Walk through any aircraft electrical system and you will find capacitors doing quiet but essential work: smoothing out voltage ripples, filtering interference, timing relay operations, and protecting sensitive avionics. For the Aviation Maintenance Technician (AMT) candidate, capacitance is one of those foundational concepts that shows up everywhere — from simple DC filter circuits to complex electronic systems. Understanding what a capacitor is, how it charges and discharges, and how to calculate its effect in a circuit will give you both the exam confidence and the real-world insight you need.
This article focuses specifically on capacitor behavior in direct current (DC) circuits, which is the most common context on the AMT General knowledge test. AC behavior introduces additional concepts (reactance, impedance) that build on this foundation, so mastering DC behavior first is the right starting point.
What Capacitance Is
Capacitance is the ability of a component or system to store an electrical charge. The unit of capacitance is the farad (F), named after physicist Michael Faraday. One farad is actually an enormous amount of capacitance, so practical capacitors used in aircraft are rated in microfarads (µF, one-millionth of a farad) or picofarads (pF, one-trillionth of a farad).
A basic capacitor consists of two electrically conductive plates (conductors) separated by an insulating material called a dielectric. The dielectric can be air, paper, mica, ceramic, or a variety of other materials. The dielectric is critically important: it not only physically separates the plates but also determines how much charge the capacitor can hold for a given voltage. The ability of the dielectric to concentrate the electric field — and therefore increase capacitance — is described by the material's dielectric constant, also called permittivity. A higher dielectric constant means more capacitance for the same physical size.
Three physical factors determine a capacitor's capacitance value:
- Plate area: Larger plates store more charge. Capacitance increases proportionally with plate area.
- Plate separation: Plates that are closer together produce a stronger electric field and greater capacitance. Capacitance increases as separation decreases.
- Dielectric material: A dielectric with a higher dielectric constant increases capacitance compared to air or vacuum.
How a Capacitor Charges and Discharges in a DC Circuit
When a capacitor is connected to a DC source through a resistor, it does not charge instantly. Instead, electrons from the negative terminal of the source accumulate on one plate, while electrons are simultaneously pulled away from the other plate toward the positive terminal of the source. This separation of charge creates an electric field across the dielectric and produces a voltage across the capacitor.
At the very first instant the circuit is closed, the capacitor acts almost like a short circuit — it has zero voltage across it and maximum current flows. As charge builds up on the plates, the voltage across the capacitor rises and the charging current decreases. Eventually the capacitor voltage equals the source voltage, at which point current stops flowing entirely. In a steady-state DC circuit, a fully charged capacitor blocks direct current — this is one of the most important and most-tested facts about capacitors.
The rate at which a capacitor charges (and discharges) is described by the RC time constant, represented by the Greek letter tau (τ). It is calculated as:
τ = R × C
Where R is the resistance in ohms and C is the capacitance in farads. The time constant τ represents the time it takes for the capacitor to charge to approximately 63.2% of the applied voltage. After five time constants (5τ), the capacitor is considered fully charged (about 99.3% of source voltage). The same relationship applies in reverse during discharge: after one time constant the voltage falls to about 36.8% of its starting value, and after five time constants the capacitor is essentially fully discharged.
Example: A 100 µF capacitor in series with a 10,000-ohm (10 kΩ) resistor has a time constant of: τ = 10,000 × 0.0001 = 1 second. The capacitor will reach approximately 63.2% of source voltage in one second and will be fully charged after about five seconds.
Capacitors in Series and Parallel
Capacitors combine in circuits differently than resistors do — and the FAA test exploits this contrast heavily.
Capacitors in parallel add directly, just like resistors in series. The total capacitance is the sum of all individual capacitances: Ctotal = C1 + C2 + C3 + … This makes intuitive sense because connecting capacitors in parallel is effectively increasing the total plate area available to store charge.
Capacitors in series combine like resistors in parallel — the reciprocal method. For two capacitors: 1/Ctotal = 1/C1 + 1/C2. The total capacitance of capacitors in series is always less than the smallest individual capacitor. Connecting capacitors in series effectively increases the plate separation, which reduces capacitance.
Types of Capacitors Used in Aviation
Aircraft maintenance technicians encounter several common capacitor types, and knowing their construction helps with identification and troubleshooting:
- Electrolytic capacitors: Offer very high capacitance in a compact package. They are polarized, meaning they must be installed with correct polarity (positive terminal to positive voltage). Reversing polarity can destroy the capacitor and may cause it to fail violently. Common in filter and power supply circuits.
- Ceramic capacitors: Non-polarized, small, and stable. Used widely in high-frequency filter and bypass applications in avionics.
- Mica capacitors: Highly stable and accurate, used where precise capacitance is required, such as in radio transmitters and tuned circuits.
- Film capacitors (paper, mylar, polyester): Non-polarized, general-purpose capacitors used in a variety of analog and timing circuits.
Why Capacitors Matter in Aircraft Systems
Aircraft technicians encounter capacitors in practical applications constantly:
- Filtering: Capacitors placed across DC power bus lines smooth out voltage ripple from generators and alternators, protecting sensitive avionics from fluctuating supply voltages.
- Noise suppression: Small capacitors are wired across relay contacts and motor brushes to suppress electrical noise (electromagnetic interference) that could disrupt radio communications and navigation equipment.
- Timing circuits: RC circuits using capacitors determine the time delay in relay and warning systems.
- Energy storage and surge protection: Capacitors can release stored charge quickly to provide a brief burst of current, protecting circuits during sudden load changes.
- Ignition systems: Capacitors (condensers) in older magneto-based ignition systems protect breaker points and improve the sharpness of the ignition spark.
Key Numbers and Rules
- The unit of capacitance is the farad (F); practical values are in µF or pF.
- After one time constant (τ = RC), a capacitor charges to approximately 63.2% of source voltage.
- After five time constants, a capacitor is considered fully charged or fully discharged (≈ 99.3%).
- A fully charged capacitor in a DC circuit blocks DC current — current is zero at steady state.
- Capacitors in parallel: Ctotal = C1 + C2 + … (total increases).
- Capacitors in series: 1/Ctotal = 1/C1 + 1/C2 + … (total is less than the smallest).
- Electrolytic capacitors are polarized — incorrect installation can cause failure or danger.
- Increasing plate area or dielectric constant increases capacitance; increasing plate separation decreases capacitance.
Common Test Traps
- Series vs. parallel confusion: Students who memorize resistor rules and apply them backward to capacitors are caught every time. Remember: capacitors in parallel ADD (like resistors in series); capacitors in series use the reciprocal formula (like resistors in parallel).
- Instantaneous vs. steady-state current: At the instant voltage is applied, a capacitor allows maximum current (acts like a short). At steady state (fully charged), it passes zero current (acts like an open). Many questions exploit this by asking about current at a specific moment in time.
- The 63.2% rule: The FAA test frequently asks what percentage of voltage a capacitor reaches after exactly one time constant. The answer is 63.2%, not 50% and not 100%.
- Polarity of electrolytics: Questions may ask what happens when an electrolytic capacitor is installed backward. The correct answer involves damage to the capacitor and potential circuit failure — not just reduced capacitance.
- Effect of plate separation on capacitance: Some test questions describe a physical change to a capacitor (moving plates closer together or farther apart) and ask how capacitance changes. Moving plates closer increases capacitance — this is counterintuitive to students who think of more space as allowing more storage.