On Thu, 31 Mar 2011 11:12:22 -0700, (Dave Platt) wrote:

Then what causes my bulk eraser to shut off when it overheats
-- then come back when it cools off?

That's a resettable *thermal* fuse, NOT a fuse designed for
overcurrent protection.

I had always thought that at least some polyfuses /were/ resettable.

Polyfuses (in the sense of the term I've always heard used) are
self-resetting, unless you hit 'em so hard that you damage them.

They aren't designed for use at mains voltages/currents, though... at
least, I haven't seen any that are.

Me neither, other thing with polyfuses is that there's approx 2:1 current
difference between trigger point and hold point. I have 16V and 30V poly-
fuses here, generally for 12V or 24V SLA battery powered gear.

The self-resetting thermal "fuses" that you'd find in a bulk eraser or
similar "not intended for a 100% duty cycle - let it cool down!"
device are (I think) of the bimetallic type, somewhat akin to what you
would find in a traditional wall thermostat.

Some are the same thing as used in clothes dryer for thermostat, 100% duty
cycle for years on end Lower temperature for thermostat and a higher
temp one for overheat safety cutout.



Grant.