Resistor Selection in Circuit Board Design

A resistor is the simplest component on the board and the one most often chosen without thought. The value is calculated, a familiar part number is reached for and the design moves on. That habit works until a circuit drifts at temperature, a part runs hot, or a production batch measures differently from the prototype. Resistor selection is a set of deliberate choices, and each of them has a consequence.

Value and Tolerance

The nominal value is the starting point, and the tolerance around it is the first real decision. A one percent part costs a little more than a five percent part and is not automatically the right answer: in a pulldown that only needs to define a logic level, the wider tolerance is irrelevant.

Where the value sets a current, a gain or a time constant, the tolerance propagates into the circuit performance. Working out the worst case across the tolerance band, rather than reading the nominal value, is what identifies the resistors that actually need to be precise. Precision in the remaining positions buys nothing and costs money.

Power Rating and Derating

The power rating of a resistor is the amount it can dissipate at a rated ambient temperature, and it falls as the ambient temperature rises. A part run at its full rating will be hot, will drift, and will have a shortened life.

Derating to roughly half the rated power is the usual practice, and the resulting temperature rise is a better guide than the rating alone. Where the resistor carries current continuously, the derating should be calculated rather than assumed, because a small chip resistor dissipating a watt will heat the board around it as well as itself. Thermal effects of that kind are covered in our component tolerance and reliability notes.

Temperature Coefficient

The temperature coefficient describes how much the resistance changes per degree of temperature change, and it is expressed in parts per million. A standard part may drift by a few hundred ppm over its operating range, which is negligible in a pulldown and significant in a measurement divider.

The coefficient also decides how a part behaves under its own self heating, because the power it dissipates raises its temperature. In a precision divider this produces a drift that appears as a gain error, and the fix is either a lower coefficient or a lower power dissipation through a higher value.

Package Size and Its Consequences

Package size sets the footprint, the available power dissipation and the ease of assembly. Small packages save board area and reduce parasitic inductance, which matters at high frequency, but they dissipate less and are harder to inspect.

The mechanical side is easy to overlook. A small chip resistor on a board that will flex can crack, and the crack may be partial and invisible, producing a resistance that changes with temperature. Where the board will be handled roughly or mounted where it can bend, a larger package or a compliant arrangement is the safer choice. Layout conditions that affect this are discussed in our PCB layout tips article.

Where the Resistor Sits in the Circuit

The same value behaves differently depending on its role. A series resistor at an input protects against overvoltage but adds noise and forms a filter with the input capacitance. A resistor in a feedback network sets gain but its tolerance and drift appear directly in that gain.

Placement matters as much as the part. A sense resistor must be connected with a Kelvin arrangement, so that the measurement does not include the resistance of the copper. A gate resistor must sit close to the device it drives, otherwise the trace inductance it was meant to damp becomes part of the loop.

Stock, Availability and the Second Source

A part that is available in volume today may not be next year, and a resistor that is specified to a fraction of a percent is more likely to be affected than a commodity part. Checking lifecycle status during selection avoids a redesign later.

Where a circuit will tolerate a range, specifying the range rather than a single part number gives the manufacturer room to substitute. Where it will not, the constraint should be stated explicitly in the BOM so that it is not lost. That discipline is described in our prototype BOM checklist.

Networks and Matching

In a resistive divider or a bridge, the absolute tolerance of each part matters less than how the parts track each other. Two resistors from the same batch drift in the same direction as temperature changes, so the ratio stays close even when the absolute values move.

That is why resistor networks and matched pairs exist. They are built on a common substrate and trimmed together, which gives a ratio stability far better than two separate precision parts could achieve, and they save board area at the same time.

Inductance and High Frequency Behaviour

A resistor is not a pure resistance. Its leads or terminations add inductance, and the body adds a small capacitance, so above a certain frequency the impedance stops rising and begins to fall.

For high frequency work the physical form matters more than the printed value. Thin film parts with short terminations and small packages have the least parasitic inductance, and a low value of a few ohms behaves more like an inductor than a resistor. Where the resistance is used to terminate a transmission line, that behaviour is part of the design rather than an afterthought.

Failure Modes to Consider

Resistors fail open, drift, or crack. An open circuit in a feedback path can drive an amplifier to its rail, and in a sensing path it can remove the feedback that a control loop depends on. Where that consequence matters, a redundant element or a circuit that fails safe is worth considering.

Drift and cracking are harder to detect because the circuit may still function. Both are found by measuring at temperature and by inspecting joints and bodies on boards that have seen thermal cycling.

Specifying Resistors on the BOM

A BOM entry for a resistor should carry more than the value and the footprint. Tolerance, power rating and temperature coefficient each change the part that arrives, and leaving them unstated invites a substitution that meets the value and misses the requirement.

Where a circuit tolerates a range, stating the range gives the manufacturer room to work and reduces the chance of a shortage becoming a delay. Where it does not, saying so in writing is what keeps the constraint alive through procurement.

The same principle applies to the parts around it. A resistor that sets the current through an LED or a reference must be checked against the tolerance of the device it serves, not in isolation, which is the same worst case discipline described in our component tolerance and reliability notes.

FAQ

Is a tighter tolerance always better? No. It costs more and only helps where the value affects the performance of the circuit. Everywhere else the money is better spent on the positions that do matter.

How much should a resistor be derated? Running at about half the rated power is a common and safe practice. Where the ambient temperature is high or the part is enclosed, derate further.

What causes a resistor to drift in service? Temperature is the usual cause, either from the environment or from self heating. Load life and humidity also shift the value over years, which is why precision work specifies the applicable stability grade.