Capacitors
| Capacitors | |
|---|---|
| What it does | Stores energy in an electric field between two conductors |
| Unit | Farad (F); in practice microfarads (µF), nanofarads (nF) and picofarads (pF) |
| Schematic letter | C |
| Reactance | Xc = 1 / (2 × pi × f × C); falls as frequency rises |
| In practice | |
| Common types | Ceramic, film, mica, electrolytic, variable |
| Key ratings | Capacitance, tolerance, working voltage |
| Series and parallel | Parallel values add; series values combine like resistors in parallel |
| Ham uses | Filters, antenna tuners, tuned circuits, power supply filtering, RF bypassing |
Capacitors are electronic components that store energy in an electric field. The simplest capacitor is two metal plates separated by an insulator, called the dielectric. A capacitor blocks direct current but lets alternating current pass, and it passes high frequencies more easily than low ones. That simple behavior makes capacitors some of the busiest parts in any radio: they smooth the output of power supplies, steer RF away from places it doesn't belong and, paired with inductors, form the tuned circuits and filters that pick one signal out of many.
Capacitance
Capacitance is the amount of electric charge a capacitor stores for each volt across it. It is measured in farads, named after the English scientist Michael Faraday. One farad is a very large amount, so practical values use smaller units:
- microfarads (µF), millionths of a farad, typical of power supply filters;
- nanofarads (nF), thousandths of a microfarad;
- picofarads (pF), millionths of a microfarad, typical of RF tuned circuits.
Capacitance increases with larger plates, closer spacing between them, and a dielectric with a higher dielectric constant than air. Small capacitors are often marked with a three-digit code in picofarads: the first two digits are the value and the third is the number of zeros, so "104" means 100,000 pF, or 0.1 µF.
Charging through a resistor takes time. Resistance in ohms times capacitance in farads gives the time constant in seconds, the time to reach about 63 percent of full charge; after five time constants the capacitor is essentially full.
Reactance
A capacitor's opposition to alternating current is its capacitive reactance, measured in ohms:
- Xc = 1 / (2 × pi × f × C), with f in hertz and C in farads
Reactance falls as frequency or capacitance rises. A 100 pF capacitor has about 227 ohms of reactance at 7 MHz but only about 114 ohms at 14 MHz, while at DC its reactance is infinite. That is why a small capacitor from a circuit to ground can bypass RF while leaving DC and audio untouched, and why a coupling capacitor in series with a signal path passes the signal from one stage to the next while blocking DC.
Series and parallel
Capacitors combine the opposite way to resistors.
- In parallel, capacitances simply add: 100 pF and 220 pF in parallel make 320 pF.
- In series, the total is less than the smallest one: 1 / C = 1 / C1 + 1 / C2 + ... Two equal capacitors in series give half the value of one, so two 100 pF capacitors make 50 pF.
Types
| Type | Typical values | Notes and uses |
|---|---|---|
| Ceramic | Under 1 pF to tens of µF | Small and cheap; bypassing and coupling. NP0 (C0G) types stay stable with temperature and suit tuned circuits |
| Film | About 1 nF to tens of µF | Polyester and polypropylene; low loss; audio, timing and filters |
| Mica | A few pF to about 10 nF | Silver mica types are stable and low-loss; a traditional choice for RF filters and oscillators |
| Electrolytic | About 1 µF to many thousands of µF | Large values in a small case; power supply filtering. Polarized: must be connected the right way around |
| Variable | A few pF to several hundred pF | Air variables, trimmers and vacuum variables; tuning and matching |
An electrolytic capacitor connected backward can overheat, swell or burst, so its marked negative lead matters.
Voltage rating
Every capacitor has a maximum working voltage, often marked WV or WVDC. Beyond it the dielectric can break down, so the rating should sit comfortably above the highest voltage the part will see. RF voltages can be surprisingly high: the tuning capacitor of a magnetic loop may see several thousand volts at 100 watts, and the capacitors in an antenna tuner matching a high-impedance load see high voltages too at full legal power. Such capacitors have wide plate spacing or are vacuum types.
Uses in ham radio
- Filters. A low-pass filter at a transmitter's output places capacitors to ground between series inductors, passing the operating frequency while shunting harmonics away. Band-pass filters use both parts to pass one band.
- Antenna tuners. The common T-network tuner uses two variable capacitors in series with the signal and an inductor to ground.
- Tuned circuits. A capacitor and an inductor together resonate at one frequency, the basis of oscillators and receiver front ends; see Resonance and Q.
- Power supplies. Large filter capacitors smooth rectified AC into steady DC.
Filter capacitors in power supplies, and especially in tube amplifiers, can hold a dangerous, even lethal, charge long after the power is switched off. Bleeder resistors are fitted to drain it, but they can fail.