I want to replace an ICL (inrush current limiting) NTC thermistor in a vintage computer PSU (for Amiga 1200). This question is similar to 'Selecting an NTC for inrush current limiting', except that question was related to a triac control circuit and so the answer was specific to that case.
Taken from a working PSU, the ICL NTC marking reads "KCC 30 OHM", which I understand means it should measure 30 Ω at 25 °C.
What I noticed while trying to find a replacement is that there are different current ratings for the same resistance. For example, using Farnell, when I filter the THC ICLs on 30 Ω, I get two in stock; 1.5 A (10 mm; 20% larger than my part) and 5 A (22 mm; about 2x the diameter of mine). The fuse is 1 A fast blow (F1AH250V), so should the THT ICL current rating be less than or more than 1 A? Does the replacement have to measure 30 Ω or can a different resistance be used to limit inrush current?
I noticed the question, 'Thermistor replacement', but that seems more relevant to finding a rare resistance rating rather than deciding on the current rating.
In case you're wondering: the ICL NTC was damaged due to using a battleshort instead of a fuse (yes, bad idea, I know), and the fault in the circuit was 3 of 4 shorted diodes in the full bridge rectifier.
Edit: The measurements for the part are...
- Diameter: 0.31 in / 8 mm
- Thickness: 0.11 in / 2.8 mm
One of the 30 Ω replacement parts is 1.5 A which measures 10 mm diameter, so perhaps I can estimate that the original part is rated around 1.2 A? (i.e. 80% of the 10 mm part). Does it work that way? i.e. Can the diameter of an ICL NTC indicate it's current rating? I noticed that 8 mm parts are rated at 10 Ω and 2 to 4 A.