SMD resistors

The resistor is one of the most widely applied passive elements, without which it would be practically impossible to develop any electronic circuit. It is used as the main component of filters or voltage dividers, among other things, as it allows to set currents as well as the difference of electric potentials across particular signal paths. The resistance is expressed in ohms[Ω]. Limiting the current flowing through a conductor involves a loss of power, and the energy is dissipated as heat. According to Ohm's law, this power is directly proportional to the voltage difference occurring at the ends of the conductor and also to the current flowing through it.

In most digital electronics applications, taking into consideration only the resistance value of a given resistor is a perfectly valid assumption. For advanced applications, especially for very high values of current oscillation frequencies in the circuit of 1GHz and above, it may also be important to take into account the internal, so-called parasitic inductance and capacitance of the component, as these can significantly change its nature. This is an issue related to the equivalent circuit diagram of a resistor, which takes into account all three parameters (electrical resistance, capacitance and inductance). In the vast majority of applications, however, only resistance will be relevant, as the other two values are usually negligible.

Resistors – mounting methods

Resistors, like most other electronic components, come in two types: through-hole (THT) and surface mount (SMD) . The latter are becoming increasingly popular, especially in modern electronic circuits, as their size is much smaller and, consequently, they can be placed on PCBs much more densely. SMD resistors, unlike those for through-hole mounting, are not soldered to a tinned hole in the board. Instead, they are soldered to a pad that is not covered with soldermask, i.e. an exposed patch of copper on the surface of the PCB. This facilitates the design process of double-sided boards, with electronic components and tracks on both their top and bottom surfaces.

SMD resistor markings

The markings of SMD resistors, unlike those for through-hole mounting, which are usually marked with 3 to 6 colour bars, have a printed code, usually made up of 3 or 4 digits. Learning how to read them without additional support material should not be difficult, however, dedicated calculators as well as apps that convert the SMD resistor code into the corresponding numerical value of the component's resistance can easily be found online, which helps prevent mistakes and makes working with these small components easier.

SMD resistor – sizes

SMD resistors are relatively small, however, their packages (cases) are standardised and shaped according to the corresponding series. It should be mentioned here that they can be denoted in metric or in imperial units. For example, the very common 0603 package size is available in both options, which can sometimes be confusing. However, markings in imperial units are most commonly used. The first two digits correspond to the length of the component (e.g. 0.06 inches), while the next two indicate its width (e.g. 0.03 inches). The most popular sizes include 0201, 0402, 0603, 0805, 1206 and 2512. Resistors may also come in cylindrical packages, such as minimelf 0204 or melf 0207. You can also find SMD power resistors enclosed in bigger packages that are less common for resistors and are able to dissipate and release significantly more heat to the environment.

How to choose the right SMD resistor?

Resistance

Choosing the right resistor should first of all depend on its main parameter, the electrical resistance value. Resistors can be connected in series or in parallel to obtain the resistance value that is required for a given project.

Power rating

Another parameter to consider is the power rating. As already mentioned, it depends very much on the size of the package, as it allows the heat released on its surface to be dissipated to the environment. This rating, expressed in watts [W], can be calculated as the product of the average current flowing through a resistor and the voltage difference at its ends. If you choose a resistor with a power rating that is too low for the conditions in the circuit, it will undoubtedly burn out sooner or later and cause an interruption in the electronic system. The most popular power ratings for SMD resistors are 0.1W, 0.125W, 0.25W, 0.5W and 1W, although there are also resistors with intermediate values as well as higher/lower values than those listed. In the case of low-power signal circuits, this parameter may even be insignificant. In such a case, the miniaturisation of the final product and therefore the choice of passive components in the smallest possible packages may be of greater importance.

Maximum operating voltage

The maximum operating voltage, which is the maximum voltage that can be applied to the ends of a resistor without fear of breakdown and permanent damage to the resistor itself, as well as to other components in the electronic circuit, can also prove very important. This can prove particularly dangerous when working with 230V AC supplied from a household mains outlet.

The last of the most pivotal parameters quoted by manufacturers of electrical resistors is their tolerance, which determines how accurately the resistance rating corresponds to the actual resistance that can be measured with a suitable measuring device. This can range from 0.1% to 10%, with the most popular resistors being those with an accuracy of 1% and 5%. This information can be of considerable importance in the case of building, for example, resistor dividers, where we need to obtain a very precise, assumed voltage value at the output, or in the case of various types of measuring systems.

Temperature coefficient

In more advanced applications, the temperature coefficient can also be an important parameter. It determines how the electrical resistance value of a given resistor varies in response to a change in its temperature. This fluctuation may be the result of power dissipated in the form of heat, but will also indirectly depend on the ambient temperature, as well as the cooling method applied in the target system. This factor is usually given in units of parts per million per degree Celsius [ppm/°C].

It is also worth realising that SMD resistors are built using various technologies, hence we can distinguish between the following types of resistors: wire wound, metal film, metal glaze, power metal, thin film, carbon film, and the most popular, thick film resistors.