Resistor Selection for PCB Design: Power, Voltage and 0 Ohm Uses
Choosing a resistor looks like the simplest task in a schematic, and it is where a surprising number of reliability problems begin. Two engineers can specify the same 10 k value and end up with one circuit that works for a decade and another that fails in the field, because only one of them considered power, voltage and temperature. Sound resistor selection means reading the datasheet parameters rather than the nominal value alone.
Parameters Beyond the Ohm Value
The two numbers printed in most schematics are the nominal resistance and, sometimes, the tolerance. Both matter, but a resistor is also defined by its power rating, its voltage rating, its temperature coefficient, its parasitic inductance and its stability over life. A part that satisfies the resistance and tolerance requirement can still fail if any of the others is exceeded, and the failure mode is usually a change in value, a crack in the body or an open circuit rather than a short.
Resistor families differ in exactly these secondary properties. Carbon film parts are cheap and stable enough for general use, metal film parts hold tolerance and temperature coefficient far better, wirewound types handle high power but add inductance, and thick film chip resistors dominate surface mount work. The choice follows the application rather than a general preference.
Power Rating and Derating
Power dissipated in a resistor is the product of the current through it and the voltage across it. A 100 ohm resistor carrying 100 mA dissipates 1 W, which no 0805 or 1206 chip can survive, because their ratings are a small fraction of a watt. The rule is to keep the dissipated power to no more than half the rated value at the working ambient temperature, and to derate further as the ambient rises.
Derating curves matter because a resistor rated at 0.25 W at 70 degrees Celsius may be rated at only 0.1 W at 100 degrees Celsius. Where the board sits inside an enclosure next to a power stage, that reduction is real. The other trap is the physical size of the copper around the part: a chip resistor relies on its terminations and the pad to remove heat, so a part surrounded by solder mask on a narrow trace runs hotter than the same part on a wide pad. Where the trace is also a current path, the trace width should be checked against the current it carries, as described in the notes on trace width and current calculation.
Voltage Rating and Insulation
The voltage rating is the maximum potential that may appear across the part, and it is often the parameter that decides a design. In an offline power supply, the discharge resistors that bleed the input capacitors are connected directly across the mains. A pair of megohm resistors in series is used to divide that potential, and each one still sees a substantial voltage. A chip resistor rated for 50 V used in that position will break down, and the failure can be a flashover that damages the board.
Resistor voltage is limited by package size, by the internal element geometry and by the spacing between terminations. Two resistors in series are frequently a better answer than one large part, because the potential is divided and the creepage along the surface is increased at the same time. Where the parts sit on a high voltage net, both the component rating and the clearance around the pads have to be verified against the applicable safety standard rather than against a general rule.
Tolerance and Temperature Coefficient
Tolerance is the spread of the initial value, while the temperature coefficient describes how far the value drifts as the part heats. A 1 percent part with a coefficient of 100 parts per million per degree Celsius changes by 0.5 percent over a 50 degree rise, so the drift can be larger than the initial tolerance. In a divider that sets a reference or a feedback threshold, that drift turns directly into a change in output.
<img src="https://www.gopcba.com/wp-content/uploads/2026/06/智能无人机PCBA-1.jpg" alt="Varistor and thermistor components in a power input protection circuit” />
For a divider, the matched pair matters more than the absolute value. Two resistors of the same type and value on the same board tend to track each other as the temperature changes, so the ratio stays closer than either individual part. Where a very tight ratio is needed, adjacent placement and, where possible, the same physical size are the practical measures.
The Zero Ohm Resistor
A zero ohm resistor is a link that a placement machine can install. It is not a perfect short; it has a small resistance and a small inductance, and that is exactly why it is useful.
The classic application is joining an analog ground and a digital ground at a single point. A ferrite bead suppresses noise over a narrow band and has to be matched to a known noise frequency, a capacitor creates a floating reference, and an inductor is large and has stray parameters. A zero ohm link provides a narrow current path that limits the loop current circulating between the two ground areas, which is why it is often the least troublesome option. The layout considerations for that arrangement are covered in the notes on mixed signal PCB design guidelines.
Other uses follow the same logic. It can act as a jumper to bypass an unused section, hold the place of a value that is still being tuned, provide a removable link for measuring supply current, and solve a routing conflict on the last layer. In high frequency circuits it also provides a low impedance connection between a plane and a pin without the via that would otherwise be needed, and without the hole that a via punches in the plane. The one habit worth avoiding is using it where a real connection is intended and the current is high, because the link carries far less current than a trace of the same width.
Varistors on the Input Stage
A metal oxide varistor protects the circuit that follows it by clamping the voltage when the supply spikes. Its resistance is high in normal operation and collapses when the voltage across it exceeds the varistor voltage, diverting the surge current away from the downstream components. The relevant parameters are the clamping voltage, the surge current capacity, the energy it can absorb and the response time.
A varistor is not complete protection. It cannot withstand sustained overvoltage, it does not limit inrush current, and it will not clear a short circuit or an over current condition. Its energy rating is finite and degrades with each large surge, so it should be treated as a sacrificial element backed by proper fusing. In an offline converter the varistor belongs immediately after the input fuse, and the layout guidance for that stage is in the notes on DC-DC converter layout and routing.
Thermistors in Protection Circuits
A thermistor changes resistance with temperature, and the sign of that change decides its role. A negative temperature coefficient device drops in resistance as it heats. Placed in series with the input, it presents a high resistance at switch-on and limits the inrush current that would otherwise stress the rectifier and the bulk capacitors, then warms up and falls to a low resistance so that the steady state loss is small.
A positive temperature coefficient device does the opposite and is used as a self resetting fuse. Under normal current it stays cool and presents a low resistance; an over current heats it, its resistance rises sharply and the current collapses. The same property gives over temperature protection, because the part responds to ambient heat as well as to its own dissipation. Both types are slow compared with a semiconductor, so they complement rather than replace a fast electronic protection circuit.
FAQ
Is it safe to run a resistor at its rated power? No. Design margin of at least two to one is normal, and the rating must be derated for the ambient temperature inside the enclosure.
Why use a zero ohm resistor instead of a trace? It can be placed automatically, measured, removed for current measurement and used to join two ground areas at one point, all of which a trace cannot do.
Can a varistor replace a fuse? No. A varistor clamps transient overvoltage but cannot interrupt a fault current, and it degrades with every large surge it absorbs.
