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Selecting a NTC thermistor inrush limiter

The NTC is a good idea when a ramping is needed.

Pablodeux said:
One benefit of installing a thermistor in a 50/60 year old tube amp is that even after its warmed up, it still provides a drop in line voltage of a couple/three volts.
My line voltage is normally 120+/- 1V. This keeps me close to the design center of 117V that most of this equipment was designed around. The higher B+ can be stressing the ancient insulation inside transformers and aging capacitor dielectrics if the thermistor is bypassed after it's warmed up.
I've seen where some folks are putting two thermistors in series to cope with higher line voltages.
Click to expand...
I did that with two CL-90 in my Dyna Stereo 70 at the fuse holder.
20210303_090314.jpg
 
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Hi-Q - that Yaquin mod doc shows a poor circuit location for fusing the B+ feeds - the fuses really really should be in the AC secondary winding part of the circuit. Also in many countries the use of a mains side fuse in the neutral feed would be banned, and that is even more apparent when mods like the NTC's are fitted to the AC mains side circuitry.
 
There is a proper way to calculate them. I prefer to select on that in normal state use between 30% and 50% of their maximum current rating. 30% is the minimum to work properly and the closer you get to 100% the more you'll be shortening its life.

Attached some screendumps of how to calculate - I cannot remember where I found them, and some specifications of common NTC thermistors.

The calculation may have come from Ametherm before they put up their calculators: https://www.ametherm.com/inrush-current/calculators
 

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I have a 300b SET amp that has a CL-70 Thermistor that had a crack in it. Some times when I would turn the amp on, I would get a arc. Today I received my order from Digi-Key, put in the new Thermistor, all is good. I know they get hot, but I didn’t think the ceramic would crack...live and learn.
 
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