nuckolls.bob(at)aeroelect
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 Posted: Tue Jan 05, 2010 6:58 am Post subject: Question about 16V POLYFUSER Radial Leaded Resettable PT |
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At 05:36 PM 1/4/2010, you wrote:
| Quote: | Polyfuses only reset themselves IF the cause that made them to trip disappears.
Carlos |
Sort of . . . the thing that trips a polyfuse is passage
of the minimum current to warm it beyond the trip point
while it is relatively cool. Once it trips, the reistance
goes to a high state where it's EASIER to keep it warm
because presumably, the power is still on, the protected
circuit is still drawing the minimal "keep it tripped"
current.
I think some folks have pointed out that depending on
the sophistication of the device on the protected circuit
the current drawn through the polyfuse in a "tripped"
state will be insufficient to keep it hot and in a
safe . . . low current condition. In some cases, one
could experience an kind of low frequency oscillation
where the circuit trips, the polyswitch cools and
resets and the process repeats.
There are so many interactive variables to consider
that I hope it's obvious that the polyfuse is NOT
a blanket replacement for fuses or breakers. Even
when all the protection dynamics are found acceptable,
there's still the problem of how do you mount these
things in a manner suited to the aircraft power
distribution system environment. Finally, one should
consider how they affect the legacy failure mode
effects study that calls for understanding how
the pilot becomes aware of a problem with the system
such that the plan-a/plan-b decision can be made.
I'm pretty confident that the designers of the
polyswitch never envisioned this device to be
a replacement in the classic applications for
breakers or fuses. The polyswitch offered designers
a NEW opportunity to add protection inside some device
where the use of a breaker or fuse was impractical.
In these situations, the designer of the appliance
can analyze the failure modes independently of the
all-up system (i.e. an airplane full of electro-
whizzies). The idea that we can simply
craft some sort of power distribution etched
circuit board to make a blanket replacement of
legacy power distribution systems is a whole new
ball game. As I've written many times . . . the
polyswitch WILL always keep a wire from catching
fire. Circuit protection safety is not the issue.
Polyswitches throw a wrench into the gears for
how we think about operating the system when the
unexpected and unwelcome event occurs.
Bob . . . [quote][b]
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