Some newcomers are shocked when they learn about a zero ohm (0R) resistor. These curiosities do actually come in handy for practical electronics design. Here are five good examples.
Bridging over tracks
In some electronics design cost is a major constraint to balance, along with circuit function. To reduce costs, reduced layer counts may be employed, e.g. two layer or even single layer PCBs. Take the single layer scenario. The layout of the components may be such that the resulting tracking is impossible. A 0R resistor 'cheats' the single layer constraint by bridging one track over another.
Component values to be determined
Sometimes component values aren't known until the circuit has been built. A good example of this is an antenna matching network. It can be difficult to simulate the entire product (electronics and mechanicals) to gauge the feedpoint impedance of the antenna. Instead, a generic matching network is designed in, using 0R resistors as placeholders. For series network elements, the 0R are fitted. For shunt elements, they are not fitted. Once the circuit has been built, the feedpoint impedance can be measured, and then the matching network can be designed (and tested!) using low-loss inductors and capacitors. If you're lucky, only a Bill Of Materials (BOM) change is needed at this point.
It's worth noting that if this board design goes into production with an Engineering Change Order (ECO) on BOM for the inductors and capacitors, the PCBA house may get confused with the old R# designators of the 0R placeholders.
Bringup or debug
For risky designs with some unknowns, it can be helpful for future you to include 0R resistors in select spots. You will be thankful for these 0R resistors.
For example, it may be helpful to debug I2C issues by having every slave fitted with 0R resistors. This allows for an easy disconnect of I2C devices to aid in debug. It also allows for a convenient tapping point for sniffing the I2C bus with flying leads.
Another example is power supply bringup. It may be useful to validate only the power supply section by disconnecting the output (or input) with 0R resistors. This allows for the validation with an external electronic load or power supply with controlled variables.
Trailing a variation of a circuit
In the scenario that you have a known-good circuit, but want to trail a variation of the circuit or alternative part, it can be a good idea to include both together on the board (space and cost permitting). 0R resistors can be used to select the power or signal path between the original known-good circuit, and the untested new circuit.
Chip strapping
Some ICs are configurable by pulling particular pins high or low, e.g. the LSB of a chip's I2C address. It is typically worthwhile to include both the pull-up and pull-down options, despite initially bing confident on the strapping option to use.
I have found using 0R for these strapping options to be controversial amongst peers:
- A co-worker argued that 0R shouldn't be used in case both the pull-up and pull-down are fitted (via BOM or PCBA mistake), and instead use something with current limiting (e.g. 10k) so to not cause a short circuit.
- My counter argument: using 0R will allow this BOM/PCBA mistake to be easily visible via the short circuit. 10k straps with the BOM/PCBA mistake may go unnoticed or cause an unintended strapping option.
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