If you are switching more than 5A through a standard panel-mount AC solid state relay (SSR), you need a heatsink. Guessing the size based on physical bulk leads to melted terminals or catastrophic short-circuit failures. The exact heatsink size is dictated by the thermal resistance path from the silicon junction to the ambient air, measured in °C/W. For a 15A continuous load on a 240VAC circuit, a standard 2.5 °C/W extruded sink will overheat; you need a profile rated at 1.0 °C/W or lower, paired with high-quality thermal interface material.
This guide walks through the exact junction-to-ambient math, interprets manufacturer derating curves, and provides a concrete decision tree to select the right hardware for your bench or control panel.
The Thermal Path Math: Sizing the SSR Heatsink
Unlike mechanical contactors, SSRs use semiconductor switches (typically back-to-back SCRs or a TRIAC). These are not perfect conductors; they exhibit a forward voltage drop ($V_f$) when conducting. For standard AC SSRs, this drop is typically 1.2V to 1.5V. That voltage drop multiplied by your load current equals pure heat dissipation ($P_D$) inside the silicon.
To size a heatsink, we use the thermal equivalent of Ohm’s Law. The goal is to keep the internal silicon junction temperature ($T_J$) below its maximum rating (usually 125°C) by managing the thermal resistance ($R_{\theta}$) across three boundaries:
- $R_{\theta JC}$ (Junction-to-Case): Internal to the SSR. Typically ~1.2 °C/W for a standard hockey-puck package.
- $R_{\theta CS}$ (Case-to-Sink): The interface between the SSR baseplate and the heatsink. Typically ~0.5 °C/W with proper thermal paste.
- $R_{\theta SA}$ (Sink-to-Ambient): The heatsink’s ability to shed heat into the air. This is the number you are solving for.
The governing equation is:
$T_J = T_A + P_D \times (R_{\theta JC} + R_{\theta CS} + R_{\theta SA})$
Worked Example: 15A Resistive Heater Load
Let’s size a heatsink for a Crydom D2425 (25A rated) switching a 15A, 240VAC heater inside a control panel.
- Power Dissipation ($P_D$): Assuming a worst-case $V_f$ of 1.5V, $P_D = 1.5V \times 15A = 22.5W$.
- Target Junction Temp ($T_J$): While the silicon survives 125°C, we will target 100°C for long-term reliability.
- Ambient Temp ($T_A$): The panel interior is 40°C (a realistic assumption for an enclosed box with other components).
Plugging in the knowns to solve for the required Sink-to-Ambient resistance ($R_{\theta SA}$):
$100 = 40 + 22.5 \times (1.2 + 0.5 + R_{\theta SA})$
$60 = 22.5 \times (1.7 + R_{\theta SA})$
$2.66 = 1.7 + R_{\theta SA}$
$R_{\theta SA} = 0.96 °C/W$
You must select a heatsink with a natural convection rating of 0.96 °C/W or lower. A standard 2-inch extruded sink (typically 2.5 °C/W) will result in a junction temperature of 156°C, instantly destroying the part.
Derating Curves and Failure Signatures
Every reputable SSR datasheet includes a derating curve. This chart plots maximum allowable load current against ambient temperature. The curve assumes you are using the manufacturer’s recommended heatsink. If you undersize the sink, the curve shifts aggressively to the left.
When an SSR exceeds its thermal limits, it rarely fails open. The most common failure signature of thermal stress is a shorted output. The internal TRIAC loses its ability to block voltage when the gate signal is removed. The load stays ON permanently, even if you remove the 3-32VDC control signal. For heating elements or motors, this is a critical safety hazard. Always use a secondary mechanical contactor or a high-speed semiconductor fuse in series with the SSR to protect against this failure mode.
According to All About Circuits, the physical symptom of impending thermal failure is often a discolored or melted plastic casing near the output terminals, or a distinct ozone/burning epoxy smell. If the SSR baseplate exceeds 85°C to the touch (measured with an IR thermometer or thermocouple on the metal base, not the plastic cover), you are operating outside the safe margin for most commercial units.
Airflow and Enclosure: What Buys You Headroom?
If the math demands a 0.96 °C/W heatsink, you have two paths: buy a massive block of aluminum, or manipulate the environment to lower the effective thermal resistance.
Forced Air Convection
Heatsink ratings are usually published for "natural convection" (still air). Adding a standard 40mm x 40mm axial fan moving 10 to 15 CFM across the fins will typically cut the $R_{\theta SA}$ value in half. A heatsink rated at 1.8 °C/W in still air will drop to ~0.9 °C/W with a 40mm fan. This allows you to use a much smaller, cheaper, and lighter extruded profile.
Enclosure Derating
The biggest mistake DIYers make is using the room temperature (e.g., 25°C) as $T_A$ when the SSR is mounted inside a sealed NEMA or IP-rated enclosure. The ambient temperature inside a sealed box is the room temperature plus the heat rise generated by the SSR and other components. A 22.5W heat source inside a small sealed plastic box can easily raise the internal ambient by 15°C to 20°C. If your shop is 25°C, your $T_A$ for the math equation is actually 45°C. Always add a 10-15°C penalty to $T_A$ for enclosed natural-convection setups, or install an exhaust fan on the enclosure.
Decision Tree: Picking Your Solid State Relay Heatsink
Use this decision matrix to terminate your design process with a specific hardware choice. This assumes a standard panel-mount AC SSR (like the Crydom Series 1 or Omron G3NA series) and a maximum ambient temperature of 40°C.
| Continuous Load Current | Mounting Environment | Required $R_{\theta SA}$ Target | Concrete Hardware Pick |
|---|---|---|---|
| < 5A | Open bench / PCB | N/A (No sink needed) | Bare SSR baseplate mounted to DIN rail or non-flammable surface. |
| 5A - 12A | Open air or ventilated panel | < 2.5 °C/W | Aavid Thermalloy 530602B02500G (or generic 2" extruded SSR sink). |
| 12A - 20A | Open air | < 1.2 °C/W | Crydom HS103DR (Heavy duty finned extrusion, ~0.6 °C/W natural). |
| 12A - 20A | Sealed Enclosure | < 1.5 °C/W (forced) | Standard 2.5 °C/W sink + 40mm 12V DC cooling fan zip-tied to fins. |
| > 20A | Any | < 0.6 °C/W | Crydom HS201DR or parallel two 25A SSRs on separate sinks to halve $P_D$ per unit. |
Never mount an SSR dry. The microscopic air gaps between the machined SSR baseplate and the extruded heatsink will add 1.0 °C/W or more to your $R_{\theta CS}$. Use a high-quality thermal compound like Arctic MX-4 or a 0.5mm thick silicone thermal pad (like Bergquist Sil-Pad). Apply a thin, uniform layer—excess paste acts as an insulator, not a conductor. Torque the M4 mounting screws to the manufacturer's spec (usually ~1.5 Nm) to ensure even pressure without cracking the internal ceramic substrate.
The Default Workbench Pick
If you are building a general-purpose AC switching module for the workbench (e.g., controlling a 1500W heater or a 1HP motor via an ESP32 or Arduino) and want to eliminate thermal math from your workflow, standardize on the Crydom D2425 paired with the Crydom HS103DR heatsink.
The HS103DR is a purpose-built, 4.1-inch long extruded aluminum profile with a thermal resistance of roughly 0.6 °C/W in natural convection. At 15A (22.5W dissipation), this combination yields a case temperature rise of only ~13.5°C above ambient. Even inside a warm enclosure on a hot summer day, the junction temperature will stay well below 80°C, keeping you far away from the derating cliff. It costs roughly $25-$30 more than a bare SSR and generic aluminum bar, but it guarantees you will never have to debug a mysterious thermal short-circuit failure at 2 AM.
