Small resistors are commonly used for current sensing and voltage regulation. By placing a small resistor in a circuit, the current in the circuit can be measured, and it can also serve as a feedback signal to control the output voltage of the power supply. This ensures that the output voltage and current of the power supply remain stable under varying load conditions.
Small resistors can also be used to control the current in switching power supplies. When the power supply output current reaches a preset value, the small resistor will cause the power supply to shut down, thus ensuring that the power supply is not overloaded.
In some applications, small resistors are also used for temperature control, such as in measuring battery power. Furthermore, small resistors can be used in radio frequency circuits as band-limited filters to reduce interference from spurious signals.
There are many types of resistors. Commonly used resistors include carbon film resistors, cement resistors, metal film resistors, and wire-wound resistors; special resistors include varistors, thermistors, and photoresistors. Different types of resistors have different characteristic parameters, and the key considerations for their use in circuits also differ. Ignoring certain special parameters of resistors in circuit design may compromise the stability and reliability of the product. A correct understanding of the various parameters of resistors and the selection considerations for different types, along with a comprehensive understanding of the true role resistors play in circuits, is essential to ensuring the functionality and performance of the product from a fundamental level in circuit design.
Basic parameters of resistor
When we talk about resistance, our first impression should be the description in physics textbooks: the opposition of a conductor to the flow of electric current is called resistance. Resistance is represented by R in circuit diagrams, and the unit is ohm (Ω). Commonly used units include ohm, kiloohm, and megaohm (represented by Ω, KΩ, and MΩ, respectively).
The main parameters of a resistor that are of interest are:
1) Nominal resistance
The resistance value indicated on the resistor.
2) Resistance deviation
The percentage obtained by dividing the difference between the nominal resistance and the actual resistance by the nominal resistance is called the resistance deviation, which indicates the accuracy of the resistor.
However, in actual circuit design, it is not enough to only focus on these two parameters. There are two other important parameters that must be given sufficient attention in the design: rated power and withstand voltage. These two parameters have a great impact on the reliability of the entire circuit system.
For example, if the current flowing through a resistor in a circuit is 100mA and the resistor's resistance is 100Ω, then according to the circuit power calculation formula P=I*I*R, the power consumed by this resistor can be calculated to be 1W. In this case, choosing a common surface-mount resistor, such as one with a 0805 or 1206 package, is unsuitable, as the circuit will experience problems due to the resistor's low rated power. Therefore, the resistor should be selected with a rated power of 1W or higher (in circuit design, the power margin when selecting a resistor should be at least twice the actual power consumed); otherwise, the power consumed by the resistor will cause it to overheat and fail.
Similarly, an inappropriate voltage rating can cause the resistor to break down, leading to a malfunction in the entire circuit system. For example, according to safety regulations (GB4943.1 standard), the input front-end design of an AC-DC switching power supply module must ensure that after the plug or connector is disconnected, the residual voltage on the input terminals L and N can decay to less than 37% of its initial value within 1 second. Therefore, in actual circuit design, if the resistor's voltage rating is lower than the high voltage at the input terminal, it will fail.
The role of resistor in a circuit
1. Basic Functions
In circuits, resistors are used as voltage dividers, current dividers, and load resistors; together with capacitors, they can form filters and delay circuits; in power supply or control circuits, they are used as sampling resistors; in semiconductor circuits, they are used as bias resistors to determine the operating point of the circuit, etc. These applications in circuits are numerous and very important.
The type of resistor to use depends on its function in the circuit and specific technical requirements. For example, carbon film resistors are sufficient for voltage-dropping and current-limiting resistors, as well as audio load resistors. Metal film resistors are preferable for applications requiring high thermal stability, such as sampling resistors in voltage regulator circuits and timing resistors in delay circuits. For shunt and voltage-dividing resistors in measuring instruments, resistors with higher precision should be selected. We won't go into further detail about these common functions.
We will focus on the role of 0-ohm resistors and special resistors in electronic circuit design, as well as the precautions for their use.
2. The role of a 0-ohm resistor in a circuit
When reviewing electronic products designed by experienced designers, we often see 0-ohm resistors in the circuit diagrams. Why design such a resistor? Why not just connect it with a wire in the circuit diagram? Why add this unnecessary extra detail?
There are several reasons, which we will briefly introduce below:
1) Single-point grounding of analog and digital grounds
We know that in circuit diagrams, all grounds must eventually be connected together and then to earth. If they are not connected, it's called a "floating ground," which creates a voltage difference, easily accumulates charge, and causes static electricity. Ground is a reference zero potential, and all voltages are derived from it; ground standards must be consistent, therefore all grounds should be shorted together. Connecting analog and digital grounds extensively can lead to mutual interference. Not shorting them is also problematic. There are four ways to solve this problem: using a ferrite bead; using a capacitor; using an inductor; and using a 0-ohm resistor.
Let's analyze these four connection methods one by one:
a) Using ferrite beads: The equivalent circuit of a ferrite bead is similar to a band-stop filter, which only significantly suppresses noise at a specific frequency. When using it, the noise frequency needs to be estimated in advance to select the appropriate model. Ferrite beads are unsuitable for situations where the frequency is uncertain or unpredictable.
b) Using capacitors for connection: Capacitors block DC and pass AC, which can easily cause floating ground;
c) Using inductors: Inductors are bulky, have many stray parameters, and are unstable;
d) Using a 0-ohm resistor: A 0-ohm resistor is equivalent to a very narrow current passage, which can effectively limit the loop current and suppress noise. Resistors have attenuation effects across all frequency bands (a 0-ohm resistor also has impedance), which is better than ferrite beads.
2) Used for current loops when bridging
When the ground plane is segmented, the shortest return path for signals is broken. The signal loop must then detour, creating a large loop area. This strengthens the influence of electric and magnetic fields, making it easier to interfere with or be interfered with. Placing a 0-ohm grounding resistor across the segmented area can provide a shorter return path and reduce interference.
3) Circuit configuration
Generally, products should not have jumpers or DIP switches. These manually operable switches can lead to user error, incorrect settings, misunderstandings, or malfunctions. To reduce maintenance costs, 0-ohm resistors should be soldered onto the circuit board instead of jumpers. Since jumpers function like antennas at high frequencies, surface-mount resistors are preferable.
4) Other uses
For debugging/testing during routing: At the beginning of the design, a resistor is often added in series for debugging, but the specific value is not yet determined. Adding such a component facilitates later circuit debugging. If the debugging results show that the resistor is not needed, a 0-ohm resistor can be added. It can also temporarily replace other surface-mount components as a temperature compensation device, often due to EMC countermeasures. Additionally, a 0-ohm resistor has lower parasitic inductance than a via, and vias also affect the ground plane (because they require drilling).
In summary:
It has no function in the circuit; it's just on the PCB for reasons such as ease of debugging or compatibility design.
It can be used as a jumper. If a section of the line is not used, you can simply attach a 0-ohm resistor (it will not affect the appearance).
When the parameters of the matching circuit are uncertain, a 0-ohm resistor is used as a substitute. During actual debugging, after the parameters are determined, the component with a specific value is used as a substitute.
When you want to test the current in a certain part of the circuit, you can remove the 0-ohm resistor and connect an ammeter, which makes it convenient to perform current testing.
If you really can't run the wiring, you can add a 0-ohm resistor.
In high-frequency signal conditions, it acts as an inductor or capacitor (depending on the characteristics of the external circuit), mainly to solve EMC problems, such as between ground and ground, power supply and IC pin.
Single-point grounding (refers to protective grounding, working grounding, and DC grounding, which are separated from each other on the equipment and each becomes an independent system).
3. The role of special resistors in the peripheral protection circuit of the power module
The most common special resistors include thermistors, humidity sensors, and varistors. Varistors play a key role in the design and application of AC-DC switching power supplies.
Varistors (MOVs) are among the most commonly used devices in electromagnetic compatibility (EMC) circuits. They are widely used in electronic circuits to protect circuits from damage caused by transient voltage surges in the power supply system. Simply put, when the upstream voltage exceeds the varistor's threshold voltage, the varistor breaks down, its resistance decreases, and it diverts current, preventing downstream circuits from being damaged or interfered with by excessive transient voltage, thus protecting sensitive electronic components. Circuit protection utilizes the non-linear characteristics of varistors; when an overvoltage occurs between the varistor's terminals, it clamps the voltage to a relatively fixed value, thereby protecting downstream circuits. The main parameters of a varistor include its varistor voltage, current carrying capacity, junction capacitance, and response time.
However, the role of varistors shouldn't be overestimated. Varistors cannot provide complete voltage protection; their energy or power tolerance is limited, and they cannot provide continuous overvoltage protection. Sustained overvoltage will damage the protection device (varistor). Varistors also cannot protect against inrush current during startup, overcurrent during short circuits, and voltage dips; these situations require other forms of protection.
A thermistor is a temperature-dependent device, generally divided into two types: NTC (negative temperature coefficient) thermistors, meaning their impedance decreases as temperature increases; and PTC (positive temperature coefficient) thermistors, meaning their impedance increases as temperature increases. Utilizing the temperature-sensitive characteristic of impedance plays a crucial role in circuit design.
In circuits, the primary function of an NTC (Network Current Tractor) is to suppress the inrush current during circuit startup. During system startup, due to the presence of power circuits, capacitive and inductive loads, a very large inrush current occurs instantaneously. If the instantaneous current withstand capability of components is not considered during component selection, the components can easily be damaged during repeated startup operations. Adding an NTC to the circuit increases the input impedance during startup, reducing the inrush current. When the system is in a stable state, the NTC generates heat, and due to its negative temperature characteristic, its impedance decreases, thus reducing losses on the NTC and overall system losses.
In circuits, a PTC (Positive Temperature Coefficient) acts as a fuse, hence its other name: a resettable fuse. During system operation, if a circuit malfunctions and experiences a large current, a PTC connected in series in that part of the circuit will effectively allow a large current to flow through it. The PTC will heat up, and due to its positive temperature coefficient (PTC), its impedance will increase significantly, thus increasing the overall circuit impedance and reducing the current in the circuit, effectively acting as a fuse. Based on its PTC characteristic, another function of a PTC is to provide over-temperature protection in circuits.
Summary of Resistor Usage in Circuits
The knowledge of resistors is vast, extending far beyond simply knowing Ohm's Law. It encompasses the unique properties of materials, such as the fact that the resistance of a resistor depends not only on temperature, material, and length, but also on its cross-sectional area. The temperature coefficient, defined as the percentage change in resistance for every 1°C increase in temperature, measures the extent to which resistance is affected by temperature. A resistor's primary physical characteristic is its conversion of electrical energy into heat; it is essentially an energy-consuming component, as current passing through it is lost as heat. Resistors typically function as voltage dividers and current dividers in circuits. Both AC and DC signals can pass through resistors. For hardware engineers to use components effectively, a deep understanding of their materials, electrical characteristics, and unique properties is essential.
In summary, although small resistors are insignificant, they play a crucial role in power supply design. They help regulate current and voltage, control current in switching power supplies, and measure current in circuits, ensuring the performance of the power supply and system. Therefore, the role of small resistors cannot be ignored when designing power supplies.
