[Elecraft] K2 Failure

David Woolley forums at david-woolley.me.uk
Sat Dec 5 05:50:16 EST 2015


I'd agree that the voltage reading do not implicate F1.

However I would also question whether F1 would produce a popping sound, 
although it might through thermal effects, or by affecting the power to 
the audio chain.

F1 isn't a circuit breaker; it is effectively a rather aggressive 
positive thermal coefficient (PTC) thermistor.  When the current gets 
too high, the temperature goes up, this causes the resistance to 
increase, which increases the heat generation for the same current, and 
you have a thermal runaway.  The resistance goes up so fast that it 
limits the current in the device severely before the device gets hot 
enough to self destruct (although its on resistance is permanently 
raised), and the actual current needed to keep it hot is quite low.

Once the current demand drops below the triggered state current, it will 
begin to cool, and at some point, as long as the current remains low, it 
will cool faster than it is being heated and the resistance will drop to 
the point where increasing the current won't cause an immediate runaway.

In the triggered state it reaches an equilibrium, letting through enough 
current to keep it in the high resistance state.  It's not like a 
bi-metal circuit breaker that has no self heating once triggered and 
will eventually reset given just time.

On the other hand, I imagine there are some load conditions in which a 
polyfuse would go into a slow oscillation.

https://en.wikipedia.org/wiki/Resettable_fuse

-- 
David Woolley
Owner K2 06123

On 05/12/15 04:36, Matt Maguire wrote:
>
> Just a comment on your observations with F1 — if you are applying power and measure 0V on both sides of F1, then that suggests that F1 is not open circuit. It may indicate that diode D10 has gone high resistance and is dropping the 13.8V when the radio tries to draw current.
>
> On 5 décembre 2015 at 2:56:47 PM, Dauer, Edward (edauer at law.du.edu) wrote:

>
> Anyone know if F1 has a reset time of just under a second? If so, then
> the “popping” noise may be F1 trying to reset itself. Which doesn’t help
> locate the downstream problem, of course. But it would strongly suggest a
> DC short somewhere rather than some configuration glitch, yes?




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