I am building a small TRIAC valve for ON/OFF control of a 300W heater. The current design problem is as follows. When the heater is cold, it's resistance is low (15Ω @ 220VAC) and the initial current is large.
Is an NTC the right way to tackle this problem? If so, what guidelines should I observe in selecting one? If not, what would be a better idea?
Best Answer
Possibly; this page from Ametherm has a lot of description and guidance on how to select one. However, the cases they describe are mostly power supplies, where the inrush current is into a capacitor, and the NTC heats up and passes full current pretty quickly.
Given that you're building a triac control circuit, I suggest looking at "triac dimmer" circuits and using one of those instead. You can then control current either by "dead reckoning" (gradually increase power over a time period that gives the desired result) or by measuring current flow (eg Hall sensor or current transformer).
You also need to consider what happens if the heater fails short-circuit. The triac needs to survive until the fuse blows or circuit breaker trips.
It may be simplest to just overbuild the triac to handle the inrush current.