As I said, stay below the rated power and they should work fine. When they go, they don't go gradually, though. They really are simply fuses on a heat sink.
As far as their RF characteristics is concerned, no argument from me. 73, Dave AB7E Ian White GM3SEK wrote: > David Gilbert wrote: > >> juergen piezo wrote: >> >>> A 2nd option could also be a dummy load using one of those very cheap >>> RF film resistors. This would have to be mounted onto the case for a >>> heatsink. >>> >>> John >>> >> Metal film resistors are notoriously fragile. They will typically meet >> their published dissipation specs just fine, but they go from being OK >> to being an open in a flash (literally) when overstressed. I personally >> would never use one in any application where the power they were >> expected to handle wasn't rigid controlled. Check with any industrial >> control manufacturer and you will hear the same story. >> >> 73, >> Dave AB7E >> > > This isn't an industrial control application where huge surges may be > encountered. > > TO220 film resistors make excellent RF dummy loads, up to at least > 50MHz. Many of these resistors are also used in RF power attenuators and > as "passive grid" loads in tetrode amplifiers. Power handling depends > on the resistor rating, but is limited mostly by the performance of the > heatsink (which is mandatory, because the power dissipation of the bare > T220 package is only a few watts). > > A 50-ohm dummy load will normally use two 100-ohm resistors in parallel. > VSWR at higher frequencies is limited mostly by layout and construction > technique. For example, see > http://tinyurl.com/inpractice/#0908 > > Turning to conventional wire-ended metal film resistors, again my > experience is entirely the opposite. I have deliberately overloaded > samples until they were glowing bright red and the paint has completely > burned off... and still they did NOT fail. Furthermore, when they cooled > down their resistance was still within a few percent of the correct > value. > > Also, it's largely a myth that tubular wire-ended metal film resistors > are "too inductive for RF". If you calculate or measure the inductance, > you'll find it's generally no more than a few tens of nanohenries, and > the effective inductance can be reduced even more by paralleling several > higher-value resistors. With care, they are even usable at 144MHz. > > > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
