2, the role of resistance in the circuit:
2.1 fundamental action:
There are electronic engineers to study impact resistance, the divider, shunt and load resistance in the circuit with it; and a capacitor and can form filter and delay circuit, sampling resistor in power supply circuit or circuit with master in semiconductor circuit for use; tube bias resistor must point on these tasks, probation very much, the use of the circuit, is also very important, do not do too much description. The following is mainly to introduce the 0 ohm resistor and special resistance in the design of electronic circuits and the use of the role of attention.
2.20 ohm resistance in the circuit on the role of:
I believe there are many new electrician, watching some elders design electronic products will often see 0 resistance circuit, why such a resistance, not even a direct drawing is good, but also self defeating? After rectification of the search and the materials, the main points are as follows:
1) a single point of ground and digital ground
All it takes is to get together, and then into the earth. If it is not connected with the "floating", there is a pressure difference, the accumulation of charge, the formation of static electricity. Ground is a reference to the 0 potential, all the voltage is derived from the reference to the ground, the standard should be different, so all kinds of places should be short together. People think that the earth can receive all the electric charge, keep the fluctuation, it is the ultimate reference point. Although some boards are not connected to the earth, but the power plant is connected to the earth, the power of the board will eventually go to power plant. If the imitate and digital ground area is directly connected, will lead to mutual interference. Nonshorting and improper, there are four ways to deal with the results: 1, convergence, convergence of the 2 magnetic beads; capacitance; convergence of 3, 4, 0 with inductance; ohmic connection.
The equivalent circuit of magnetic equivalent bandstop limiter, only to a certain frequency of the noise has obvious restraint effect, use demand after measure in order to select the appropriate type of noise frequency. Not sure about the frequency or unpredictable situation, magnetic incompatibility; blocking capacitor through turn formed floating inductor; large volume, spurious parameters, no fluctuation; 0 ohm is equivalent to the current narrow smooth road, can effectively limit the current loop, the noise without restraint. Have a role in the resistance attenuation of all frequency bands (0 ohm impedance, it also has strong magnetic ratio).
2) used in the current loop
When the electric contact stereo signal after the formation of the shortest way back breaking, at this time, the signal loop had to detour loop area is large, the effects of electric and magnetic fields becomes strong, easily disturbed / disturbed by. In the contact area is connected across a 0 ohm resistor, can return short supply routes, reduce trouble.
3) set up the equipment display circuit
In general, do not show jumper products and dial switch. Sometimes the user will touch settings, easy to cause misunderstanding, in order to increase the protection cost, use 0 ohm jumper welding on the board instead of. The vacancy in high frequency equivalent antenna jumper, with patch resistance effect.
4) other uses
The wiring cross line debugging / testing: in the beginning of the design, to a series resistor is used for debugging, but also not sure with the value added of such a device in the future before the convenient circuit debugging, debugging if the consequences do not need to add resistance, resistance to add a 0 ohm. Temporarily replace other patch device as a temperature compensation device, more time is out of the needs of EMC solutions. In addition, the 0 ohm resistance is smaller than the parasitic inductance of the hole, and the through hole can also affect the earth (as a result of digging holes). Summarized as follows:

1, in the circuit does not have any function, just on the PCB in order to facilitate debugging or compatible design and other reasons.
2, can be used if a line jumper, no, the resistance can be directly affixed (not affect appearance)
3, when mating circuit parameter uncertainty, with a 0 ohm alternative, when debugging must practice parameters, with detailed numerical element substitution.
4, would like to test the current consumption of a local circuit, you can go to lose 0 ohm resistor, connected to the current meter, so that it is convenient to measure current.
5, in the wiring, if the real cloth nothing more than to go, you can also add a 0 ohm resistance.
6, in the high frequency signal, an inductor or capacitor (related to the internal circuit characteristics, secondary processing) EMC scores. Such as ground and ground, power supply and Pin IC.
7, single point grounding (refers to the maintenance of grounding, the task of grounding, DC grounding in equipment to leave each other, each become independent system).
2.3 special resistance in the power module core protection circuit
The most rare special resistance has a pressure sensitive resistance and thermal resistance, this in the AC-DC switch power supply design and use of the crux of the role, understand the characteristics of the two types of resistance and the role of the details of the:
MOV is one of the most commonly used devices in the circuit electromagnetic compatibility EMC, and is widely used in the electronic circuit to protect the circuit damage due to the transient voltage gradient of the power supply system. The characteristics of simple understanding of the threshold voltage is a voltage higher than front varistor, varistor breakdown, pressure sensitive resistance decreased and the current will be diverted, to avoid the excessive instantaneous voltage level has been destroyed or disturbed, thus maintaining sensitive electronic components. The nonlinear characteristics of circuit protection is the application of varistor, when overvoltage appears in varistor between the poles, the varistor voltage can be clamped to a fixed voltage absolute value, thus completing the maintenance of secondary circuit. The main parameters of the pressure sensitive resistance are: voltage, current capacity, junction capacitance, echo time, etc..
However, don‘t want to affect the varistor is too big, the varistor is not to provide perfect voltage maintenance, energy or power to accept the varistor is infinite, cannot continue to maintain the supply voltage. Continue over voltage will destroy the maintenance and installation (pressure sensitive resistance), and the formation of equipment damage. The varistor cannot supply maintenance: local as well as the impact of current, on a short circuit when current, voltage sags, protection of these conditions demand other methods.
The thermistor is a coherent device with temperature, the general is divided into two kinds, NTC negative temperature coefficient thermistor temperature is higher, the impedance is smaller; PTC positive temperature coefficient thermistor temperature is higher, the greater the resistance. Application of resistance to temperature sensitive features in the circuit design has played a major role.
NTC mainly for the starting current circuit starts in the process of control circuit, when the system is started in the process, because the system external power circuit, capacitive load and rational, and has very large impact current in the day will start. If the circuit device selection process on device transient current to anti, so fragmented in repeated startup operation process, it is easy to cause the device to be breakdown, and in NTC in the circuit, the output circuit is equal to the start output impedance increases the impact of current progress, piecemeal in wave form, because the NTC have a fever according to its characteristics, negative temperature, impedance decreased, thus the loss in NTC also decreased, increased the loss of system.
PTC in the circuit can play a role in the fuse, so it has another name for the self recovery fuse. In system operation process, circuit show abnormal, leading to emergence of the current, if the local circuit in series with a PTC, then it is equal to PTC in a large current flow, PTC have a fever, according to the positive temperature characteristics, the impedance will become very large, so that all the larger impedance circuit, thereby the loop current is smaller, the impact fuse. According to the characteristics of its positive temperature, PTC is another role in the circuit to complete the over temperature maintenance.