TRIAC Heatsink Sizing Calculator
Calculate how much heat your TRIAC will dissipate, what junction temperature it will reach, and whether a heatsink (and how big) is required for safe operation. Built specifically for AC dimmer and switch modules like the RobotDyn 1-channel and 16/24 A dimmers.
Why this matters: a TRIAC with no heatsink can hit its 125 °C junction limit at as little as 2 A. Most cooked TRIACs in DIY projects aren't due to overcurrent — they're due to thermal runaway from underestimated dissipation.
TRIAC dissipation — W
Required RθSA — °C/W
Junction temp (with chosen sink) — °C
Heatsink size —
How to read the results
- TRIAC dissipation — heat the TRIAC generates, roughly
VT × 0.9 × IRMSfor full-on phase conduction. For phase-cut at lower dimming levels, dissipation is lower; this calculator uses the worst case (full conduction). - Required RθSA — the heatsink's thermal resistance must be at most this value to keep junction temp ≤ 100 °C (safety margin below the 125 °C datasheet limit). Lower RθSA = bigger / better heatsink.
- Heatsink size recommendation — translation of RθSA into real-world heatsink categories (clip-on / finned / forced air).
Heatsink RθSA reference
| RθSA | Heatsink type | Approximate size |
|---|---|---|
| > 60 °C/W | No heatsink (TO-220 in still air) | — |
| 30 °C/W | Small clip-on aluminum | ~1 cm², 10 g |
| 15 °C/W | Medium clip-on with fins | ~5 cm², 30 g |
| 8 °C/W | Standard TO-220 finned heatsink | ~15 cm², 80 g |
| 4 °C/W | Large finned heatsink | ~40 cm², 200 g |
| 2 °C/W | Very large extruded heatsink | ~100 cm², 500 g |
| < 1 °C/W | Forced air required (fan + heatsink) | — |
Tips that the math doesn't show
- The TRIAC tab is electrically live. It's connected to MT2, which is at mains potential. If your heatsink is exposed (not inside a grounded enclosure), use a silicone insulator pad or mica + paste — adds thermal resistance but prevents electrocution.
- Vertical mounting of heatsink fins improves natural convection by ~30 % vs. horizontal. Cheap insurance.
- Start-up inrush on incandescent lamps can be 10× the steady-state current. The thermal mass of the heatsink absorbs the transient — don't undersize.
- De-rate at high ambient. If your enclosure runs at 50 °C inside (common for closed boxes near heating elements), you have only 75 °C of headroom to TJ max — much less than the 100 °C the calculator assumes from 25 °C ambient.
- For continuous loads > 10 A on the High-Load Dimmer, install a thermal cutoff on the heatsink — if the TRIAC overheats, the cutoff breaks the AC supply before fire.
Related
- AC Dimmer Power Calculator — figure out RMS current for input to this calculator
- RobotDyn AC Dimmer 1-Channel — uses BTA16-600B (default in this calculator)
- RobotDyn 16/24A High-Load Dimmer — uses BTA24-600B, needs serious cooling above 10 A
- ESP-01 SSR AC Relay — uses BT136-600B for small loads