What Are Zero-Ohm Resistors and How Are They Used In Circuit Design?
In this FEQ (Frequent Engineering Question), we'll take a look at the advantages of zero-ohm resistors and where they're most suitable in design.
What Is a Zero-Ohm Resistor?
The term “zero-ohm resistor” is simply a convenient way of referring to a jumper that is packaged like a normal resistor (nowadays, usually a surface-mount resistor). Why would anyone want to use these components?
Designing Extensible PCBs
First, they provide an easy way to deal with uncertainties that arise during schematic design. Sometimes you just can’t be sure about how a circuit should be configured until you’ve had a chance to do some experimentation, and zero-ohm resistors provide a relatively convenient form of post-fabrication configurability.
I used zero-ohm resistors on the C-BISCUIT robot control board to maintain some flexibility in the power-supply circuit.
Second, designers often need to incorporate extensibility into their PCBs.
For example, you might be designing a somewhat generic ADC-plus-microcontroller board that will receive a 5 V analog input signal from an external amplifier module. You suspect, however, that someone will eventually want to use this board in a higher-voltage system, and you decide to address this need in advance by incorporating two input paths, one for the 5 V signal and one for higher voltages.
By placing a zero-ohm resistor in series with each input path and specifying one of the parts as “DNI” (“do not install”) or “DNP” (“do not populate”), you can ensure that the PCB will come back from the assembly house in the desired state—that is, it will be ready for immediate use in the 5 V system yet can be adapted for use in the higher-voltage system after a small amount of soldering work.
Advantages of Zero-Ohm Resistors
There are certainly other ways of accomplishing this type of functionality, and the use of zero-ohm resistors might seem rather primitive. An SPDT mechanical switch would do the trick, or you could try a voltage-controlled implementation based on solid-state switches or MEMS switches. You might even consider adding a clever circuit that attempts to detect the voltage amplitude and route the signal accordingly.
The fact is, though, that in some situations it is hard to beat the simplicity, low cost, and excellent electrical performance offered by the zero-ohm resistor.
Other solutions introduce various complications: high on-state resistance, distortion caused by input-voltage-dependent variations in on-state resistance, human error when setting the switch position, additional debugging and testing of firmware, and so forth.
Zero-ohm resistors are effective, reliable, inexpensive, and fully compatible with automated assembly.