I have a habit of reusing and repairing items that many would throw away or abandon in their garages. With a bit of work, there’s a treasure trove of items available to revive for all kinds of hobbies and interests. For me, this also extends to electronics – I’m particularly fond of restoring old NES and computer systems.
However, anyone who has spent time tinkering with electronics knows they can be booby-trapped. For example, sufficiently large capacitors can carry a nasty shock even after being disconnected from sources for amply long periods.
PCB capacitor identification helps people like me avoid reaching for the burn salves. More importantly, from a design standpoint, it can teach about the core functionality of the capacitor and its various functions within a circuit.
Polarity: The Defining Quality of PCB Capacitor Identification
The first step of discerning capacitors begins with polarity. Polarity, in its strictest sense, refers to the balance of charge throughout a component, though generally, it refers to a susceptibility to pin orientation in the plane. From the former perspective, polarity can be treated as a vectorized parameter, indicating both direction and quantity. When all of the partial charges of the individual pins are summed, a polarized component will have a net charge remaining, while a non polarized component will not. Capacitors can be found in non polarized and polarized forms (the latter referred to as electrolytic capacitors), with the general distinction being the total capacitance of the conductor. A small conductor, typically nanofarad and below, is nonpolar and can be installed “backward” without any change in performance, while capacitors measuring in the microfarads and up are likely to exhibit polarity.
Electrolytic capacitors require some additional demarcation for safe layout design and installation. A polarized component installed backward (in the case of a 2-lead component) will inevitably lead to catastrophic failure of the component and a short in the circuit. Greater capacitances can result in more destruction, leading to smoke, shock, and potentially fire hazards. In order to prevent
damage to the board and surroundings, electrolytic caps will indicate direction with a longer leg for the anode (positive side) on a through-hole package and a visual indicator on the exterior wrap for the cathode (negative side).
|Non Polarized||Polarized (electrolytic)|
|Package size||Small (typically modern)||Large|
|Package style||Chip (SMT); ceramic (TH), film(TH) axial (TH)||Cylinder (SMT/TH), tantalum/niobium chip (SMT)|
|Visual indicators||N/A||Longer anode leg (TH), cathode leg indicator (TH), polarity indicator (SMT)|
There are numerous ways to differentiate between polarized and nonpolarized capacitors.
At a glance, the polarity of capacitors can usually be determined by the size and shape of a package. Standard surface mount chips and non-cylindrical through-hole packages are almost universally nonpolarized, while larger SMT packages like tantalum chips and cylindrical capacitors (whether SMT or TH) are polarized and include the aforementioned distinguishing visual features.
The general thought process should be that a larger capacitance component needs additional dielectric material to achieve the required material properties. At a minimum, polarized components should have their anode pin properly identified on the legend layer of the PCB. When in doubt, refer to the datasheet or a product catalog.
How Different Capacitors Fill Different Roles
Diving deeper, the necessary parameters of a capacitor will influence the selection, as will its role in the circuit. Capacitors perform various functions, primarily as energy stores but also as rudimentary filters, signal conditioners, decouplers, and much more. While capacitance correlates to the capacitor’s polarity, it also indicates the charge density at a particular voltage.
The equation for capacitance:
C = εAd,
where ε, A, and d represent the permittivity, plate area, and gap between plates, respectively, underlining the role of materials, as well as the physical design of the capacitor, determines its defining property.
It’s also worth noting that, like many core electrical features, capacitance varies as a function of temperature. Building on that same line of thought, an increase in temperature beyond the operational temperature range more quickly degrades capacitors in terms of usable life span and rated storage capacity. However, temperature is not the only variable factor influencing capacitance. Both frequency and voltage can impact the storage capabilities of a device. The time constant, or the time of a charge or discharge event of a capacitor, illustrates the negative correlation between frequency and capacitance. At the same time, the voltage can impact the dynamic permittivity of the material.
The material composing capacitors and the manufacturing process it undergoes will ultimately determine the role a capacitor is best suited for:
- Ceramic: Extremely flexible in terms of design options, ceramic capacitors represent an excellent “one-size-fits-most” tool. Their shape is highly moldable and provides capacitance with a low presence of parasitics. Because of this, they are well-suited for high-frequency applications like filtering, bypassing, and decoupling over a wide range of voltage ratings.
- Film: Film capacitors are similar to ceramic, but their construction offers some notable advantages. A metalized foil layer applied to the plastic dielectric enhances reliability by repairing itself during dielectric breakdown conditions. Instead of the component failure due to the removal of a conduction path between the terminals, the metal foil can melt and fill in any gaps that form in the conduction layers. Contact between the conductive layer and terminals also ensures minimal resistive losses, meaning these capacitors excel in low-power applications.
- Electrolytic: The densest capacitance-by-volume selection, electrolytic capacitors can fulfill almost any role efficiently. Due to polarity, additional precautions must be taken during layout and assembly. Such precautions will prevent the development of a reverse voltage that would destroy the capacitor and damage its surroundings.
From Procurement to Placement, Your Contract Manufacturer Has Capacitors Covered
PCB capacitor identification can aid designers in understanding the purpose of various subcircuits through the intuition of the capacitor’s role. It’s common for simple designs to have capacitor counts numbering into the hundreds. This value only escalates for more dense and complex boards. Still unsure how to choose the best capacitor to maximize performance in your circuit? At VSE, we take the guesswork out of design with a team of engineers committed to building electronics for our customers. Coupled with our professional manufacturing partners, we design and manufacture boards that meet technical specifications while exceeding expectations.