain isolation role. This connection is also called single point grounding.

 

2. up and down resistors

Pull-up is to embed the uncertain signal at a high level through a resistor, and the resistor plays a current limiting role at the same time, and pull-down is the same.

Pull-up is the injection of current into the device, pull-down is the output current.

The strength is only different from the resistance value of the pull-up resistor, and 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 output current channels for open-collector output circuits.

(1) The role

(a) Determine the level

Some applications do not want to appear high resistance state, can be in a stable state through the pull-up resistor or pull-down resistor, to avoid the use of false trigger, 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 of external signals on the bus or control lines.

(d) Level shifting, commonly used in TTL-CMOS simple and unidirectional signals.

(E) Provide current for OC and OD gates to output level "1" state.

(2) Selection of upper and lower resistors

(A) Balance between driving ability and power consumption.

(B) Drive requirements of lower-level circuits.

(c) Setting of high and low levels.

(d) Frequency characteristics.

In general, the minimum value of the resistance is to meet the requirements of the output terminal, and the maximum value is to meet the requirements of the input terminal.

(3) the concept of absorbing current, pulling current and sinking current

Pull current: pull or discharge, active output current, is the output current from the output port (pull-down resistor).

Sprinkling current: Sprinkling is charging, passive input current flows into the sink current from the output port (pull-up resistor).

Suck current: it is the active suction current, which is the current flowing from the input port. The current is the output port is originally the output current, but the external forced input current.

 

3. current limiting resistor

The current limiting resistor can be known from the literal meaning to limit the current. Of course, it can also be called a voltage dividing resistor from different angles. In our actual circuit, R in the LED circuit shown in the following figure is the current limiting resistor of the LED. If there is no such resistor, the LED needs 3V voltage, then if VCC is 3V, the LED lamp can work normally, and if VCC is 5V, the LED lamp may be damaged. So if you want this 3V LED lamp to work at 5V, we will introduce a current limiting resistor or a voltage dividing resistor. However, some people who have studied electronics for several years may not know how to get this value, and can only take it by trying. In fact, this value is obtained according to Ohm's law. First of all, we know that the rated voltage of this LED lamp is 3V, but the LED lamp still needs a certain current to light up. This parameter can be obtained according to the datasheet of the LED. Assuming that a current of 10mA is required, then we can find out the value of the resistor R.

LED circuit

 

4. feedback resistance

In the adjustable switching power supply, the output voltage is obtained by dividing the voltage by the feedback resistor. When designing the product circuit, the resistor here is recommended to use a high-precision resistor, at least above 1% accuracy, otherwise it will lead to large deviation of the output voltage of mass-produced products, resulting in poor control and other quality problems. However, sometimes the desired voltage will find that the calculated resistance value is not available in all E series, which can only take close values. However, if we want to output this value with high precision, we can design it according to the following figure.

power supply feedback circuit

 

5. voltage and current sampling

In general, we choose to sample the resistor with high precision and good thermal stability. If the branch current that needs to be sampled is large, its power must be considered, but the greater the power, the larger the volume. So we can choose multiple small power resistors in parallel to use in high current situations.