DIAC

The DIAC is a symmetrical breakover device that fires a sharp pulse when its voltage reaches ~30 V. Learn its I–V characteristic, negative resistance, and how RC–DIAC networks set a TRIAC's firing angle in classic dimmers.

A DIAC is a two-terminal device that blocks until the voltage across it reaches its breakover voltage (typically 28–36 V) in either polarity, then snaps into conduction with a drop of several volts. That snap dumps a capacitor's charge into a TRIAC's gate as a clean, fast trigger pulse.

The classic analogue dimmer — potentiometer, capacitor, DIAC, TRIAC — is still built by the million because it needs no microcontroller.

An analogy

A DIAC is like a pressure-relief valve that stays shut until the pressure reaches a set point, then opens fully and suddenly, releasing a burst.

Symmetric breakover

Below V_BO only a tiny leakage current flows. At ±V_BO the DIAC avalanches and its voltage falls back by ΔV while current rises — a negative-resistance region. It behaves identically for both polarities, so one DIAC triggers both half-cycles of a TRIAC.

RC phase-shift trigger

A potentiometer and capacitor form a phase-shifting network across the supply. The capacitor voltage lags the supply by φ = arctan(ωRC) and is smaller in amplitude. When it reaches V_BO the DIAC fires, discharging the capacitor into the TRIAC gate. Larger R → more lag and a smaller amplitude → later firing → lower power.

Hysteresis ('snap-on') and double time constants

After the DIAC fires, the capacitor voltage drops, so the next half-cycle starts from a different initial condition: turning the pot up from 'off', the lamp snaps on at a mid brightness. Adding a second RC stage and a resistor reduces this hysteresis and extends the control range.

Replacing DIACs

MCU-based dimmers replace the RC–DIAC network with zero-cross detection and timed gate pulses, enabling remote control and LED compatibility. Silicon bilateral switches (SBS) offer lower, more precise breakover voltages when accuracy matters.

Formulas

• Capacitor voltage amplitude: V_C = V_m / √(1 + (ωRC)²)
• Phase lag: φ = arctan(ωRC)
• Firing angle (estimate): α ≈ φ + arcsin(V_BO / V_C)
• Trigger pulse charge: Q = C · ΔV

Worked example: Firing angle at R = 100 kΩ

Given: V = 230 V RMS, 50 Hz; R = 100 kΩ, C = 100 nF; V_BO = 32 V

Find: Capacitor voltage, phase lag and firing angle.

• ωRC = 314.2 × 100e3 × 100e-9 = 3.14
• V_C = 325.3/√(1 + 3.14²) = 98.7 V; φ = arctan 3.14 = 72.3°
• α ≈ 72.3° + arcsin(32/98.7) = 72.3° + 18.9° = 91.3°

Answer: α ≈ 91° → about half power

Worked example: Trigger pulse energy

Given: C = 100 nF; Fold-back ΔV = 10 V

Find: Charge delivered to the gate.

• Q = C·ΔV = 100e-9 × 10 = 1 µC
• Over ~5 µs → average ~200 mA gate pulse, well above I_GT

Answer: ≈ 1 µC, ~200 mA pulse

Common mistakes

• Wrong: Connecting a DIAC directly across the mains. — Right: It conducts every half-cycle above ±32 V and will overheat; it must be in series with an RC network feeding a gate.
• Wrong: Expecting smooth control from 'off'. — Right: Simple RC–DIAC circuits show hysteresis ('snap-on'); use a two-stage RC network to reduce it.

Applications

• Analogue dimmers: Wall dimmers and fan regulators with a single knob use the RC–DIAC–TRIAC circuit.
• Fluorescent lamp starters & ballasts: DIACs start self-oscillating half-bridge ballasts in CFLs.
• Trigger circuits: Relaxation oscillators and simple SCR/TRIAC triggers use a DIAC's sharp breakover.

Key takeaways

• DIAC: two-terminal, symmetric breakover (~30 V), negative resistance after.
• RC network sets firing by phase lag and amplitude: α ≈ φ + arcsin(V_BO/V_C).
• Discharging C into the TRIAC gate gives a sharp trigger pulse.
• Beware snap-on hysteresis; digital dimmers replace DIACs today.

Going further

• Relaxation oscillator: A DC source charging a capacitor through R, with a DIAC across it, produces a sawtooth whose period ≈ RC·ln(V_s/(V_s − V_BO)). It is used in simple timers and pulse generators.
• EMI from phase control: Each firing produces a fast current step with broadband noise; dimmers include an LC filter (a small toroid choke plus X-capacitor) to meet conducted-emission limits.
• Thermal drift: V_BO has a small temperature coefficient; combined with capacitor tolerance, analogue dimmers drift slightly as they warm, which precision controls avoid with digital timing.