DIAC
Thyristor
A two-terminal trigger device that blocks until about ±32 V, then avalanches and dumps a sharp pulse — used almost exclusively to fire triacs cleanly and symmetrically in dimmers.
Schematic symbol
Thyristor (SCR)
How diac appears in a circuit diagram.
Types & variants
Key specs
Breakover voltage
V±28–36 V for a DB3 — where it fires.
Breakover current
µACurrent at the trigger point.
Peak pulse current
AWhat it can dump into the triac gate.
Symmetry
VMatch between + and − breakover — asymmetry causes lamp flicker.
Markings
Tiny glass/plastic axial part marked DB3 (or ST2/BR100). No polarity — it's bidirectional.
Standard values
DB3 at 32 V covers nearly every dimmer; that's effectively the only 'value'.
How to choose
If the triac circuit calls for a diac, use a DB3 unless the design says otherwise. Symmetry spec matters for flicker-free dimming.
Pinout & package
2 leads, axial (DO-35 style). Either way round.
Example circuits
- Classic RC + diac + triac lamp dimmer
- Fan speed regulator
- Relaxation-oscillator trigger
Common failures
Shorts or drifts asymmetric after surges — symptom: dimmer stuck full-on, flickering, or dead at low settings.
How to test
A multimeter can't reach 32 V: it reads open both ways (which only proves it isn't shorted). Bench test: through a 10 kΩ resistor from a variable supply, it snaps into conduction near ±32 V.
Substitutes
Another DB3; a SIDAC for higher-energy triggering. Some triacs (quadracs) have the diac built in.