Re: Can a 1W resistor handle 50W for 7msec?

In article <[EMAIL PROTECTED]>,
[EMAIL PROTECTED] (Brian) writes:

>Watson A.Name - "Watt Sun, Dark Remover" <[EMAIL PROTECTED]> wrote in
>message news:<[EMAIL PROTECTED]>...
>> 
>> But what happens if the relay fails to close in 7 mSec?  OOPS!
>> 
>> Or fails to close at all?  BIG OOPS!
>
>I think the resistor becomes a fuse.

   Brian, probably NOT for a single 7 millisecond event...unless it is one
   of the less-than-quarter-Watt SMDs.  Using quarter-Watt carbon
   compositions (cost about 5 cents), it is very easy to prove or
   disprove on the bench with a few items from the junk box.

   Conventional fuses CAN be a danger in "normal" applications if
   mounted horizontally.  I've come across three situations where
   horizontally-mounted fuses rated greater than 0.5 Amperes did
   NOT fully melt-and-separate the fusible link inside.  In each case
   there was sufficient voltage across the gap to arc over and continue
   current flow.  Mounting the fuse vertically insures that a melted
   fusible link separate enough for HV B+ not to arc over.

   The first such incident involved the plate supply for an AN/TRC-1
   VHF transmitter (FM, 70 to 100 MHz, 829 in the PA) which had
   (apparently, from post-mortem by in-the-know others of more
   experience than I in 1954) a problem in its plate supply transformer.
   The front-panel mounted fuse (horizontal) did not fully seprate its
   fusible link, the short continued, the plate supply transformer
   kept heating up and eventually boiled out some of the now-melted
   potting compound.  Potting goo spread over some of the internal
   tube multiplier stages' wiring, then cooled down to encapsulate
   all that wiring.  Compound had to be melted with an iron, then
   picked out, little gob by little gob.  Some of the carbon composition
   resistors lost their color bands, those having to be repainted to
   restore the coding.  [TM had photos of the wiring, resistors did not
   have to be removed to check values]  Some gas must have been
   generated inside the transformer can since it sheared off two of
   the 8-32 mounting screws and bent the tough aluminum chassis
   slightly.  Took several weeks of one person on each shift having
   to play with goo removal.

   In 1959 in the Electronic Warfare Lab at Ramo-Wooldridge, one of
   my tasks was the measurement of Traveling Wave Tubes (S-band)
   as 5-octave output mixers.  About 2 KV (?) of supply voltage.  The
   TWTs of that time would occasionally arc over inside.  A series fuse
   was in the B+ line but encased in a plexiglass box.  On a arc-over
   the fuse literally exploded...the reason for the box full enclosure.
   Way too much voltage for that 3AG fuse type but nothing else was
   available then to protect the supply, no quick crowbar circuits or
   other things semiconductor for protection.

   The last incident was an old fuse-in-the-AC-plug that seemed to be
   popular in the 60s to 70s (but seldom used now).  Fuses were
   horizontal when the plug was inserted into a wall socket.  Friend's
   house damn near caught on fire before a mains circuit breaker kicked
   out...but not before the plastic plug had melted enough to expose
   the AC mains wires.  The fuses just didn't separate their links enough
   physically once the links melted...arc-over continued with resulting
   hotness and things.  We slightly modified a ground-fault protector
   (new product for the mid-1970s) to trip-out on over-current and that
   worked okay before we fixed the piece of test equipment that caused
   the over-current on the AC.  Those 3AG fuses were rated for 250 V
   service, should have worked.  Didn't.

   Len Anderson
   retired (from regular hours) electronic engineer person