[amsat-bb] Re: FT-847 Disaster - THE PAIN!

[John Wright]

>
>
>[snip]
>
> > In the commercial world of radio's (R/T) the Tait/Motorola/etc rigs have
> > all sorts of "idiot proofing" built in....the most useful ones being
> > FAST blow fuses or circuit breakers combined with a very large (>100
> > amp) suicide diode in the main power leads to remove power really,
> > really fast in the event of a major problem.
> >
>
>Just remember, as soon as you make something "idiot-proof", nature will
>invent a better idiot...

A fuse and sacrificial diode are all very well, BUT.. Fuses take time 
to rupture, and a power transisistor always has, and always will be 
the fastest fuse on 3 legs... or 4 or whatever.
For higher power items, I prefer the Idiot-Proofing method of a big 
relay!... Car accessory type of thing, in series with the input line, 
and the coil energized with a smaller diode!.. Wrong polarity, no 
coil current, so no power to item. It also has a zero volt-drop 
advantage. Smaller items are catered for with a SERIES diode, the 
0.7V volt drop rarely has any effect on smaller items, as they tend 
to be internally regulated to lower voltages anyway.