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  • The difference between zero-ohm resistance and direct wire connection

    What is a zero ohm resistor? 

    A Zero-ohm resistor is a wire link used to connect traces on a printed circuit board and is packaged in the same physical package format as a resistor. This design format allows it to be placed on the circuit board using same automated equipment used to place other resistors, instead of using other a separate machine to install a jumper or other wire. The resistance is approximately zero (typically its 10-50 mΩ maximum is specified). A percentage tolerance would make sense as it would be specified be specified as percentage of ideal value of zero-ohm, so it is not specified.

    What is a direct-wire-connection? 

    A Direct-wire-connection describes the use of a wire to aid in flow of currents of any ratings as per the size of the conducting wire. Therefore, we will not connect directly using wire instead of a zero-ohm resistor because initially one uses zero-ohm resistor to allow traces on same side of PCB to cross to another trace, one trace has a zero-ohm resistor while the second trace runs in between the leads of the resistor, hence avoiding contact with first trace.

    Figure 1 Zero-Ohm Axial-Lead Resistor

    Figure 1 Zero-Ohm Axial-Lead Resistor

    Figure 2 Zero-Ohm Surface-Mount Resistor

    Figure 2 Zero-Ohm Surface-Mount Resistor

    What are the main advantages of zero ohm resistors?

    In fact, zero-ohm resistors and direct wire connections are still different, and zero-ohm resistors have many uses in circuit design and technological advantages.

    1. Debugging and compatibility of PCB Board or Circuit

    When designing a PCB, we need to consider as much compatibility as possible, because a PCB board is physically fixed after physical printing and copper processing. If the compatibility problem is not fully considered in the design, it will bring a lot of inconvenience to the engineer during the debugging phase of the PCB board. Here, the zero-ohm resistor will play a key role for debugging because a zero-ohm resistor is used to connect PCB traces.

    For example, a pin of a microchip has two functions, such as driving a buzzer and driving an LED. However, these two functions cannot work at the same time. In order to realize which period can be selected for driving on the same circuit board, it is possible to add zero-ohm resistance to the line connecting the buzzer and the LED, and which zero on the path is soldered. In this example, circuit functionality decides whether to drive the buzzer or the LED light.

    1. Convenient wiring

    The main factor a zero-ohm resistors are used is because components in most printed circuit boards (PCB) are inserted by automatic insertion machines. In some instances, it may be necessary to short two points on the PCB, in which a wire would normally be placed between the two points. But, these automatic insertion machines can only handle components such as resistors, and not jumper wires. The separate jumper wire machine would have to be used to insert the jumper wires and/or jumper wires can be installed manually by a person. However, instead zero-ohm resistors are used in place.

    Zero-ohm resistors are ideal in that they can be more easily removed than the jumper wires. Further, if after the design there are changes needed to be made, zero-ohm resistor could be easily removed and new component be put in its place conveniently.

    1. Reserve Resistance Position

    If at the circuit design stage, a certain position is uncertain, how much resistance should be connected to the resistor? At this point, the soldering position of the resistor can be left at this position and the zero-ohm resistor can be soldered. In the actual circuit debugging, it is convenient to change the resistance of different resistance values, and then connect the appropriate resistance after debugging the determined resistance parameter.

    1. Convenient Current Measurement

    After designing any circuit system, it is usually necessary to test power consumption of the entire circuit during operation. It is conventional practice to calculate the power consumption by taking measurements of current and then using measured current and voltage to determine power consumptions. Here, the method of determining the current is usually to measure the current of the circuit using current instruments.

    At this time, if a zero-ohm resistor is placed where the current needs to be measured, the resistor is removed when the measurement is needed, and the ammeter or multimeter is connected. For normal operation, solder directly to the zero-ohm resistor.

    1. Electrical Noise Reduction

    Due to the characteristics of the zero-ohm resistor itself, the loop current can be effectively suppressed, thereby suppressing noise. In fact, zero-ohm resistance has negligible impedance; only superconductors can effectively achieve zero impedance. Therefore, the zero-ohm resistance actually plays a role in attenuation in all frequency bands and range.

    1. Circuit Effective Operation

    Many pins are often seen on many boards and need to be terminated with a jumper cap. Or use the DIP switch to control whether the circuit is closed. Although these two methods are more convenient during the debugging phase, it is best to use them as little as possible when making products. Since the vacant pins are equivalent to the antenna in the high-frequency circuit, it is easy to disturb the signal.

    In addition, the DIP switch is easily disturbed by unsuspecting people, causing circuit system errors. Therefore, for safety reasons, it is best to use a zero-ohm resistor instead of a pin and a dip switch. It can avoid misuse and reduce maintenance costs.

    1. Acting as a capacitor inductor

    In the high-frequency circuit system, a zero-ohm resistor can be used as a small capacitor or inductor when it matches the characteristics of the external circuit, which can solve the EMC problem well. For example, between ground and ground, or between the power supply and the chip pins.

    1. Ground wire isolation

    In chip design, the ground of analog circuit is called AVSS, and the ground of the digital circuit is called VSS. AVSS and VSS are usually separated inside the chip. Since the bottom line is separated, the current signals can be prevented from interfering with each other during operation of the analog circuit and the digital circuit.  However, the ground wires at the board level are usually connected together at the end. At this point, AVSS and VSS ground pin are first connected through the zero-ohm resistor and then connected together to play an isolation role where the analog circuit and the digital circuit current signals are prevented from interfering with each other.

    How to achieve a strong advantage of a zero ohm resistor link by direct wire connection?

    Robust advantage of Zero-ohm resistor link over direct wire connection.

    Practically, zero-ohm resistors are useful as configuration jumpers, but caution is taken for PCB designs which may use zero-ohm resistors to select between options which require larger trace currents in the design. For such case, it is better design practice to specify a low-ohm resistance such as 0.001Ω to 0.003Ω resistor, rather than generic zero-ohm resistor in which its actual resistance may be higher and tolerance not given. The low-ohm resistors are easily obtained with 5% or 1% tolerances on a maximum specified resistance and can be safely utilized to pass higher currents.

    For example; a surface mounted 0805 size resistor of 0.003 ohms, rated at 0.5 watt theoretically can safely pass upto 12.9 Amperes of current.

    In real practical case, when approaching power limit for the given package, it’s always good practice to either use an even lower-ohm product or go up one package size larger (same power efficiency but cost may be kept lower). The cost associated with taking up more board space for the larger package may also be a consideration.

    In this example, for 12 Amperes to pass through jumper, a lower resistance or a bigger surface-mount package (such as a 1206) is usually specified. In contrast, a worst-case zero-ohm real-world jumper with 0.05-ohm impedance in a similar 0805 package could only pass 3.1 amperes maximum. The use of specific tolerance resistances is a much safer design practice for higher currents than zero-ohm option, although the bill-of-materials cost can be higher for low-ohm devices.


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