Choosing a Resistor - Avoid the Common Mistakes

A resistor is a two-terminal passive component. Theoretically simple, but choosing the right resistor involves more than just specifying a few top-tier parameters.

Resistors are relatively simple components; passive two-terminal devices with their basic behavior defined by Ohm’s Law serving many functions such as current limiting, current sensing, capacitor bleeders, voltage balancing, signal attenuation, and rise/fall time control, to give a few examples.

Common Mistakes to Avoid When Choosing a Resistor

Once the needed resistance value has been considered, the user might think that most of the work associated with resistor selection is done – but it is not, there’s much more to selection than identifying just a few basic parameters. The reality is that there are several factors that go into choosing the right resistor type for the application, and it’s easy to overlook them.  

Among the factors that are easy to neglect:

  • Power Rating

By their nature and the basic P = I2R relationship, resistors are self-heating components, therefore the resistor must be able to handle the dissipation and still meet specifications. (Figure 1) And, in extreme cases, the resistor may need a heat sink.

At the same time, over-specifying and selecting a resistor with extreme safety margins adds needless cost and real estate. If operating in high temperature environments, look cautiously at the derating curves on the vendor datasheet. Also, note whether the temperature axis relates to ambient air, termination or heats ink temperature.

The WSMHP series of non-inductive thick-film resistors is available in TO-263 SMT packages, with ratings to 35 W.

Figure 1. The WSMHP series of non-inductive thick-film resistors is available in TO-263 SMT packages, with ratings to 35 W.

  • Pulsed loads

Even if the average power that the resistor disperses is within its rating, bursts of higher-power pulses can surpass that rating by several orders of magnitude. If this is important for the application, a resistor with a specified pulse performance should be selected; thick film and wire wound are the most common technologies for pulse resistors. Careful analysis of pulse levels and duty cycles is imperative to ensure that the mean power is within the power rating.

  • Temperature Coefficient of Resistance (TCR)

Self-heating, in addition to other heat sources in the vicinity, will cause a resistor’s ohmic value to change. For some situations, such as a basic pull-up resistor, the change may be of little concern. However, if that resistor is being used to measure current flow via voltage drop, the TCR-induced change could severely affect the accuracy of the reading. Similar concerns apply to critical resistors in many instrumentation applications. Do the calculations and select a resistor with low-enough TCR.

  • Allowing for cooling

Since they are heat sources, resistors will frequently need cooling, usually via passive convection but sometimes using forced air. Be sure to provide a clear path for the cooling airflow and ensure that the resistor is not shadowed by larger nearby components which will block that much needed flow. If you are using the PC board as a heat spreader, don’t let other heat-generating components near the resistor diminish the heat-absorbing potential on which you are relying.

  • Voltage rating

Resistors are often used in higher-voltage applications, with potentials across their leads reaching into hundreds or thousands of volts. Ensure that the resistor is suitable for the circuit voltage, (Figure 2) independent of the current and power levels, or you may experience arcing and outright failure.

For high-voltage applications, various resistor families are available with lead spacing and other attributes needed to support circuit functions in the tens of kilovolts.

Figure 2. For high-voltage applications, various resistor families are available with lead spacing and other attributes needed to support circuit functions in the tens of kilovolts.

  • Resistor mounting

Many higher-dissipation resistors are larger than typical small ICs and other passives, so how and where the resistor will be placed is an issue worth considering. Is the resistor a surface-mount device, or through-hole? Does it need mechanical support or to be placed where the PC board is not flexing?

  • Self-inductance

The inductance of the resistor is not a problem for DC applications, but it may become one at higher frequencies. Resistors are available with very low self-inductance for situations which are sensitive to this parameter.

  • Adequate connections

As current levels rise, it’s necessary to plan for solid, low-resistance connections for unimpeded current flow and minimal IR drop, avoiding reliance on thin PC board tracks. Ensure that the resistance of the current-carrying leads to the resistor is insignificant compared to the resistor’s value.

Don’t overlook the possible need for Kelvin (four-wire) connections. If you are trying to accurately measure the voltage across the resistor, as well as reserving a place for the amplifier (often a differential or isolated unit which senses the voltage) then this needs to be located close to the resistor itself, to minimize noise pickup.

  • Finally, and perhaps most difficult; choosing a suitable resistor type.

Once you get beyond the basic low-power, small chip resistors which are used in non-critical applications, there is a world of distinctive resistor types, including fusible wire wound, steel substrate, and silicon-based networks among others. Some have a very low TCR; some are mechanically rugged, (Figure 3) others are resistant to contaminants like Sulphur, some can handle pulse overloads, while others have well-defined, open-circuit failure modes.

The extremely rugged, high dissipation WDBR resistor series uses multilayer construction built on a stainless-steel substrate to make it highly immune to cracks and fracturing which could result from thermal extremes and vibration.

Figure 3. The extremely rugged, high dissipation WDBR resistor series uses multilayer construction built on a stainless-steel substrate to make it highly immune to cracks and fracturing which could result from thermal extremes and vibration.

Conclusion

Unless your resistor application is routine, your smartest, low-risk option is to work with the application specialists at a resistor vendor such as TT Electronics, because they have the experience and expertise to assist with every unique application of resistors.

This information has been sourced, reviewed and adapted from materials provided by TT Electronics plc.

For more information on this source, please visit TT Electronics plc.

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