Role of 0Ω or 100mΩ Resistors in PCB Designs

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?

The explanation I’ve seen is for tuning. You probably don’t need a resistor there, but you might, so leave room for one.

Leaving it unpopulated might create an open circuit, but a zero-ohm resistor is nearly harmless.

I don’t know the reason for very-low ohm, unless they’re acknowledging that zero doesn’t quite exist, and trying to better define some ratio.

“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.

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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.

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