Ed
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If this "polyfuse" being discussed is what I know as a positive
tempco thermistor (PTC) current limiter, it could be either
detrimental or ideal for the application - it all depends on the
resistance vs. temperature characteristic. PTCs for protection are
commonly used as "resettable" fuses, but they are not quite a drop-in
replacement. They rely on ambient temperature plus rise from
self-heating to reach the curie point, so will have higher cold
resistance than a comparably-rated one-time fuse. Also, they never
truly "open," but can reach a fairly high resistance. For thermal
protection only, a relatively high current PTC with the right
tripping temperature will have low resistance at normal temperature,
but still zoom up in resistance beyond a certain temperature, even
with small current. If the "proper" fuse rating is applied, or
tripping temperature is too low, the PTC may have too much series
resistance at the normal operating temperature, and affect the
voltage regulation or operation of the circuit it protects. There are
many different PTC types and temperature characteristics available
for various applications, but for this one I wouldn't worry about
over-current protection, which can be done externally and as
accurately as necessary, and just optimize for the over-temperature
aspect, which must be inside the oven.
