What is a Zero-ohm Resistor?

A zero-ohm resistor, also referred to as a jumper resistor, is a specialized component with a nominal resistance value of zero ohms. It is commonly used in PCB design and other applications as a versatile, idealized component. Despite being labeled ‘zero ohms,’ the actual resistance of a zero-ohm resistor is not exactly zero, but very close to it. Zero-ohm resistors typically come in two packaging types: axial lead and surface-mount configurations.

Functions of a Zero-ohm Resistor

Debugging and Compatibility

In PCB design, hardware engineers often need to address compatibility issues. For instance, a specific pin on a chip might have dual functions—such as driving either a buzzer or an LED—but these functions cannot operate simultaneously. To choose between the two, a zero-ohm resistor can be added in the circuit paths for the buzzer and the LED. By soldering the zero-ohm resistor on either the buzzer’s or the LED’s path, engineers can select which module to drive.

As illustrated in the diagram below, engineers can select the signal path and solder the corresponding component, leaving the unused path unconnected.

In embedded system configuration, zero-ohm resistors are also used for selecting different boot sources via on-board settings.

Used as Jumpers

Zero-ohm resistors can replace traditional jump wires to reduce wiring complexity, improve aesthetics, and prevent high-frequency interference that might arise from using jump wires.

Bridging Circuits During Routing

During PCB layout and routing, certain connections may be difficult to establish, particularly on smaller boards with dense connections. In such cases, a zero-ohm resistor can be used to bridge existing traces without adding extra layers, thus helping to reduce production costs.

Reserved for Debugging

When precise circuit parameters are not yet determined, zero-ohm resistors are often used as placeholders during testing. For example, if there are two potential voltage supply options—5V and 10V—uncertainty about their effect on the circuit might require a zero-ohm resistor to connect them temporarily during debugging, allowing engineers to select the power supply without altering the design.

Measuring Circuit Power Consumption

When measuring the power consumption of a circuit or a chip, a zero-ohm resistor can be placed in series with the power supply. After completing the prototype, the resistor can be removed, and the current can be measured directly between the two solder points. This helps determine the actual current consumption of the circuit, which is essential for power calculations. If measuring current in specific sections of the circuit, a zero-ohm resistor can facilitate accurate current measurement, especially for high-current applications.

As shown in the diagram below, a zero-ohm resistor is placed between the power supply terminal and the measured circuit.

Acting as Capacitors or Inductors in High-Frequency Signals

At high frequencies, a zero-ohm resistor, in combination with external circuit characteristics, can function as a small capacitor or inductor. This can help address EMC issues, particularly between ground connections or between the power supply and chip pins.

Serving as Fuses for Overcurrent Protection

PCB traces typically have a high fusing current, making them difficult to fuse during short circuits or overcurrent events. However, zero-ohm resistors have a lower current-carrying capacity, causing them to melt first in the event of an overcurrent, thereby disconnecting the circuit and preventing more severe damage.

Single-Point Grounding for Analog and Digital Grounds

In mixed signal circuits, especially those involving both digital and analog components, it is often necessary to have separate grounds that are connected at a single point. A zero-ohm resistor can serve as the link between these two grounds, rather than directly connecting them, which helps avoid issues such as floating ground and static charge accumulation, particularly during processes like large-scale copper pouring.

Alternatives to address this issue include: 1) ferrite beads, 2) capacitors, 3) inductors, and 4) zero-ohm resistors.

Used in Current Loop for Bypassing

If a ground plane is split, the return path for a signal might be disrupted, increasing the loop area and making the circuit more susceptible to electric and magnetic interference. By placing a zero-ohm resistor across the split, the return path is shortened, reducing potential interference.

As illustrated in the diagram below, when two signals communicate over different ground planes, placing a zero-ohm resistor between them ensures the signals follow the shortest path.

Configuration Circuit

This function is similar to jumpers or DIP switches, but zero-ohm resistors are soldered into place to prevent inadvertent modifications by end users. By installing resistors in different positions, the circuit’s functionality or address settings can be adjusted.

Noise Suppression

Because of its properties, a zero-ohm resistor can effectively suppress loop currents and attenuate noise. While a true zero impedance can only be achieved by superconductors, a zero-ohm resistor provides a degree of attenuation across various frequency bands.

What is the difference between a zero-ohm resistor and a wire?

While there are similarities, there are key differences between a zero-ohm resistor and a wire. As noted in the functions discussed earlier, zero-ohm resistors have a range of capabilities. In some applications, such as circuit bridging, zero-ohm resistors can serve the same function as wires. However, in surface mount technology (SMT), machines can easily recognize zero-ohm resistors, making them more convenient for manufacturing compared to wires, which might not be as easily detected by SMT equipment.

Is there a difference in the accuracy of zero-ohm resistors?

Yes, zero-ohm resistors come in various accuracy ratings. For example, they may be specified with an accuracy of ±1% or ±5%. Additionally, they are available in a variety of package sizes, such as 0603, 0805, 1206, 1210, 1812, etc. The accuracy of the resistor influences its size and form factor, as shown in the diagram below.

How much current can a zero-ohm resistor handle?

The selection of a zero-ohm resistor’s rated power depends on the current in the circuit. The typical resistance of a zero-ohm resistor is around 50 milliohms with a tolerance of approximately ±5%. Based on its rated power, the current capacity of a zero-ohm resistor can be calculated.

For different package sizes, the rated current calculation is as follows:

0402 1/16W: 1/16 = I * I * 0.05, which gives I = 1.118A

0603 1/8W: 1/8 = I * I * 0.05, which gives I = 1.58A

0805 1/4W: 1/4 = I * I * 0.05, which gives I = 2.236A

The specific current capacity of a zero-ohm resistor also depends on how well it dissipates heat on the PCB.