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What Are Zero-Ohm Resistors Used For?

If you are not familiar, the term “zero-ohm resistor” might seem like some sort of joke to play on new or naïve circuit designers. After all, they are under constant pressure to simplify their designs and reduce the bill of materials (BOM). However, the zero-ohm resistor is not a joke at all. For proof, consider that at the time of this writing, Digi-Key has 150,000 of one version in stock. Clearly, they are being used.

So, why would you even want one? What can it add to a design that wouldn’t be better served by getting rid of it, thus saving cost and board space?

There are at least three reasons where this apparently “useless” component makes sense. Two are related to design, test, and manufacturing, while the third… well, let’s just say it has little to do with these.

1: Printed Circuit Board Layout

Figure 1: This single-sided phenolic pc board from a 2010 microwave oven contains the power supplies (low and high voltage), transformer, and power devices; note the use of top side jumpers along the top edge about one-third of the way from the left side. (Image source: Amazon.com)

Let’s start with an older reason which is still viable. About fifty years ago, in the early days of this new thing called “printed circuit boards,” or pc boards, our standard FR4 glass epoxy board clad on two sides did not exist. Instead, those earliest boards were made of pressed phenolic paper with copper on only one side. Components were inserted by hand, which was feasible since most of them were large such as vacuum tube sockets, discrete transistors, passives, transformers, and connectors.

It took real skill to lay out a board’s wiring using traces on only one side, and there were times when it was simply impossible. The solution: wire jumpers were added to “bridge over” areas to enable the connection between two traces. As machine-based insertion took over, the basic wire jumper was replaced with a discrete, standard body zero-ohm resistor to serve the same function.

Single-sided phenolic boards and their jumpers are still in use. Even modern appliances such as coffee machines or microwave ovens still use single-sided phenolic boards when there are larger components to mount, such as transformers, and use jumpers to solve topology problems (Figure 1).

2: Circuit and board flexibility

Zero-ohm resistors still have a place in our modern multilayer FR-4 board design. There may be cases where the layout routing is so complicated that some path connections simply cannot be completed. The solution is to “buy” an extra layer for a few cents in that critical spot via a zero-ohm resistor.

These resistors can also ease the reconfiguration of a circuit’s interconnection and operation. They enable complete electrical separation between a board’s subcircuits for debugging and testing, as it is easier to unsolder/solder even a tiny SMT zero-ohm resistor than to cut and then try to restore a hair-thin PCB trace. They also can be used to short-out circuit functions such as extra filter stages that are not needed in all configurations or may have to be disabled for test and calibration cycles.

Another use is for these resistors to enable a single pc board layout to be tailored to different configurations even after the board is populated and soldered. In the simplest case, consider a signal path that needs either zero ohms or 10 ohms (Ω) in a damping or snubber circuit, with the correct value determined by the specifics of the load that the product will be driving. The board can be laid out to accommodate a single resistor of either zero ohms or 10 Ω, and the appropriate value put on the BOM for that assembly run or inserted and soldered by hand. Alternatively, the circuit and pc board can be designed with both the zero-ohm and 10 Ω resistors in parallel, and the zero-ohm one removed if 10 Ω is the correct value.

There is another option: create two pc board layouts, one with a resistor in place and one without any resistor. However, it is cheaper, smarter, and better inventory management to have just one board and insert/remove the zero-ohm resistor as needed.

3: A thin veil over the schematic

Finally, there’s a less obvious rationale for zero-ohm resistors: to complicate and conceal a circuit’s function and confuse someone trying to trace out and thus reverse-engineer a design. While this was more commonplace in the early days of simpler single-sided boards with mostly analog circuitry, it is still done for lower density areas such as power functions. When scoping out a circuit diagram this way, the first step is to trace out the schematic, followed by trying to identify the various components and their roles. Slipping in a few zero-ohm resistors makes that second step more complicated (it’s somewhat analogous to the “dirty” software trick of using  NOPs to adjust program and loop timing).

Zero ohms, multiple packages

Zero-ohm resistors are available as both single and multiple units. For example, the SR1-0805-000 from NTE Electronics, Inc is a single chip resistor in a standard 0603 1.5 × 0.8 millimeter (mm) (0.06 × 0.03 inches) surface mount technology (SMT) package (Figure 2).

Figure 2: The SR1-0805-000 from NTE Electronics, Inc is a zero-ohm resistor in an 0603 package that looks and handles like any other SMT chip component. (Image source: NTE Electronics)

For situations where multiple zero-ohm resistors are needed in proximity to one other, the Panasonic EXB-28VR000X array with four resistors in an 0804 package is available (Figure 3).

Figure 3: Panasonic’s EXB-28VR000X is a four-resistor zero-ohm array in a standard 0804 package. (Image source: Digi-Key Electronics, using source material from Panasonic)

Interestingly, zero-ohm resistors have two unusual attributes in their specifications. First, they do not have a tolerance specification. That number is normally called out as plus-or-minus some percentage of the nominal value of the resistor, which is meaningless at zero ohms. Second, these jumpers do have a maximum power rating, which seems unnecessary as their dissipation defined by I2R and R here is 0 ohms. However, even a zero-ohm resistor is not perfect: most do specify a maximum actual resistance such as 50 milliohms (mΩ), which mandates maximum current ratings (Table 1).

Table 1: Even a zero-ohm resistor is not perfect: most do specify a maximum actual resistance, such as 50 mΩ, which mandates maximum current ratings. (Table source: Panasonic)

Conclusion

The zero-ohm resistor is an excellent example of a component whose function seems unnecessary at first, and perhaps even useless. However, it is quite useful for designers who are aware of it and understand how it can help solve circuit and layout problems at very low cost with no or minimal complications. For those reasons, vendors offer them in various configurations.

Recommended Reading:

Use Fine-Pitch Board-to-Board Connectors to Optimize System Packaging

Printed Circuit Boards: So Much Responsibility, So Little Respect

Use Direct Plug-in Insulation Displacement Connectors to Streamline Assembly and Lower BOM

About this author

Image of Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

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