What is the role of ohmic resistors in electric vehicle chargers?
I believe that there are many new electricians. When looking at some electronic products designed by predecessors, they often see that there is a 0Ω resistor on the circuit. Why should we design such a resistor? Baidun searches and organizes the data, the main points are as follows:
1) Analog ground and digital ground are grounded at a single point
As long as it is the ground, it will eventually be received together, and then enter the earth. If they are not connected together, they are "floating ground", there is a voltage difference, and it is easy to accumulate charges and cause static electricity. The ground is referenced to 0 potential, and all voltages are derived from the reference ground. The standard of the ground should be the same, so the various grounds should be shorted together. The earth is believed to be the ultimate ground reference point, capable of absorbing all electrical charges, remaining stable at all times. 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 into the ground. If the analog ground and the digital ground are directly connected in a large area, it will cause mutual interference. There are four ways to solve this problem: 1. Use magnetic beads to connect; 2. Use capacitors to connect; 3. Use inductors to connect; 4. Use 0 ohm resistors to connect.
The equivalent circuit of the magnetic bead is equivalent to a band-stop wave limiter, which can only significantly suppress the noise at a certain frequency. The frequency of the noise needs to be estimated in advance in order to select the appropriate model. In the case of uncertain or unpredictable frequency, the magnetic beads do not fit together; the capacitor blocks the direct traffic, causing floating; the inductance is large and has many stray parameters, which are unstable; 0 ohm resistance is equivalent to a very narrow current path, which can effectively limit loop current, so that noise is suppressed. Resistors have attenuation in all frequency bands (0 ohm resistors also have impedance), which is stronger than magnetic beads.
2) For current loops when connected across
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, and the influence of the electric field and the magnetic field becomes stronger, and it is easy to be disturbed/disturbed. Connecting a 0 ohm resistor across the partition can provide a short return path and reduce interference.
3) Configure the circuit
Generally, jumpers and DIP switches should not appear on the product. Sometimes users will tamper with the settings, which is easy to cause misunderstandings. In order to reduce maintenance costs, 0 ohm resistors are used instead of jumpers to be welded on the board. The vacant jumper is equivalent to the antenna at high frequency, and the chip resistor is effective.
4) Other uses
For cross-line debugging/testing during wiring: At the beginning of the design, a resistor should be connected in series for debugging, but the specific value cannot be determined yet. After adding such a device, it is convenient to debug the circuit later. If the result of debugging does not need to add a resistor , add a 0 ohm resistor. Temporary replacement of other SMD devices as temperature compensation devices is more often due to the need for EMC countermeasures. In addition, 0 ohm resistors are smaller than the parasitic inductance of vias, and vias also affect the ground plane
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