I'm about to replace a relay with a discrete triac design. The load is a (resistive) heater element and power 1200W @ 240V (RMS current ~5A). I will use zero crossing detection and max switching frequency of 1Hz. Environment temperature < 50C. The triac is fired from an mcu via optocoupler.
I'm looking for advices to create a robust solution from a practical point of view and in particular from field experience.
What rating should the triac have for V and A.
Is it possible/advisable to avoid a heat sink?
Voltage surges are common in the region, how will that effect component selection?
Can a (fast) fuse be effective in protecting the triac from short circuit and power surge? If not, are there known working solutions for circuit protection.
As high as you can afford. 25A or 40A would be good. No triac you can afford will be as rugged (or efficient) as a relay contact.
Not a hope. Dissipation will be in the 5W range- read the datasheets and do some calculations. Do not run the triac junction too hot. It won't last long and you need the delta between operating temperature and absolute maximum to absorb the temperature surge from a short or other abuse.
Use at least a 600V triac, preferably 800V, and consider an MOV.
Yes, a fast fuse can protect a generously overrated triac from a short circuit. The more demands you place on the fuse, the more expensive and hard-to-find it becomes, and the more likely your customer will replace it with a penny or whatever.
Keep in mind that triacs are much more likely to fail 'on' than properly rated relays or contactors are.