The BTA24 is a high-current triac rated at 25A RMS and 800V, housed in the industry-standard TO-220AB insulated package. Manufactured by STMicroelectronics and compatible brands, it is one of the most widely used power semiconductors for heavy-duty AC switching and phase control applications. This guide covers everything you need: full specs, pinout, how it works, typical applications, equivalent parts, and the best places to buy it.
What Is the BTA24?
The BTA24 is a 4-quadrant bidirectional thyristor (TRIAC) designed for general-purpose mains AC power switching. The “BTA” prefix indicates an internally isolated tab (rated at 2500V RMS), meaning the metal heatsink mounting tab is electrically isolated from the internal circuitry. This makes it safe to bolt directly to a grounded heatsink without an insulating pad, simplifying thermal management in industrial designs.
BTA24 Technical Specifications
| Parameter | Value |
|---|---|
| Repetitive peak off-state voltage (VDRM) | 800 V |
| RMS on-state current (IT RMS) | 25 A |
| Non-repetitive surge current (ITSM) | 250 A |
| Gate trigger current (IGT) | 50 mA |
| Gate trigger voltage (VGT) | 1.3 V |
| Peak on-state voltage drop (VTM) | 1.55 V |
| Holding current (IH) | 70 mA |
| Junction-to-case thermal resistance (Rth j-c) | 1.7 K/W |
| Operating junction temperature (TJ) | -40°C to +125°C |
| Tab isolation voltage | 2500 V RMS (UL certified) |
| Triggering quadrants | 4 quadrants (standard) / 3 quadrants (Snubberless W-suffix) |
| Package | TO-220AB (insulated) |
BTA24 Pinout (TO-220AB)
Viewed from the front face (label side facing you), the TO-220AB package has 3 pins:
- Pin 1 — T1 (MT1 / Main Terminal 1): Connected to neutral or one leg of the AC mains.
- Pin 2 — T2 (MT2 / Main Terminal 2): Connected to the load or the other AC mains leg. Also internally connected to the heatsink tab.
- Pin 3 — Gate (G): Control terminal. A trigger pulse here turns the device on.
Note: Unlike the BTB24 (non-isolated), the BTA24 tab is isolated — it is NOT electrically connected to T2. You can mount it on a grounded heatsink without an insulating pad.
How Does the BTA24 Work?
The BTA24 functions as a bidirectional AC switch. In the off state, it blocks voltage in both directions. When a sufficient gate trigger pulse (50 mA minimum at 1.3V) is applied, it enters conduction and remains on until the load current drops below the holding current (70 mA) — which happens naturally at each AC zero crossing.
Being a 4-quadrant device, it can be triggered with positive or negative gate signals regardless of the polarity of the main terminal voltage, giving circuit designers maximum flexibility.
Key Applications
- ⚡ Static relays and contactors: Replacing mechanical relays in high-current AC loads (motors, heaters, compressors).
- 🌡️ Heating regulation: Phase-angle control for industrial furnaces, water heaters, and heating elements up to 25A.
- ⚙️ Induction motor soft-start: Limits inrush current when starting large AC motors.
- 💡 High-power light dimmers: For stage lighting, industrial dimming panels.
- 🔧 Power tools speed control: Variable speed control in drills, saws, and grinders.
- 🏭 Industrial automation: Solenoid valve drivers, pneumatic actuator control, conveyor motor switching.
BTA24 vs BTB24 — What Is the Difference?
| Feature | BTA24 | BTB24 |
|---|---|---|
| Tab isolation | Yes (2500V RMS) | No (tab = T2) |
| Heatsink insulator needed | No | Yes |
| Thermal resistance (Rth j-c) | 1.7 K/W | 1.0 K/W |
| UL1557 certified | Yes | No |
Equivalent and Substitute Components
- BTA24-600B — Same part, 600V version (lower voltage rating).
- BTB24-800B — Non-isolated tab version, identical ratings.
- T25-800B — D²PAK surface-mount equivalent.
- BTA26-800B — Higher current version (25A to 40A upgrade path).
- MAC228A8 — Older equivalent in TO-220 package.
🛒 Where to Buy the BTA24 25A 800V
Mercado Livre — BIOSCHEMATICS Store
Original unit, ready to ship, 30-day return guarantee:
🟡 Buy BTA24 25A 800V TO-220 on Mercado Livre
AliExpress — Bulk Packs
Best options for buying in quantity:
- 📦 10 pcs BTA24-800B TO-220 Triac 25A 800V — AliExpress
- 📦 5 pcs BTA24-800B BTA24-800 TO-220 Original — AliExpress
- 📦 10 pcs BTA24-800B 25A 800V TO-220 Genuine — AliExpress
- 📦 5 pcs BTA24-800B BTA24 Triacs 25A 800V — AliExpress
Practical Tips and Design Notes
- Heatsink is mandatory at currents above 5A. Despite the isolated tab, thermal resistance is 1.7 K/W, and at 25A the junction will overheat quickly without adequate cooling. Use thermal paste for best results.
- Snubber circuit: For inductive loads (motors, solenoids, transformers), add an RC snubber (typically 100Ω + 100nF in series) between T1 and T2 to suppress high dV/dt transients that can cause false triggering.
- Use BTA24W (Snubberless) for inductive loads if you want to avoid the snubber circuit entirely. The W suffix version has enhanced commutation performance.
- Gate drive: The 50 mA gate trigger current means it cannot be driven directly from a microcontroller I/O pin. Always use an optocoupler (MOC3041, MOC3061) or a gate driver IC for isolation and sufficient drive current.
- Voltage derating: For reliable long-term operation, derate the voltage to 80% of VDRM. For 240V mains (peak ~340V), the BTA24-600B is technically sufficient, but the 800V version provides better surge protection margin.
Conclusion
The BTA24 25A 800V TO-220 is a robust, industry-proven triac for demanding AC power control applications. Its isolated tab, 4-quadrant triggering, and 25A current capacity make it the go-to choice for industrial-grade static relays, motor starters, and heating controllers. Whether you are repairing commercial equipment or designing a new system, the BTA24 delivers the reliability and safety margins that the job demands.