| ??? 02/23/10 11:44 Read: times |
#173461 - Thanks Andy Responding to: ???'s previous message |
Andy Neil said:
Kiran V. Sutar said:
1. For SMPS we need to put a Thermistor to limit the in-rush current. So what should be the type NTC or PTC ? Think about it - what do the initials "NTC" and "PTC" stand for; and what do they mean...? NTC : Negative Temperature Coefficient: Resistance reduces as Temp.increases. PTC : Possitive Temperature Coefficient: Resistance increases as Temp.Increases. Andy Neil said:
Kiran V. Sutar said:
2. for SCR: What is the theory of using a thermistor in such an application? The text you quoted tells you the purpose of the thermistor; Uderstanding what "NTC" and "PTC" means will show you how this is achieved. I never read anything about this, when I studied about Thermistors. I was expecting more than 1-line information about this theory. Till now, I knew that Thermistors are used for measuring the temperature(based on change in resistance). But in an SMPS Application: you are using a thermistor for limiting the in-rush current !!! and that too 1~2 Amps !!! What about the wattage of Thermistor? It is not mentioned in the datasheet (or I might have missed it) In SCR/TRIAC Application: you are using it to limit dI/dt rating. Is the dI/dt rating dependant on the Temperature? So how does a thermistor fit into this ? And can a thermistor pass a current of 25Amps@230VAC? What about the wattage of thermistor? Andy Neil said:
Kiran V. Sutar said:
3. My Full Load Current is Max.25Amps.(@ 230VAC) so can a Thermistor pass this much current That depends entirely on the specific thermistor that you use - its Datasheet will answer this question directly! Can you give me a part number or link to datasheet so I can refer to it? I have refered to some datasheets of thermistors, but they do not specify the current that can pass through them. How to select it? Andy Neil said:
Are you confusing limiting the inrush to the power supply with limiting the dI/dt of a triac? There are 2 separate things that I wanted to know: 1. for SMPS which thermistor to use ? NTC or PTC? 2. for SCR/TRIAC: Can a thermistor be used for limiting the dI/dt of SCR/Triac for 25Amps@230VAC? and what is the parameter that helps in selecting the Thermistor for this application? The fact is, I do not know the theory of using the thermistor for SCR/TRIAC or even the SMPS application. So if you can post some links about it, it will be of great help. Thank you. |
