1.5 Capacitors: Construction, Ratings and Safety

1.5 Capacitors: Construction, Ratings and Safety

ACMA Exam Focus: Syllabus items 4.9, 4.10 and 4.11 — recall the farad and the two-conductors-and-an-insulator construction, state how plate area, plate separation and the dielectric change the value, explain breakdown voltage and polarised capacitors, and describe the danger of the charge a capacitor still holds after switch-off.

---

The farad and the units you will actually meet

Capacitance is the ability of a component to store electric charge. It is defined by

$C = \frac{Q}{V}$

where $Q$ is the stored charge in coulombs and $V$ is the voltage across the component. The unit is the farad (F), named after Michael Faraday: a capacitor of one farad holds one coulomb of charge for every volt applied to it.

One farad is an enormous amount of capacitance, so almost every part in a VK station is labelled in a sub-multiple.

Learn the ladder in both directions: 1 µF = 1000 nF = 1 000 000 pF. Older circuit diagrams and surplus gear label microfarads mfd and picofarads mmfd.

What a capacitor is made of

Every capacitor is two conducting surfaces (the plates) separated by an insulator (the dielectric). Connect a voltage source and electrons are pushed onto one plate and pulled off the other until the resulting charge imbalance produces a voltage equal to the source. The energy is stored in the electric field in the dielectric, not in the metal.

Because the plates never touch, a capacitor blocks direct current while still passing alternating current — the plates simply charge and discharge in step with the applied waveform. That one property explains most of its uses in a transceiver: coupling audio between stages without passing the DC bias along with it, bypassing RF to chassis while leaving a supply rail intact, and setting the frequency of a tuned circuit.

The three factors that set the value

Written together, $C \propto \varepsilon_r A / d$. The examination asks for the direction of each effect rather than the arithmetic: increase the area or the permittivity and you gain capacitance, increase the spacing and you lose it. A variable capacitor demonstrates the first two directly — rotating the moving vanes out of mesh reduces the overlapping area and therefore the capacitance, while transmitting types use wide plate spacing so they can survive high RF voltages.

Common dielectrics in a station

• Ceramic — 1 pF to about 1 µF, non-polarised; the standard RF bypass and decoupling part.
• Silvered mica — 1 pF to about 10 nF; very stable and low loss, used in VFOs and power-amplifier tank circuits.
• Polyester and polypropylene film — 1 nF to about 10 µF; audio coupling, timing and snubber duty.
• Aluminium electrolytic — 1 µF to 10 000 µF and beyond, polarised; power-supply reservoirs, useless at RF.
• Tantalum — 0.1 µF to about 100 µF, polarised; compact low-voltage decoupling on logic boards.
• Air-spaced variable — a few pF to a few hundred pF; tuning, and the valve-era antenna tuning unit.

Breakdown voltage and working voltage (item 4.10)

Every capacitor carries a DC working voltage (WVDC) rating. Exceed it and the dielectric suffers breakdown: the insulator is punctured, the part becomes a short circuit or fails in a puff of smoke, and whatever it was serving usually goes with it.

The trap the examiners set is the difference between root-mean-square and peak. A capacitor charges towards the peak of the applied waveform, and for a sine wave the peak is 1.414 times the RMS value.

Worked example — a mains-derived rail. Australian mains is 230 V RMS at 50 Hz. Rectified straight into a reservoir capacitor, the peak is

$V_{peak} = 1.414 \times 230 = 325\ \text{V}$

so a 250 V part, and even a 350 V part, is unsafe here. Fit a 400 V or 450 V capacitor and you have headroom for mains surges and for the higher voltage present at no load. The same trap appears at radio frequencies in a different guise: the RF voltage swinging across a transmitter tank circuit can be several times the DC supply rail, which is why tank capacitors have widely spaced plates.

Polarised capacitors (item 4.10)

Aluminium electrolytic and tantalum capacitors achieve their very high capacitance per unit volume because their dielectric is an extremely thin oxide layer, formed electrochemically on the anode foil. That oxide survives only while the applied voltage keeps the correct polarity, so these parts are polarised and must be connected the right way round.

• Can-type electrolytics carry a printed stripe filled with minus signs down the negative side; radial types have a longer positive lead.
• Connect one backwards and the oxide degrades, leakage current rises sharply, the electrolyte heats and boils, and pressure bursts the scored vent — noisily, and with hot electrolyte.
• Never place a plain electrolytic directly across an AC source, where the polarity reverses every half-cycle. Use a bipolar (non-polarised) electrolytic or a film capacitor instead.

Stored charge is the real hazard (item 4.11)

The energy held in a charged capacitor is

$E = \tfrac{1}{2}CV^2$

in joules, with $C$ in farads and $V$ in volts. Energy rises with the square of the voltage, so doubling the voltage quadruples the stored energy.

Worked example. A linear amplifier supply uses a 100 µF reservoir charged to 450 V:

$E = 0.5 \times (100 \times 10^{-6}) \times 450^2 = 0.5 \times 0.0001 \times 202\,500 \approx 10.1\ \text{J}$

The danger is not the number by itself. It is that the capacitor still holds that charge after the equipment is switched off and unplugged. With no discharge path the terminals can sit near full voltage hours later, and even a shock that is not directly fatal triggers an involuntary muscular reaction, so the fall or the secondary contact does the damage.

The controls are simple, and they are examinable:

1. Bleeder resistor — a high-value resistor of adequate power and voltage rating, permanently connected across the reservoir and sized to discharge it within a set time after switch-off. It can fail open with no outward sign, so it is never the only safeguard.
2. Shorting stick — an insulated rod carrying a suitably rated resistor, typically a few hundred ohms to a few kilohms, with a heavy lead to chassis earth. The resistor limits the peak current so the discharge is controlled rather than explosive.
3. Verify, then link — measure zero volts, then clip a shorting link across the terminals and leave it in place while you work, because a capacitor can recover some voltage through dielectric absorption.
4. Order of work — switch off, unplug, wait, discharge with the stick, verify with a meter, fit the link. Keep one hand in your pocket so that no current path crosses your chest.

Capacitors are rarely used one at a time. The rules for finding the value of a series, parallel or series-parallel group are dealt with in the next section, alongside the matching rules for inductors.