How big is the box? How big is your heat sink? I'm assuming it's sealed/closed (no airflow)?
The trick with external heat sinks is that airflow or convection helps suck away heat from the heat sinks.
Assuming your box is 100% closed, install the heat sink inside and what happens is that the heat sink gets hot sucking heat out of SSR which then heats up the inside components (reducing their lifespan to some degree). The whole inside hit some max ambient temp and stays there as heat hits the sides of the box and outside air currents/convection helps keep the whole box cool, but everything inside runs hotter than if you have the heatsink outside.
It would likely be fine, but outside is always better.
Kal
Kal
Ok, I'll give it a try and see what the temps end up peaking at. It wouldn't be any harder at the stage I'm at now to move it out later. thanks, kal