Mook Johnson wrote:
Phil Allison wrote:

The same amp has a novel feature not seen by me before. In faulty condition, it drew about 10mA of AC current while having no DC rails and there was a faint smell of something hot.

Turns out, it has a soft start circuit consisting of a relay and a PTC in parallel. No, not an NTC - a *PTC* !!

A coin sized device made by "Epcos" that has 6 ohms cold resistance and jumps to circa 20kohms if overheated - and it can tolerate 240VAC.

So, if the DC supply does not come up in the first 100mS, the PTC goes high and that is that. Brilliant.



I believe some loudspeakers use that trick in tweeter and midrange
crossovers. They called them "polyswitch" to protect them from too much
continuous power.

** Been using Polyswitches for decades.

They are not suitable for 240VAC use and have far too low cold resistance values like 0.1 ohms.

The Epcos PTC has 6ohms cold resistance and can be connected directly across the AC supply with NO harm despite dissipating 10kW for a few milliseconds.


..... Phil