In a DC circuit, the steady-state voltage drop across a fully charged capacitor equals the source voltage, effectively acting as an open circuit. In an AC circuit, however, the voltage drop across a capacitor is dynamic, determined by its capacitive reactance and the circuit current. For home electrical systems—specifically HVAC compressor motors, ceiling fans, and power factor correction banks—this AC voltage drop is the critical mechanism that creates the phase shift necessary to generate starting torque. Misunderstanding this drop leads to misdiagnosed motor failures, blown start windings, and incorrect component substitutions.
The Physics: Calculating Voltage Drop Across a Capacitor
To understand the voltage drop across a capacitor, you must separate DC transient behavior from AC steady-state behavior. In DC, once the capacitor charges (typically within 5 time constants, or $5\tau$), current stops flowing, and the voltage drop across the component matches the supply voltage.
In AC systems, the capacitor continuously charges and discharges. The opposition to AC current flow is called capacitive reactance ($X_C$), measured in ohms. The formula is:
X_C = 1 / (2 * π * f * C)
Where f is frequency in Hertz and C is capacitance in Farads. Once you have the reactance, the voltage drop across the capacitor ($V_C$) follows Ohm's Law: V_C = I * X_C.
Imagine a 40 µF (0.000040 F) motor run capacitor on a 240V, 60Hz residential AC circuit. The start winding draws 2A of current.
1.
X_C = 1 / (2 * 3.1416 * 60 * 0.000040) = 66.3 Ω2.
V_C = 2A * 66.3 Ω = 132.6VBecause the voltage drop across the capacitor and the voltage drop across the motor winding are 90 degrees out of phase, they do not add up arithmetically to 240V. Furthermore, back-EMF from the spinning motor can push the actual measured voltage drop across the capacitor terminals well above the 240V line voltage—often reading 300V to 350V on a multimeter.
Capacitor Types: Which Dielectric for Which Job?
Selecting the correct capacitor requires matching the dielectric material to the electrical stresses of the application. Using a DC-biased ceramic capacitor in an AC motor circuit will result in catastrophic failure. Here is the selection criteria for common passive applications.
| Type | Construction / Dielectric | Typical Tolerance | Tempco (Temp Stability) | Typical Use Case |
|---|---|---|---|---|
| Metallized Polypropylene (Film) | Plastic film, self-healing | ±3% to ±10% | Highly Stable (-40 to +85°C) | HVAC motor run caps, ceiling fans, AC snubbers |
| Electrolytic (Aluminum) | Aluminum foil, liquid/solid electrolyte | -10% to +50% | Poor (High temp degradation) | Motor start caps, DC power supply filtering |
| MLCC (Ceramic C0G/NP0) | Multi-layer ceramic, Class 1 | ±1% to ±5% | Ultra-Stable (±30ppm/°C) | RF filters, precision timing, high-freq decoupling |
| MLCC (Ceramic X7R/Y5V) | Multi-layer ceramic, Class 2 | -20% to +80% | Poor (Y5V drops 80% at 85°C) | General DC decoupling, bulk bypass on PCBs |
| Tantalum | Tantalum pentoxide, solid MnO2 | ±10% to ±20% | Moderate | Space-constrained DC filtering (Note: fails short, fire risk) |
Which type for which job? For continuous AC voltage drop applications (like keeping a compressor running), always use Metallized Polypropylene Film. For momentary high-torque DC/AC bursts (like starting the motor), use Electrolytic Motor Start capacitors. For PCB-level DC smoothing, use Aluminum Electrolytic or X7R Ceramics.
Decoding the Markings: What the Codes Mean
Physical space on a capacitor is limited, leading to cryptic marking schemes. Misreading these codes is a primary cause of bench errors.
1. The 3-Digit Ceramic Code (Picofarad Base)
Small ceramic disc and MLCC capacitors use a three-digit code where the first two digits are significant figures and the third is the multiplier (number of zeros), expressed in picofarads (pF).
- 104: 10 followed by 4 zeros = 100,000 pF = 100 nF = 0.1 µF.
- 222: 22 followed by 2 zeros = 2,200 pF = 2.2 nF.
- 471: 47 followed by 1 zero = 470 pF.
2. Letter Suffixes (Tolerance)
A trailing letter indicates tolerance. J = ±5%, K = ±10%, M = ±20%, Z = +80%/-20%.
3. Motor Capacitor Markings (MFD and VAC)
HVAC and home electrical capacitors print values plainly. MFD is an older term for microfarads (µF). You will often see dual ratings like 45/5 MFD, meaning 45 µF for the compressor (HERM) and 5 µF for the fan motor. The voltage rating will explicitly state VAC (e.g., 370VAC or 440VAC).
Failure Modes and Visual Symptoms
Capacitors degrade over time due to heat, voltage stress, and dielectric breakdown. Recognizing the visual and electrical symptoms of failure saves hours of troubleshooting.
- Electrolytic (DC Filtering / Motor Start):
- Visual: Bulging top vent, crusty brown electrolyte leakage at the base, or a ruptured rubber bung.
- Electrical: Equivalent Series Resistance (ESR) spikes. The capacitance value drops. In a power supply, this manifests as high AC ripple on the DC rail.
- Film (HVAC Motor Run):
- Visual: The metal can swells or deforms. Sometimes there is no visual cue, but the internal pressure switch pops.
- Electrical: They typically fail 'open' (loss of capacitance). The motor will hum loudly, overheat, and trip the thermal overload or breaker. Less commonly, they fail 'short', which instantly trips the branch breaker.
- Ceramic (MLCC):
- Visual: Micro-cracks along the edges, often invisible to the naked eye. Caused by PCB flexure (the piezoelectric effect makes them sensitive to mechanical stress).
- Electrical: Intermittent shorts or leakage current that causes logic brownouts in microcontrollers like the ESP32 or Arduino.
Safe Substitution: When You Don't Have the Exact Part
When a 40µF 370VAC motor run capacitor fails on a Friday night and your parts bin only has a 45µF 440VAC, you need to know the rules of substitution to avoid burning out a $1,500 compressor.
- Voltage Rating: You can always go UP, never DOWN. A 440VAC capacitor can safely replace a 370VAC capacitor. The higher voltage rating simply means a thicker dielectric, which slightly increases the physical size but improves longevity.
- Capacitance (µF/MFD): You must stay within the manufacturer's tolerance, typically ±5% to ±10%. Substituting a 45µF cap for a 40µF cap (a 12.5% increase) will increase the current through the start winding. Over time, this excess current will overheat and melt the winding. If you must combine capacitors to get the exact value, wire them in parallel to add capacitance (C_total = C1 + C2) or in series to reduce it.
- Start vs. Run: Never substitute a 'Start' capacitor for a 'Run' capacitor. Start capacitors are designed for a maximum of 3 seconds of energization. If left in the circuit continuously via a faulty potential relay, an electrolytic start capacitor will overheat and explode.
For deeper diagnostic techniques on testing these components in the field, the Electronics Tutorials guide on AC Capacitance provides excellent baseline theory on how phase angles affect real-world measurements.
Frequently Asked Questions
Why is the measured voltage drop across a capacitor higher than the supply voltage?
This is a common point of confusion for DIYers testing HVAC systems. If you measure 240V at the disconnect but read 320V across the run capacitor terminals, your meter is not broken. In an LC (inductor-capacitor) circuit like a motor winding and a capacitor, the back-electromotive force (back-EMF) generated by the spinning motor rotor induces a voltage that adds to the line voltage. Because the inductive and capacitive voltage drops are out of phase, the localized voltage drop across the capacitor can easily exceed the nominal supply voltage by 20% to 40%.
Does a capacitor cause a voltage drop in a DC power supply?
In a steady-state DC circuit, a capacitor blocks DC current entirely, meaning the voltage drop across it simply equals the source voltage. However, during the initial power-on transient, the capacitor acts as a dead short, causing a massive momentary voltage drop across the wiring and power supply's internal resistance (inrush current). In power supply filtering, the capacitor doesn't 'drop' voltage; rather, it fills in the voltage 'valleys' of the rectified AC waveform, smoothing the DC output and reducing ripple.
How do I measure the AC voltage drop across a motor run capacitor safely?
Use a CAT III or CAT IV rated digital multimeter. Set the meter to AC Volts. Place one probe on the 'C' (Common) terminal and the other on the 'HERM' (Compressor) or 'FAN' terminal. Keep your hands clear of the terminals and ensure the HVAC access panel safety switch is bypassed safely or the panel is off while probing. Expect to see a reading significantly higher than your nominal line voltage (e.g., 300V-350V on a 240V system). If the reading is exactly 0V, the capacitor is shorted or the power supply is dead. If the reading is line voltage (240V) but the motor isn't running, the capacitor has likely failed open internally.
What happens to the voltage drop across a capacitor if the frequency increases?
Because capacitive reactance ($X_C$) is inversely proportional to frequency ($f$), increasing the frequency decreases the reactance. In a series AC circuit, a lower reactance results in a smaller voltage drop across the capacitor and a larger share of the voltage dropped across other series components. This principle is the foundation of AC high-pass and low-pass audio crossover filters.
