| ??? 09/20/10 23:38 Read: times |
#178614 - Soft start, again... Responding to: ???'s previous message |
Erik said:
use an ohmmeter to measure the resistance of a cold heating element and compare that to the rating (w). That will tell you the ratio of resistance cold/hot. Exactly! As a very simple soft start you can turn on the heater using a pre-resistor or NTC for limiting of inrush current, which is later, after the preheating period, shorted by the main triac. Finally, when the end temperature is reached you can turn on and turn off the heater during so and so many mains periods. In many cases a phase angle control of heater isn't actually needed then, especially if the thermal inertia of heater is big. Controls using a current limiting element for the preheating period are extremely robust, because the triac and the heater have only to withstand a much lower inrush current. I think here of occurrences when the zero crossing detector is running ill for some reason... Kai Klaas |
