8.3 Diodes, SCRs & Triacs

8.3 Diodes, SCRs & Triacs

Quick Answer: Diodes pass forward, block reverse (within PIV). Rectifiers (half-wave, full-wave, bridge) make DC from AC. SCRs use anode, cathode, gate and latch after a gate pulse. TRIACs act like two SCRs back-to-back with terminals gate, anode 1, anode 2. RF specialties: PIN switches, hot-carrier VHF/UHF mixers/detectors, varactors for tunable C, tunnel diodes with a negative-resistance region.

Key topics 023–024 finish this chapter’s power and RF semiconductor toolkit: controlled rectifiers for AC power and the many diode types inside receivers, transmitters, and supplies.

PN junction diode basics

A PN junction diode joins p-type and n-type semiconductor. Forward bias (p more positive than n by roughly the barrier voltage) injects carriers and conducts. Reverse bias widens the depletion region and blocks current except tiny leakage—until reverse voltage hits breakdown.

Two most common diode specifications (from electrical-principles crossover): maximum forward current and peak inverse voltage (PIV).

Structural categories (pool)

Structurally, the two main categories of semiconductor diodes: junction and point contact.

• Junction diodes: planar PN junctions—workhorse rectifiers, switching, zeners, many signal diodes.
• Point-contact diodes: cat-whisker style construction historically used for microwave detection; still a classification the pool tests.

Rectifier circuits

Half-wave

One diode conducts on one AC half-cycle. Output pulse rate equals line frequency (60 Hz in → 60 Hz ripple). Simple but inefficient use of the transformer and large filter C needed.

Full-wave center-tap

Transformer secondary center-tapped; two diodes alternately conduct. Ripple frequency 2× line. Each diode must withstand high reverse voltage relative to the full secondary.

Bridge (full-wave bridge)

Four diodes; no center tap required. Both half-cycles delivered to the load; ripple 2× line. Widely used in radio power supplies.

Peak inverse voltage (PIV)

PIV is the maximum reverse voltage the diode must survive each cycle. In rectifiers, PIV depends on topology and secondary voltage peaks. Undersized PIV → reverse breakdown → shorted diode → blown fuses or damaged transformers. Always derate for surges and elevated temperature.

Freewheeling (flyback) diodes

When you switch current through a relay coil, motor, or other inductor, opening the switch collapses the magnetic field and induces a high-voltage spike (Lenz’s law / back EMF). A freewheeling diode (flyback diode) placed reverse-biased across the inductor in normal drive polarity provides a path for inductive current when the switch opens, clamping the spike and protecting transistors, contacts, and SCRs. This is standard practice on DC relay drivers in radio installations.

SCRs (silicon-controlled rectifiers) — 3-C-023

Terminals

Three terminals of an SCR: anode, cathode, and gate.

Not base/collector/emitter (BJT), not gate/source/sink (FET wording), not dual-base UJT naming.

Latching gate control

An SCR is a four-layer PNPN device that blocks in the forward direction until a gate pulse (with anode positive relative to cathode) triggers it into conduction. Once on, it latches—stays conducting even if the gate signal is removed—until anode current falls below the holding current (usually at AC zero-cross or when DC is interrupted). That latching behavior makes SCRs excellent for controlled rectifiers, crowbars, and power switching, but poor as linear amplifiers.

Parallel reverse-polarity connection

How might two similar SCRs be connected to safely distribute the power load of a circuit? In parallel, reverse polarity.

That arrangement handles opposite half-cycles of AC (one SCR for each polarity path), sharing the AC load safely in the pool’s framing. Same-polarity parallel without care risks unequal current sharing; series stacks raise voltage rating but need balancing networks in practice.

Where you might find an SCR

A circuit that might contain an SCR: a light-dimming circuit (phase-controlled AC). Not a filament circuit of a tube radio as the pool’s correct choice, not a “shunt across a transformer primary” as the defining answer, and not a bypass capacitor to ground.

TRIACs

Which device acts as two SCRs connected back to back, facing opposite directions, sharing a common gate? A TRIAC.

Transistor fabricated as two complementary SCRs in parallel with a common gate terminal: again, a TRIAC.

Three terminals of a TRIAC: gate, anode 1, and anode 2 (sometimes labeled MT1/MT2—main terminals—in datasheets; Element 3 uses anode 1 / anode 2 wording).

TRIACs shine in AC control: lamp dimmers, heater controls, and some motor controls. Snubbers and zero-cross drivers matter in real hardware to limit dV/dt false triggering—exam level focuses on identity, terminals, and dual-SCR model.

flowchart TB
  subgraph SCR_path [SCR half-cycle path]
    A1[Anode] --> G1[Gate trigger]
    G1 --> K1[Cathode]
  end
  subgraph TRIAC_model [TRIAC model]
    MT[Anode 1 / Anode 2 bidirectional]
    GT[Common gate]
    MT --- GT
  end

Specialty and RF diodes (3-C-024)

PIN diodes at RF act nearly like current-controlled resistors when forward-biased (low loss “on”) and high impedance when reverse-biased (“off”)—ideal for T/R switching and stepped attenuators without mechanical relays.

Hot-carrier diodes switch extremely fast with low charge storage—excellent for high-frequency mixing and detection where ordinary junction diodes smear RF energy.

Varactors reverse-bias a junction to change depletion width → capacitance. More reverse voltage → typically less C. Radio front ends and PLLs exploit that electrically tuned C.

Tunnel diodes exhibit a region where increasing voltage decreases current (negative resistance). That region can be biased for oscillation or amplification—a unique pool fact pair: negative-resistance characteristic and amplification/oscillation capability.

Putting power and RF diodes to work in one station

1. Bridge rectifier + filter + three-terminal regulator → clean 13.8 V radio rail.
2. Freewheeling diode across antenna-tuner relay coil → protects the driver transistor.
3. SCR crowbar on an overvoltage sense line → hard-clamps a runaway supply (latches until fuse opens).
4. TRIAC dimmer on a non-RF AC accessory load (never casually on a precision RF filament without design care).
5. PIN diode T/R switch between PA and receiver input.
6. Hot-carrier diodes in a VHF mixer or detector stage.
7. Varactor in a VCO or front-end peaking network.

Exam-day checklist for 023–024

1. Diode forward/reverse + PIV / forward current ratings.
2. Rectifier topologies and 2:1 full-wave ripple frequency vs line.
3. Freewheel diode for inductive kick.
4. SCR terminals anode, cathode, gate; latches until current collapses.
5. Two SCRs for AC load: parallel, reverse polarity.
6. TRIAC = dual SCR model; terminals gate, anode 1, anode 2; light dimmers.
7. Categories junction and point contact.
8. PIN → RF switch; hot-carrier → VHF/UHF mixer/detector; varactor → voltage-variable C; tunnel → negative resistance / amp-osc.

Clear this section and you have completed Element 3 Topic 3-C key topics 019–024—the passive and power-device foundation before bipolar transistors and FETs in the next chapter.