Choose resistors carefully! Different resistors can play different roles in
circuit design
Resistors are one of the most commonly used devices in electronic products.
Basically, as long as it is an electronic product, there will be resistance
inside. Resistors can be used as voltage dividers, shunts and load resistors in
circuits; together with capacitors, they can form filters and delay circuits;
they can be used as sampling resistors in power supply circuits or control
circuits; they can be used as bias in semiconductor tube circuits. Set the
resistor to determine the working point; use resistors with special properties
such as varistor and thermistor to prevent surge voltage, suppress inrush
current, achieve over-temperature protection, etc. Resistors are the most common
components and are also indispensable components in circuits. Choosing and using
resistors properly is crucial to the stable operation and reliability of the
product.
0 Preface
There are many types of resistors. Commonly used resistors include carbon
film resistors, cement resistors, metal film resistors, wire-wound resistors,
etc.; special resistors include varistor, thermistor, photoresistor, etc.
Different types of resistors have certain differences in their characteristic
parameters, and the points that need to be considered when using the circuit are
also different. For engineers who are new to circuit design, they may ignore
some special parameters of resistors, resulting in product stability and
reliability not being guaranteed. Only by correctly understanding the various
parameters of resistors and the precautions for selection, and comprehensively
understanding the real role of resistors in the circuit, can we ensure the
quality of the product from the most basic circuit design at the bottom.
1Basic parameters of resistors
Engineers who are new to hardware circuit design may have their first
impression of resistance. The resistance of conductors to current described in
physics books is called resistance, represented by the symbol R, and the units
are ohms, kiloohms, and megohms, respectively. , KΩ, MΩ said. The main
parameters of concern are 1), nominal resistance value: the resistance value
marked on the resistor; 2), allowable error: the percentage of the difference
between the nominal resistance value and the actual resistance value and the
nominal resistance value. Value deviation, which represents the accuracy of the
resistor. In the design of the circuit, it is not enough to only pay attention
to these two parameters. There are two other important parameters that must be
paid attention to in the design: rated power and withstand voltage value. These
two parameters have an important impact on the reliability of the entire system.
The impact is very large.
For example, if the current flowing through the resistor in the circuit is
100mA and the resistance is 100Ω, then the power consumption on the resistor is
1W. It is not appropriate to choose commonly used chip resistors, such as
package 0805 or 1206, because the rated power of the resistor is small. And
something went wrong. Therefore, the rated power of the selected resistor must
be above 1W (the power margin of the resistor selected for circuit design is
generally more than 2 times), otherwise the power consumed on the resistor will
cause the resistor to overheat and fail.
Similarly, if the withstand voltage value is not selected appropriately,
the system design will fail due to breakdown of the resistor. For example: at
the input front end of the design of the AC-DC switching power supply module,
according to the requirements of the safety standard GB4943.1, after ensuring
that the plug or connector is disconnected, the residual voltage on the input
terminals L and N is within 1S. attenuates to 37% of the initial value.
Therefore, during design, resistors with one or two MΩ level impedances are
generally used for energy discharge, and the input end is high voltage, that is,
both ends of the resistor must withstand high voltage. When the resistor If the
withstand voltage value is low and the input terminal is high, failure will
occur. Table 1 below shows the parameters of common SMT thick film resistors.
You should check with the manufacturer of the device when making the final
selection.
2The role of resistors in circuits
2.1 Basic functions
Electronic engineers have all learned the basic functions of resistors,
which are used as voltage dividers, shunts and load resistors in circuits;
together with capacitors, they can form filters and delay circuits, and can be
used as sampling resistors in power circuits or control circuits. ; Used as a
bias resistor in semiconductor tube circuits to determine the operating point,
etc. For these functions, there are many applications in the circuit and they
are also very important, so we will not describe them too much. The following
mainly introduces the role of 0Ω resistors and special resistors in electronic
circuit design and the precautions for their use.
2.The role of 20 ohm resistor in the circuit
I believe that many new electricians will often see a 0Ω resistor on the
circuit when looking at some electronic products designed by their predecessors.
Why should we design such a resistor? It would be better if we just connect it
to the drawing board. Why are we superfluous? Through searching and organizing
the information, the key points are as follows:
1) Single-point grounding of analog ground and digital ground
As long as it is the earth, it will eventually be connected together and
then enter the earth. If they are not connected together, they will float and
there will be a voltage difference, which will easily accumulate charges and
cause static electricity. The ground is referenced to 0 potential, and all
voltages are derived with reference to the ground. The ground standards must be
consistent, so various grounds should be shorted together. The earth is believed
to be able to absorb all electrical charges, remain stable, and is the ultimate
ground reference point. Although some boards are not connected to the ground,
the power plant is connected to the ground, and the power on the board will
eventually return to the power plant and into the ground. If the analog ground
and digital ground are directly connected over a large area, mutual interference
will occur. It is not appropriate to short-circuit. There are four ways to solve
this problem: 1. Connect with magnetic beads; 2. Connect with capacitor; 3.
Connect with inductor; 4. Connect with 0 ohm resistor.
The equivalent circuit of the magnetic bead is equivalent to a
band-rejection limiter, which only has a significant suppression effect on noise
at a certain frequency point. When using it, you need to estimate the noise
frequency in advance to select an appropriate model. For situations where the
frequency is uncertain or unpredictable, the magnetic beads do not match; the
capacitor blocks DC and causes floating ground; the inductor is large, has many
stray parameters, and is unstable; the 0 ohm resistor is equivalent to a very
narrow current path, which can effectively limit Loop current suppresses noise.
Resistors have attenuation effects in all frequency bands (0 ohm resistors also
have impedance), which is stronger than magnetic beads.
2) Used for current loop when bridging
When the electrical ground plane is divided, the shortest return path of
the signal is broken. At this time, the signal loop has to be detoured, forming
a large loop area. The influence of the electric field and magnetic field
becomes stronger, and it is easy to interfere/be interfered with. Connecting a 0
ohm resistor across the partition can provide a shorter return path and reduce
interference.
3) Configure circuit
Generally, jumpers and DIP switches should not appear on the product.
Sometimes users will tamper with the settings, which can easily cause
misunderstandings. In order to reduce maintenance costs, 0 ohm resistors should
be soldered to the board instead of jumpers. The empty jumper acts as an antenna
at high frequencies, and chip resistors work well.
4) Other uses
For cross-line debugging/testing during wiring: When starting the design, a
resistor should be strung in series for debugging, but the specific value cannot
be determined yet. Adding such a device will facilitate future circuit
debugging. If the debugging results do not require adding a resistor , just add
a 0 ohm resistor. Temporarily replacing other patch devices as temperature
compensation devices is more often due to the need for EMC countermeasures. In
addition, the 0 ohm resistor has smaller parasitic inductance than the via hole,
and the via hole will also affect the ground plane (because the hole needs to be
dug).
Summarized as follows
1. It does not have any function in the circuit, it is just on the PCB for
debugging convenience or compatible design reasons.
2. It can be used as a jumper. If a certain section of the line is not
used, just attach the resistor directly (does not affect the appearance)
3. When the matching circuit parameters are uncertain, replace them with 0
ohms. During actual debugging, determine the parameters and replace them with
components with specific values.
4. When you want to measure the current consumption of a certain part of
the circuit, you can remove the 0 ohm resistor and connect an ammeter to
facilitate the measurement of current consumption.
5. When wiring, if it cannot be laid, you can also add a 0 ohm
resistor.
6. Under high-frequency signals, it acts as an inductor or capacitor
(related to external circuit characteristics), mainly to solve EMC problems.
Such as between ground and ground, power supply and ICpin.
7. Single-point grounding (meaning that protective grounding, working
grounding, and DC grounding are separated from each other on the equipment and
each becomes an independent system).
2.3 The role of special resistors in the peripheral protection circuit of
the power module
The most common special resistors are varistor and thermistor, which play a
key role in the design and application of AC-DC switching power supply. Learn
about the characteristics and specific functions of these two resistors:
Varistor MOV is one of the most commonly used devices in circuit
electromagnetic compatibility EMC. It is widely used in electronic circuits to
protect circuits from possible damage due to instantaneous voltage mutations in
the power supply system. Its characteristics are generally understood as when
the front-end voltage is higher than the turn-on voltage of the varistor, the
varistor is broken down, and the resistance of the varistor is reduced to shunt
the current to prevent the subsequent stage from being damaged or interfered by
excessive instantaneous voltage. This protects sensitive electronic components.
Circuit protection utilizes the nonlinear characteristics of the varistor. When
overvoltage occurs between the two poles of the varistor, the varistor can clamp
the voltage to a relatively fixed voltage value, thereby protecting the
subsequent circuit. The main parameters of the varistor are: varistor voltage,
current capacity, junction capacitance, response time, etc.
However, don’t overestimate the role of the varistor. The varistor cannot
provide complete voltage protection. The energy or power that the varistor can
withstand is limited, and it cannot provide continuous over-voltage protection.
. Continuous overvoltage can destroy protective devices (varistors) and cause
damage to the equipment. The parts that varistor cannot provide protection
include: inrush current during startup, overcurrent during short circuit,
voltage sag, etc. These situations require other methods of protection.
The thermistor is a temperature-related device, generally divided into two
types. NTC is a negative temperature coefficient thermistor, that is, the higher
the temperature, the smaller the impedance; pTC is a positive temperature
coefficient thermistor, which means the higher the temperature, the smaller the
impedance. The bigger. Utilizing the sensitivity of impedance to temperature
plays an important role in circuit design.
NTC in the circuit mainly suppresses the starting current during the
circuit startup process. When the system starts, due to the presence of power
circuits, capacitive and inductive loads inside the system, a very large inrush
current will appear at the startup moment. If the instantaneous current
resistance capability of the device is not considered in the circuit device
selection process, the device will easily be broken down and damaged during
multiple starts of the system. Adding NTC to the circuit is equivalent to
Increasing the input impedance reduces the inrush current. When the system is in
a stable state, due to the heating of the NTC, the impedance decreases according
to its negative temperature characteristics, thereby reducing the loss on the
NTC, reducing the overall loss of the system.
pTC can play the role of a fuse in the circuit, so it has another name as a
self-restoring fuse. During the operation of the system, an abnormality occurs
in the circuit, resulting in a large current. If there is a pTC in series in
this part of the circuit, it means that a large current flows through the pTC,
and the pTC generates heat. According to its positive temperature
characteristics, its The impedance will become very large, making the impedance
of the entire loop larger, thereby making the current in the loop smaller,
acting as a fuse. According to its positive temperature characteristics, another
function of pTC is to implement over-temperature protection in the circuit.
3 Conclusion
The knowledge of resistance covers a lot, not only knowing Ohm's law and
applying it well, but also including materials and their special properties. For
example, the resistance value of a resistor element is generally related to
temperature, material, length, and cross-sectional area. , the physical quantity
that measures the effect of temperature on a resistor is the temperature
coefficient, which is defined as the percentage change in the resistance value
when the temperature increases by 1°C; the main physical characteristic of the
resistor is that it converts electrical energy into heat energy, which can also
be said to be an energy consumer. Components, when current passes through them,
losses occur, which are expressed in the form of heat energy; resistors usually
play the role of voltage divider and current shunt in circuits; for signals,
both AC and DC signals can pass through resistors, etc. As a hardware engineer,
if you want to use components with ease, you need to have an in-depth
understanding of their materials, electrical characteristics and
particularities.
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