SCR (Thyristor)
Thyristor
A four-layer semiconductor device that acts as a switch, conducting current when triggered and staying on until the current drops below a threshold. Used for power control and switching applications.
Schematic symbol
Thyristor (SCR)
How scr (thyristor) appears in a circuit diagram.
Types & variants
Key specs
Voltage rating
Volt (V)Maximum voltage it can block when off. Must exceed circuit peak voltage.
Current rating
Ampere (A)Maximum average current it can handle while conducting.
Gate trigger current
mAMinimum current needed at the gate to turn on the device.
Holding current
mAMinimum current needed to keep the device conducting after trigger.
Forward voltage drop
Volt (V)Voltage across the device when conducting. Typically 1-1.5V.
Markings
Through-hole: Usually printed with part number and voltage/current ratings. SMD: Often marked with logo or code that needs datasheet lookup. Terminal identification: A (anode), K (cathode), G (gate).
Standard values
Not applicable - SCRs are specified by voltage/current ratings, not resistance. Common voltage ratings: 200V, 400V, 600V, 800V, 1000V. Current ratings: 1A, 8A, 25A, 40A, 100A+.
How to choose
1) Determine circuit voltage and current requirements. 2) Select voltage rating > peak circuit voltage (add margin). 3) Select current rating > max expected current. 4) Check gate trigger current matches your driving circuit. 5) Consider package type for your application.
Pinout & package
TO-220 — front view, tab at back
Check first: Shown for the common BT151/TYN family, where the tab is the anode. Some SCRs use a different order — confirm with the datasheet before wiring mains.
3 terminals: Anode (A), Cathode (K), Gate (G). Packages: TO-92, TO-220, TO-247, stud mount, SMD (DPAK, etc.). Pinout varies by package - always check datasheet.
Example circuits
- AC dimmer circuit with phase control
- DC motor speed controller
- Overvoltage protection crowbar circuit
- Solid-state relay
- Battery charger control circuit
Common failures
Short circuit (burnt between A-K), gate damage, failure to trigger (high resistance between G-K). Visible signs: cracks, discoloration, or burn marks on the package.
How to test
1) Set multimeter to diode test. 2) Check A-K: Should show open one way, ~0.5-1V drop other way. 3) Check G-K: Should show diode drop one way. 4) Trigger test: Apply gate current while measuring A-K resistance - should drop to low value. 5) Turn-off test: Reduce current below holding current - should return to high resistance.
Substitutes
Higher voltage/current rating SCRs can be used in lower applications. TRIACs for AC circuits. MOSFETs/IGBTs for high-frequency switching. Always ensure gate drive compatibility.