I’ve noticed that some schematics incorporate 0Ω or 100mΩ resistors. What role do these resistors play, and why are they included in PCB designs?
Typically, when we want to measure the current drawn by a load, we place a jumper pin across the PCB trace and then measure the current across the pin using a multimeter. Incorporating resistors for this purpose might seem like a waste of PCB real estate. Is this the primary reason why 100mΩ resistors are used instead of jumper pins?
If so, are there any considerations we should bear in mind when placing such mΩ resistors on the board to ensure they don’t impact the signal or behavior of the circuit?
“Zero” ohm resistors have many useful applications. First, they can act like jumpers, allowing other traces to pass beneath the component, allowing denser routing and maybe prevent adding more layers. Wire jumpers were very popular (and necessary) in the days of single-sided PCB’s. As automated assembly became more common, component makers created a wire jumper that was packaged like a resistor and allowed pick–and-place machines to handle and insert them the same as any resistor. They quickly found other uses such as configurement and allowing a board to be used for multiple applications. They could also function as shunts for current monitoring circuits. They are also useful on breadboards. For the most part you can treat them just like traces.
The typical uses for 0 Ohms and mOhms size resistor has been stated by others, but from a practical point of view, there are limitations as to how “similar” they are to each other.
Suppose you have a part of your circuit that may be powered from either of two voltages, the choice being made during board manufacture, the use of a 0 Ohms resistor to select which supply sounds like a good choice. But how much current can a 0 Ohms resistor handle?
Taken at face value, any current is fine because with a resistance of 0 Ohms, whatever the current you get no voltage drop, so no power dissipated in the component. Reality is different though. a 0402 size 0 Ohms resistor may be limited to as little as 1 Amp (read the data sheets). This could be a show-stopper if say this resistor happened to be selecting a 1.5V or 1.35V supply for an FPGA where the current could be much higher than 1 Amp.
Resistors designed for current sensing are expecting to handle significantly higher currents than standard 0 Ohms resistors. This is why they can easily be 10x more expensive than the 0 Ohms standard resistor.
In the end, it all comes down to how much current are you expecting to flow through the part and knowing that the part is able to handle the current. Also if you have to use a mOhms range current sense resistor, that the voltage drop it will cause shall not adversely affect your circuit.
I’ve observed 0 ohm resistors being used in various scenarios, particularly in calibration and testing processes. For instance, if you initially design an RC lowpass filter on a board but later determine it’s unnecessary, you can replace the resistor with a 0 ohm resistor and omit the capacitor.
This selective assembly of noise-reducing circuits is quite common, especially in complex consumer electronics like DTV receivers. Manufacturers often leave out decoupling capacitors during production and add them later based on quality testing results. In some cases, sensitive instrumentation devices have custom denoising circuits, meticulously fine-tuned during manufacturing.
Another interesting use of 0 ohm resistors can be used as a soldered-down DIP switch to select specific features for a device. This method allows for flexibility in configuring device functionalities.
Zero-ohm resistors serve several practical purposes in PCB design:
These resistors act like configurable jumpers, allowing different circuit variants without needing to modify the PCB. Simply place or omit the 0 Ω resistor during assembly to connect or disconnect parts of the circuit.
A 0 Ω resistor can act as a bridge over existing traces, enabling single-layer designs where a double-layer PCB might otherwise be required. This reduces manufacturing costs and complexity, especially for simpler boards.
During development, low-resistance shunt resistors can be placed to measure current through specific paths. For production, these are often replaced with 0 Ω resistors to maintain the connection without cutting traces. This is particularly useful for precise current measurements on finalized PCBs, where layout or materials might impact results.
While 0 Ω resistors might seem trivial, their practical applications make them a valuable tool for design flexibility, cost optimization, and testing. When using them, ensure they can handle the expected current to avoid issues, especially for power-heavy circuits.
Zero-ohm resistors and low-value resistors (e.g., 0.1 Ω or 1 Ω) serve different purposes in PCB design and testing. Here’s how they are commonly used:
In high-volume production, zero-ohm resistors are often used as links on single-sided boards because they can be placed by automated component insertion machines. While less relevant for modern multi-layer boards, this approach is cost-effective in specific applications.
Low-value resistors like 0.1 Ω or 1 Ω are typically used as shunt resistors to measure current. The voltage drop across these resistors provides a way to calculate current, but their impact on the circuit must be minimized:
Ensure the voltage drop is a small fraction of the total circuit voltage. For example, in a 5V circuit with 1A current, a 0.1 Ω resistor drops 0.1V (2%), which is usually acceptable.
Use resistors with values low enough to avoid affecting circuit operation but high enough to allow accurate measurement.
In noisy environments, measuring the voltage across shunt resistors can produce inaccurate results. Adding a capacitor (e.g., 10 µF or 100 µF) in parallel with the resistor or filtering the measurement signal using a series resistor and capacitor can improve accuracy.
Placing the sense resistor with one side grounded simplifies measurements by providing a stable reference point. This can be particularly convenient for development and testing.
Zero-ohm resistors, on the other hand, are versatile and can also act as configurable jumpers or facilitate routing, as others have mentioned. Their low cost and ease of use make them a valuable tool in both prototyping and production.
0Ω resistors are often used to enforce single-point connections between planes. This ensures proper isolation and helps maintain signal integrity while adhering to design rules.