[time-nuts] HP10811-60111 oven failure + repair

ed breya eb at telight.com
Fri Oct 21 14:28:20 EDT 2011

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.


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