RadioBanter - View Single Post

November 27th 03, 10:16 PM

In article ,
(Brian) writes:

Watson A.Name - "Watt Sun, Dark Remover" wrote in
message om...

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