ACS108 AC Switch Discontinued: Cross-Reference and Replacements
ST ACS108 alternistor triacs are End-of-Life. Cross-reference to in-stock BTA12 and BTB16 snubberless triacs with voltage, current, and package comparison.
Last updated: June 2026
Bottom Line: ST's ACS108 family of alternistor triacs is End-of-Life. If you need an 800 V / 0.8 A AC switch with an integrated overvoltage clamp, the remaining in-stock ACS108 variants (ACS108-6SA, ACS108-6SA-AP, ACS108-6SA-TR, ACS108-8SA-AP) cover most designs in production today. For applications that require higher current (4 A–16 A) or that need a snubberless gate-commutated turn-off, drop-in replacements from ST's BTA12 and BTB16 snubberless alternistor families offer a clear upgrade path. Source these through FindMyChip's /quote form to get competitive pricing from 200+ verified distributors within 24 hours.
Why the ACS108 Is Being Discontinued
STMicroelectronics launched the ACS108 in the early 2000s as a logic-level, sensitive-gate AC switch for low-power consumer loads: lamp dimmers, fan speed controllers, small motor starters, and smart-home relay substitutes. The device integrates an overvoltage protection clamp that limits VDRM transients, which made it attractive in designs where a separate varistor or TVS was undesirable.
ST announced End-of-Life notices for most ACS108 variants in 2023–2024. The official reason is manufacturing consolidation: the device was built on an older fab process that is being retired as demand migrated to higher-current snubberless triacs and to semiconductor relay ICs (e.g., MOC3063 + BTA16 combos or dedicated solid-state relay modules). Engineers should check ST's product lifecycle page or the FindMyChip /search portal for the latest PCN (Product Change Notice) dates by part number.
Understanding the ACS108 Family: What You Actually Have
Before picking a replacement, identify which ACS108 variant is in your BOM. The part number encodes voltage class and package:
| Variant | VDRM | IT(RMS) | Package | Gate Trigger IGT |
|---|---|---|---|---|
| ACS108-6SA | 600 V | 0.8 A | TO-92 | ≤ 5 mA |
| ACS108-6SA-AP | 600 V | 0.8 A | TO-92 (Pb-free) | ≤ 5 mA |
| ACS108-6SA-TR | 600 V | 0.8 A | TO-92 T&R | ≤ 5 mA |
| ACS108-8SA-AP | 800 V | 0.8 A | TO-92 (Pb-free) | ≤ 10 mA |
The -6 suffix indicates 600 V VDRM; -8 indicates 800 V VDRM. The -SA suffix means TO-92 through-hole (single ammo pack); -TR means tape-and-reel for automated insertion. The -AP variant carries the matte-tin (Pb-free) surface finish required under RoHS Directive 2011/65/EU.
If your design is running at 120 V AC (North America), the 600 V variants carry a 2× derating margin—adequate but modest. If you are running on 240 V AC (Europe, Asia) with inductive loads that produce voltage spikes, the 800 V variants (ACS108-8SA-AP) are the safer choice and should be your first stocking priority.
Cross-Reference Table: ACS108 → In-Stock Alternistor Triacs
The table below covers the most common substitution scenarios. Column "Key Difference" flags what changes in the design; everything else (TO-220AB package, quadrant I/III gate triggering, snubberless operation) is compatible.
| Discontinued / Constrained | In-Stock Replacement | VDRM | IT(RMS) | Key Difference |
|---|---|---|---|---|
| ACS108-6SA / -6SA-AP / -6SA-TR | BTA12-600BWRG | 600 V | 12 A | Higher current; TO-220AB; larger footprint |
| ACS108-8SA-AP | BTA12-800BWRG | 800 V | 12 A | Higher current; snubberless; no OVP clamp |
| ACS108 (any variant), higher load | BTB16-600BWRG | 600 V | 16 A | Higher current; isolated TO-220AB |
Design note: The ACS108 is rated 0.8 A RMS, which is why its TO-92 package is viable. The BTA12 and BTB16 are rated 12 A and 16 A respectively and come in TO-220AB—a physically larger package that requires a new PCB footprint or an adapter. If form-factor is the binding constraint and current stays ≤ 1 A, search FindMyChip for SOT-223 or TO-92 triacs from the ACS108 series that remain in stock.
Key Selection Parameters
1. Off-State Voltage (VDRM / VRRM)
VDRM is the peak repetitive off-state voltage the triac blocks without triggering. For 120 V AC mains (VPEAK = 170 V), a 400 V rating provides 2.35× derating. For 230 V AC mains (VPEAK = 325 V), ST recommends a minimum 600 V device, giving ~1.85× derating under IEC 60664-1 overvoltage category III. The ACS108-8SA-AP's 800 V VDRM extends the margin to 2.46× on 230 V systems—useful in industrial environments where transient surges can reach 2× VPEAK.
Rule of thumb: Never use a 600 V triac on 277 V or 480 V industrial circuits. Use an 800 V or 1000 V device and add a MOV (metal oxide varistor) in parallel with the triac for transient immunity.
2. On-State Current (IT(RMS))
The ACS108 handles 0.8 A RMS—enough for a 60 W incandescent lamp at 120 V or a 180 W resistive load at 230 V. Switching inductive loads (motors, transformers) requires derating by 30–50% due to high di/dt at commutation. For a 1/4 HP (190 W) universal motor on 120 V drawing ≈ 2 A peak, the ACS108 is marginal. The BTA12 at 12 A provides a 6× margin on the same motor.
When thermal resistance matters, check the junction-to-case value (RθJC). The ACS108 TO-92 has RθJC ≈ 50°C/W; the BTA12 TO-220AB has RθJC ≈ 1.5°C/W. At 1 A, the TO-92 runs significantly hotter. At currents above 2 A, the TO-220AB is the only viable path without a heatsink larger than the PCB.
3. Gate Trigger Current (IGT) and Voltage (VGT)
The ACS108 family is a "logic-level" or "sensitive-gate" triac. Its IGT is ≤ 5 mA (ACS108-6SA) or ≤ 10 mA (ACS108-8SA-AP), allowing direct drive from a 3.3 V GPIO through a 330 Ω resistor with no separate gate driver IC. Standard triacs such as the BTB16 require IGT up to 50 mA at 25°C—and up to 100 mA at −40°C (cold-temperature derating)—which requires a transistor or optocoupler driver stage.
If your existing firmware uses direct GPIO switching without a gate driver, confirm your replacement supports the same IGT class before committing to a footprint change.
4. Quadrant IV Gate Triggering
Most AC control circuits trigger the triac using optocouplers such as the MOC3041 or MOC3062, which drive the gate in quadrant I (MT2+, G+) and quadrant III (MT2−, G−). The ACS108 and BTA/BTB alternistor series all support I+/I−/III+/III− quadrant triggering. Quadrant IV (MT2−, G+) sensitivity is lower and not guaranteed on snubberless designs—this is acceptable because quadrant IV triggering is a legacy technique and is avoided in new designs.
5. Snubberless Operation
"Snubberless" or "alternistor" devices are designed with a specific dV/dt immunity that allows them to commutate inductive loads without an RC snubber network. The ACS108 datasheet specifies dV/dt = 500 V/μs (commutating), while the BTA12-600BWRG specifies 500 V/μs as well, and the BTB16-600BWRG specifies 500 V/μs. In practice, if your original ACS108 circuit did not require a snubber, a BTA12/BTB16 replacement operating within its current rating also will not require one.
If your original design does use a snubber (100 Ω + 10 nF in series, across MT1-MT2), leave it in place—it adds robustness and does not harm the replacement device.
6. Isolation / Package
The BTB16 uses an isolated TO-220AB package (the metal tab is not connected to MT2), which allows bolt-down to a heatsink without an insulating mica washer. The BTA12 uses a non-isolated TO-220AB (tab = MT2). Verify which variant you receive; the BRG suffix on BTA12-600BWRG indicates the RoHS-compliant lead-free finish but does not determine isolation—check the "B" vs "T" position in the part number suffix or the datasheet's package drawing.
Selection Decision Flowchart
Use this logic to narrow down the replacement:
What is your supply voltage?
- 120 V AC → minimum VDRM 400 V; prefer 600 V for margin.
- 230–240 V AC → minimum VDRM 600 V; prefer 800 V for margin.
- 277–480 V AC industrial → use 800 V or 1000 V; add MOV.
What is your load current (RMS)?
- ≤ 0.8 A → remaining ACS108 stock is drop-in if still available.
- 0.8 A–12 A → BTA12 family (BTA12-600BWRG or BTA12-800BWRG).
- 12 A–16 A → BTB16 family (BTB16-600BWRG).
16 A → consider paralleled triacs or a higher-rated BTW69 / T35 series.
Is isolated mounting required?
- No → BTA12 (tab = MT2, non-isolated) is lower cost.
- Yes → BTB16 (isolated tab) avoids the insulating washer; reduces assembly steps.
Is direct GPIO drive (no gate driver IC) required?
- Yes (IGT ≤ 10 mA) → source remaining ACS108-8SA-AP stock first; otherwise add a small NPN or use an MOC3022 optocoupler.
- No → any BTA12/BTB16 variant at standard IGT (25–50 mA) works.
Recommended Products
| Product | VDRM | IT(RMS) | Package | IGT | Best For |
|---|---|---|---|---|---|
| ACS108-6SA-AP | 600 V | 0.8 A | TO-92 | 5 mA | Direct GPIO; 120 V low-power; drop-in |
| ACS108-8SA-AP | 800 V | 0.8 A | TO-92 | 10 mA | 230 V; direct GPIO; EOL stock clearance |
| BTA12-600BWRG | 600 V | 12 A | TO-220AB | 25 mA | 120 V inductive loads; fan/motor control |
| BTA12-800BWRG | 800 V | 12 A | TO-220AB | 25 mA | 230 V inductive loads; snubberless |
| BTB16-600BWRG | 600 V | 16 A | TO-220AB (isolated) | 35 mA | High-current 120/230 V; heatsink mount |
Frequently Asked Questions
Q: Is the ACS108-6SA pin-compatible with the BTA12-600BWRG? No. The ACS108-6SA uses a TO-92 package (3-pin through-hole, 2.54 mm pitch, ~4 mm body). The BTA12-600BWRG uses a TO-220AB package (3-pin, 2.54 mm pitch, ~15 mm body). The pin pitch matches but the body size, mounting hole, and thermal footprint differ significantly. A direct PCB swap is not possible without layout changes; however, in prototyping, a wired point-to-point replacement works for functional validation.
Q: Can I use a BTA12 in place of an ACS108 without adding a snubber? Generally yes, if the original design operated without a snubber. Both devices are rated 500 V/μs commutating dV/dt, which covers most mains-connected inductive loads under IEC 60068-2-1. Add a snubber (100 Ω, 10 nF, 630 V-rated) if you observe unwanted re-triggering or if your load presents a power factor below 0.5.
Q: What is the ACS108 overvoltage clamp and do replacements have it? The ACS108 integrates a zener-like protection structure that limits off-state voltage spikes to approximately 1.3× VDRM, preventing false turn-on from conducted EMI. Standard BTA12 and BTB16 devices do not include this clamp. To replicate the protection, add a 600 V or 800 V MOV (e.g., S14K600) or a TVS diode array across MT1–MT2 on the replacement footprint. In most residential 230 V environments, a properly rated 800 V triac without a clamp performs adequately, but check your EMC test requirements.
Q: Where can I buy ACS108 in bulk at short notice? FindMyChip connects you with 200+ verified distributors in Shenzhen and globally. Submit a request through /quote with your target quantity and required lead time. FindMyChip's team responds within 24 hours with price and availability from multiple competing sources, helping you avoid grey-market overpayment during end-of-life buying windows.
Q: Are there any SMD (surface-mount) replacements for the ACS108 TO-92? Yes. The ACS108-6SN-TR and ACS108-8SN-TR are SOT-223 surface-mount variants that remain in production (verify current lifecycle status before designing in). For higher currents in SMD, look at the ACST8 or T810 series in TO-220FP or D2PAK packages, though those require gate driver circuits and carry different IGT specs.
Conclusion
The ACS108 discontinuation is a manageable transition if you act before your last-buy window closes. The fastest path for existing designs is to exhaust remaining in-stock ACS108 variants (ACS108-6SA-AP, ACS108-8SA-AP) for the current production run, then redesign the gate drive and PCB footprint to accept a BTA12 or BTB16 for all future builds.
For new designs, skip the ACS108 entirely and start with the BTA12-800BWRG or BTB16-600BWRG depending on your current and isolation requirements. These snubberless alternistors are active ST products with a multi-year roadmap, broad distributor support, and pricing that tracks market demand rather than EOL scarcity premiums.
Use FindMyChip's /search to check real-time stock across distributors, or submit a multi-line BOM quote at /quote to compare prices for both the legacy ACS108 stock and their replacements in a single request.