I have a SMPS circuit. There is a serie connected NTC at Neutral input. I know NTC is used for inrush current limiting purposes. But there is a 20ohm 2W resistor which is parallel this NTC and I don't know what is the aim for it?
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1\$\begingroup\$ Your photos are very good. The problem is also very well presented. I think this is a 5 star question. \$\endgroup\$– tlfong01Commented Jan 5, 2021 at 1:10
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This may be done to get a specific value of the NTC temperature co-efficient. Not all values of resistances are available as standard, and if you need a really specific value of resistance it is far cheaper to change its resistance by adding a resistor in series or parallel than buy a specialized NTC resistor
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3\$\begingroup\$ I am a NTC newbie, and the first thing I found reading the catalogs is that the range of NTCs to choose is bit wide. eg, the current rating jumps to 5A, then to 10A, 25A etc. I know it is not very critical to be so choosy, but I never thought of using other components to "modify" the spec. Of course I think this trick applies to not just NTC, but all components in general. Your 5 star engineering design answer explains why me, the old technician, have never been promoted to engineer. But looking back, I have no regrets, because I enjoy technician work, and hate enggr's management duties. \$\endgroup\$– tlfong01Commented Jan 5, 2021 at 1:20
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2\$\begingroup\$ @tlfong01 We engineers certainly stand on the shoulders of giants like you. The NTC trick is quite niche, and the only reason I know of it because I was asked to design a project that had a similar design requirement, and had a tight budget. Very glad that people found my insight useful! \$\endgroup\$– AdiCommented Jan 5, 2021 at 14:28