8.1 Capacitors & Photoconductive Devices

8.1 Capacitors & Photoconductive Devices

Quick Answer: Capacitance depends on plate spacing and the dielectric constant of the material between the plates (plus plate area in the full formula). Coupling capacitors block DC and pass AC; bypass capacitors give AC a low-impedance path to ground. Light on a photoconductive material increases conductivity (resistance falls). An optoisolator/optocoupler is an LED plus a photosensitive device.

Topic 3-C (circuit components) starts where pure R/L/C math left off: real parts you select, rate, and place in RF and power circuits. Key topics 019–020 cover photoconductive devices and capacitors—two building blocks that appear from bias networks to audio tone controls to optical isolation on control boards.

Photoconductive devices (3-C-019)

What light does to conductivity

What happens to the conductivity of photoconductive material when light shines on it? It increases.

Photons free additional charge carriers in the material. More free carriers → lower resistance → higher conductivity. The opposite choices (conductivity decreases, stays the same, or “becomes temperature dependent” as the defining answer) are distractors.

Photoconductive effect (pool definition): the increased conductivity of an illuminated semiconductor junction (or photoconductive solid). It is not primarily defined as “conversion of photon energy to electromotive energy” in the Element 3 wording, and it is not decreased conductivity under light.

What changes noticeability in crystalline solids? The photoconductive effect produces a noticeable change in the resistance of the solid—not capacitance, inductance, or specific gravity.

Photosensitive semiconductor junction under illumination: junction resistance decreases. Same story restated for a junction-style photosensor: light → more carriers → less resistance.

Optoisolators and optocouplers

Optoisolator description: an LED and a photosensitive device (phototransistor, photodiode, photoresistor-class sensor, etc.) optically coupled, usually in one package, so the input and output circuits do not share a galvanic (wire) connection.

Optocoupler description: the same essential pair—an LED and a photosensitive device. Element 3 treats optoisolator and optocoupler as the same functional idea under different names.

Why marine/aviation/radio techs care:

1. Ground loops and noisy digital rails stay isolated from sensitive RF or analog sections.
2. High-voltage control boards can switch low-voltage logic safely.
3. Fault isolation limits damage if one side fails short to a different potential.

Photoresistors (LDRs) in plain language

A photoresistor (light-dependent resistor, photoconductive cell) is a two-terminal resistance that falls as light intensity rises. Designers use LDRs in light meters, dusk-to-dawn switching, and simple AGC or display brightness schemes. Exam focus stays on the conductivity up / resistance down rule and the LED + photosensor package definition—not on exotic spectral curves.

Capacitor fundamentals (3-C-020)

What sets capacitance

Factors that determine the capacitance of a capacitor (pool-level): the distance between the plates and the dielectric constant of the material between the plates. The complete parallel-plate model also includes plate area:

[ C = \varepsilon_r \varepsilon_0 \frac{A}{d} ]

• Larger area A → more C
• Smaller spacing d → more C
• Higher relative dielectric constant (\varepsilon_r) → more C

Element 3’s listed correct pair emphasizes spacing and dielectric constant. Voltage on the plates and “amount of charge” describe operating state via (Q = CV), but they are not the geometric/dielectric factors that set the capacitor’s C value as a component.

Construction types you must recognize

Polarity trap: electrolytic capacitors reverse-biased can vent, heat, or fail short. Ceramic, mica, air, and most film types are non-polarized (within voltage ratings).

Series and parallel combinations

Worked parallel example: three 0.01 µF ceramics in parallel → 0.03 µF. More parallel surface area effectively.

Worked series example: two equal 20 µF electrolytics in series (with equalizing resistors in real high-voltage designs) → 10 µF, with each seeing roughly half the series string voltage if matched—useful when you need higher voltage withstand than one can alone.

Voltage ratings and ESR

Working voltage (WVDC / voltage rating): never operate a capacitor above its rated DC (or AC RMS) voltage. In rectifier filters, the capacitor sees peak DC after charging—size the rating for peak, ripple, and temperature derating, not only the AC RMS nameplate of the transformer.

ESR (equivalent series resistance): the unavoidable resistive loss in leads, plates, and electrolyte. High ESR causes heat under ripple current, soft regulation, and noisy RF rails. Low-ESR electrolytics and ceramics are preferred on switching supplies and RF bypass networks. ESR is why two capacitors with the same µF marking can behave very differently at RF.

Self-resonance and parasitics: every real capacitor has lead inductance. Above self-resonant frequency it stops looking like a pure C. That is one reason RF boards use multiple values in parallel (e.g., 0.1 µF + 0.001 µF) so some device remains low-impedance across a wide spectrum.

RF roles: coupling and bypass

Coupling capacitor

Purpose of a coupling capacitor: it blocks direct current and passes alternating current. Stages keep independent DC bias while sharing an AC signal path. It does not “block AC and pass DC,” and its primary exam purpose is not “increase/decrease resonant frequency” as the defining answer (though every C can shift resonance when it is part of a tuned network).

Bypass capacitor

A bypass capacitor provides a low-impedance path for AC to ground (or to the return rail), keeping RF or audio AC off a bias or supply node. DC bias remains; AC is shunted. In schematic reading questions, the bypass is the cap from a bias/emitter/source or supply pin to ground for the AC component.

Schematic hooks from the pool (Figures 3C4 / 3C5 concepts)

Even without the printed FCC figures in front of you, the pool trains these ideas:

1. A small variable capacitor placed in a neutralization or parasitic-suppression location can decrease parasitic oscillations (classic PA / amplifier stabilization theme).
2. One labeled component provides a signal ground path (AC return).
3. One labeled capacitor is the bypass (AC shunt).
4. A 1 µF capacitor on a potentiometer tone network is used to adjust tone (RC roll-off with the pot)—audio tone control, not “neutralize the amplifier” as the primary answer.

Putting photoconductors and capacitors on one board

Imagine a marine transceiver control panel:

• Optocoupler isolates a microprocessor GPIO from a high-side switch driving a relay.
• Ceramic bypass on every IC rail kills digital hash before it reaches the RF deck.
• Electrolytic after the bridge rectifier stores energy and reduces ripple (watch WVDC and ESR).
• Coupling film or ceramic moves AF between stages without shifting DC bias.
• Mica or NP0 ceramic in a VHF tank keeps temperature drift low.
• Optional LDR senses cabin light for display dimming—resistance falls as the cabin brightens.

Exam-day checklist for 019–020

1. Light on photoconductor → conductivity up, resistance down.
2. Photoconductive effect → increased conductivity of illuminated semiconductor; changes resistance of the solid.
3. Optoisolator / optocoupler → LED + photosensitive device.
4. Capacitance factors → plate distance + dielectric constant (and area in full physics).
5. Coupling → blocks DC, passes AC; bypass → AC to ground.
6. Parallel C adds; series C reciprocal; observe voltage rating and ESR in real designs.

Master these hooks and you clear the first slice of Topic 3-C before transformers, regulators, and power semiconductors.