[Elecraft] K3 - Metal Film Resistors for Dummy Loads

[David Gilbert]

Actually, no ... it isn't the same thing. 

In the vicinity of their rated dissipation, semiconductors (at least 
their bulk characteristics) and non-film resistors (solid carbon or 
wirewound) have a gradual exponential probability of failure with 
increasing power dissipation.  Metal film resistors have a very sharp 
(high order) exponential probability of failure with increasing power 
dissipation.  As I said before, if you stay below the rated power 
dissipation of a metal film resistor it will hold up well, but if you 
exceed that by any significant amount, even briefly, you'll turn it into 
a blown fuse that will make your rig very unhappy.  A semiconductor 
(again, talking bulk here) and a solid carbon resistor will typically 
merely degrade under the same level of abuse without catastrophic failure.

Commercially manufacturing such devices taught me a lot.

73,
Dave   AB7E



David Cutter wrote:
> Putting several in parallel on a suitable outboard heatsink is not 
> expensive and, properly rated, will cater for any load.  As with 
> semiconductors, the trick is to not exceed the "junction" 
> temperature.  As always, follow the manufacturer's recommendations 
> then add your own fiddle factor.  MIL 217 taught me a lot.
>
> David
> G3UNA