A capacitor is a passive electrical component, symbolized by the letter C.
Capacitors are constructed using two conductive metal plates, referred to as electrodes or armatures.
These plates are separated by an insulating material called a dielectric.
Due to the insulating dielectric, no DC current flows through a capacitor:
I = 0 → R = \frac{V}{I} = ∞
When a capacitor is connected to a circuit:
Negative charge flows from the voltage source and accumulates on one of the capacitor's armatures.
Positive charge flows from the voltage source and accumulates on the other armature.
This process is called charging the capacitor. Once fully charged, if the connection is broken, a voltage (VC) remains across the capacitor's armatures, equaling the source voltage (V{DC}).
4.2 Capacitance
The ability of a capacitor to store electric charge is called capacitance, denoted by the symbol C.
The unit of capacitance is the Farad (F).
The Farad is a large unit, so smaller multiples are commonly used:
Farad (F)
MilliFarad (mF)
Microfarad (µF)
Nanofarad (nF)
Picofarad (pF)
Capacitance Conversions
1F = 1000 mF = 1 * 10^3 mF
1F = 1,000,000 µF = 1 * 10^6 µF
1F = 1,000,000,000 nF = 1 * 10^9 nF
1F = 1,000,000,000,000 pF = 1 * 10^{12} pF
Practice Exercises
Convert the following:
100 nF = ? µF
2,000,000 pF = ? µF
300,000,000 pF = ? mF
400,000 µF = ? F
4.3 Stored Energy and Voltage
The energy provided by a capacitor when discharging is the energy stored within it, calculated using the formula:
W = \frac{1}{2}CV^2
Where:
W is the energy in Joules (J).
C is the capacitance.
V is the voltage.
A strong electric field between the plates causes electrical charges to accumulate in the dielectric, creating a potential difference (voltage).
Therefore, when using a capacitor for charging and discharging, the applied voltage must be less than the capacitor's rated voltage.
4.4 Ceramic Capacitors
Ceramic capacitors are widely used in electronic devices because they are efficient and cost-effective to manufacture.
These capacitors are made from ceramic materials and come in various shapes and sizes depending on the manufacturing process; they are non-polarized.
Ceramic materials have a high dielectric constant, good insulation properties, and are effective insulators for electrostatic applications.
Characteristics of Ceramic Capacitors
High performance, stable, and reliable.
Used in general applications where circuit stability is required with minimal losses.
Applications of Ceramic Capacitors
Mobile phone circuits.
High voltage lasers.
Miniature voltage converters.
X-ray machines.
High-frequency circuits.
Printer circuits.
Reading Values on Ceramic Capacitors
Instructions provided on how to read the codes printed on ceramic capacitors to determine their capacitance value.
SMD Ceramic Capacitors
SMD (Surface Mount Device) ceramic capacitors are used in compact circuits to minimize size and improve stability.
These SMD capacitors do not have their capacitance or voltage ratings printed on them.
To determine the capacitance and voltage, a handheld LCR (Inductance, Capacitance, Resistance) meter is used, or information is searched using Google.
4.5 Mica Capacitors
Mica capacitors offer stability, reliability, and high precision.
Their capacitance ranges from 20pF to 10µF.
They exhibit good performance in high-frequency applications.
Characteristics of Mica Capacitors
Stable capacitance value.
Good performance at high temperatures.
Withstand high voltages.
Low losses.
Efficient operation.
Good dielectric insulation.
Applications of Mica Capacitors
High-frequency circuits (RF).
LC circuits.
Tuning circuits.
Power circuits.
Radios, Transmitters, and High-Speed circuits.
Signal filtering.
Reading Values on Mica Capacitors
Instructions provided on how to read the values printed on mica capacitors to determine their capacitance.
4.6 Electrolytic Capacitors
Electrolytic capacitors are polarized, meaning they have a positive (+) and a negative (-) terminal, which are indicated on the capacitor body.
Characteristics of Electrolytic Capacitors
Electrolytic capacitors act as voltage stabilizers, voltage filters, and signal isolators.
Their capacitance ranges from 1µF to 47mF, and they operate with a relatively low voltage in DC circuits.
They are commonly used in power supplies, computer motherboards, and various other circuits.
They are typically used in DC voltage circuits.
Reading Values on Electrolytic Capacitors
Information typically printed on electrolytic capacitors includes:
Manufacturer's name
Maximum voltage
Polarity
Capacitor series
Capacitor value (in Farads, µF, ρF, or nF)
Maximum temperature
Reading Values on SMD Electrolytic Capacitors
Instructions provided on how to read the values on SMD electrolytic capacitors.
4.7 Supercapacitors (also called Ultracapacitors)
Supercapacitors have a large capacitance, ranging from 1 to 20 Farads.
They have a high charge storage capacity and operate with high voltages.
They typically operate in temperatures ranging from -55°C to 125°C.
They are relatively low in cost to manufacture and use.
They are often used for energy storage and with DC voltage circuits.
Characteristics of Supercapacitors
Used for strong noise filtering.
Sometimes used in DC signal filtering applications to filter AC signals that have already passed through.
Used with sensors such as temperature sensors, oil level sensors, etc.
Used in the vehicle power systems to provide auxiliary power and to amplify audio signals.
For two capacitors in series:
CT = \frac{C1 C2}{C1 + C_2}
Parallel Connection:
CT = C1 + C2 + \cdots + Cn
4.9 Applications of Capacitors
Capacitors are used for:
Amplification (to improve sound quality), providing high stability.
Filtering signals in circuits.
Storing and releasing voltage.
Generating frequencies.
4.10 Capacitor Testing and Inspection
A capacitor's condition (good or bad) can be checked using an ohmmeter.
Two methods:
In-circuit testing
Out-of-circuit testing
In-Circuit Testing
Used for capacitors with values less than 1µF.
The faulty capacitor is removed from the circuit and tested with a multimeter.
Steps for In-Circuit Testing
Ensure the multimeter's voltage range is greater than the capacitor's rated voltage.
Connect one probe of the multimeter to the positive terminal of the circuit and leave the other probe free.
Connect the multimeter's negative probe to the free terminal of the capacitor.
Interpretation of In-Circuit Testing Results
Case 1: If the multimeter shows a slight deflection, displaying a voltage value that gradually decreases to zero, the capacitor is likely good. Reversing the polarity and repeating the measurement should yield similar results. After testing, short the capacitor's terminals. If no deflection is observed, the capacitor is good.
Case 2: If the multimeter shows a slight deflection that does not decrease or increases steadily, the capacitor is faulty.
Case 3: If the multimeter shows a slight deflection that decreases but not to zero, the capacitor is leaky.
More on Capacitor Testing with AC
For AC capacitors, a multimeter may not always be necessary.
Connect one probe of an AC power meter to the AC source and the other probe to one terminal of the capacitor. Connect the remaining terminal of the capacitor back to the AC source.
Interpretation of AC Testing Results
Case 1: If the power meter shows a high current, the capacitor is shorted. Shorting the capacitor terminals should result in no deflection, indicating a faulty capacitor.
Case 2: If the power meter shows a low current, shorting the capacitor terminals and observing no deflection indicates a good capacitor.
Out-of-Circuit Testing
Before testing, ensure:
The capacitor is disconnected from the circuit.
The capacitor is discharged.
Possible scenarios:
The capacitor is open (internal disconnection).
The capacitor is shorted (dielectric breakdown).
The capacitor is leaky (imperfect insulation, some current goes through the Di-electric material).
Out-of-Circuit Testing - cases
Case 1:
If upon repeatedly reversing ohmmeter polarity, the meter shows no deflection, the capacitor is open.
Case 2:
If upon repeatedly reversing ohmmeter polarity, the meter shows a deflection that does not decrease, the capacitor is shorted.
Case 3:
If upon repeatedly reversing ohmmeter polarity, the meter shows a deflection that increases and then decreases in approximately the same manner, the capacitor is leaky or has a lot of electric flow between the connected point.
Case 4:
Connect the (+) and (-) leads of the ohmmeter, the meter should deflect and return quickly to its original numbers.
Reverse the leads, the meter should deflect more slowly and also return to its original numbers.
If you still get no deflection with these testing methods listed above, the capacitor is good.
Review Questions
Define a capacitor.
Describe the characteristics of a capacitor.
What types of capacitors are there?
What is the role of a capacitor?
Compare and contrast ceramic and electrolytic capacitors.