"John Robertson" wrote in message
...
Not an expert on instantaneous surges in resistors (you could ask my
daughter - she is in third year physics ;-) but I would suggest that
you consider using Flame-Proof resistors in this application. these
are resistors that open internally and do not burn up your board
when
they fail...sometimes known as a fuse resistor....

John :-#)#

On 24 Nov 2003 22:50:40 -0800, (Jason Hsu)
wrote:

It's part of a design for a T/R sense circuit for a noise
cancellation
device.

I won't bore you with too many details. One thing I noticed in a
design I'm looking at is that 1W resistors could be subject to as
much
as 50W of power apiece during the time it takes for a relay to
respond. This response time is 7msec. The overall duty cycle will
be
low (well under 1%).



Can the 1W-51 ohm resistors handle this 50 RF volts 0-peak (about
50W
PEP) for .007 sec? 50W over .007 seconds is .35 Joules. .35W for
1
second is also .35 Joules, which a 1W resistor should have no
trouble
handling. Can the resistors be damaged during that .007 seconds?

No, it will not be damaged.

Also, how much time does it take to damage a toroid? If it can
handle
X units of flux density continuously, how much flux density can it
handle for .007 seconds with a low overall duty cycle (like well
under
1%)?

This is an entirely different situation. The maximum flux density is
fixed by the core material, and cannot be exceeded, no matter how much
current you feed through the coil.

Jason Hsu, AG4DG
usenet AAAAATTTTT jasonhsu.com

Roger Gt wrote:

Check for wire wound resistors, they are most likely to be capable of a high
pulse of current.

Can the 1W-51 ohm resistors handle this 50 RF volts 0-peak (about 50W
PEP) for .007 sec?

I strongly suspect that a wire-wound resistor is totally
unsuitable for this RF application.

Dana

Roger Gt wrote:

Check for wire wound resistors, they are most likely to be capable of a high
pulse of current.

Can the 1W-51 ohm resistors handle this 50 RF volts 0-peak (about 50W
PEP) for .007 sec?

I strongly suspect that a wire-wound resistor is totally
unsuitable for this RF application.

Dana

Hi, good point. It should be Non Inductive winding of course. It has been
done!
I have two dummy loads I made this way. But they are Ni-Chrome in oil!

"Dana Myers" wrote in message
...
Roger Gt wrote:

Check for wire wound resistors, they are most likely to be capable of a
high
pulse of current.

Can the 1W-51 ohm resistors handle this 50 RF volts 0-peak (about 50W
PEP) for .007 sec?

I suspect that a wire-wound resistor is
unsuitable for this RF application.
Dana

Hi, good point. It should be Non Inductive winding of course. It has been
done!
I have two dummy loads I made this way. But they are Ni-Chrome in oil!

"Dana Myers" wrote in message
...
Roger Gt wrote:

Check for wire wound resistors, they are most likely to be capable of a
high
pulse of current.

Can the 1W-51 ohm resistors handle this 50 RF volts 0-peak (about 50W
PEP) for .007 sec?

I suspect that a wire-wound resistor is
unsuitable for this RF application.
Dana

Find out the specs on the candidate resistor(s). Resistors have a maxium
current and voltage rating. Even for short durations, exceeding the maximum
voltage could pose a problem for you. I always opt for conservative ratings...

Find out the specs on the candidate resistor(s). Resistors have a maxium
current and voltage rating. Even for short durations, exceeding the maximum
voltage could pose a problem for you. I always opt for conservative ratings...

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

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

Anyway, we had problems with the 7805 regulators shutting down from
overtemp. They were mounted on the PCB with a small heatsink.

Hi all,
overheating should not be neglected. In my professional life I
encountered a device made by a famous company, which had a 7805 on a
heatsink, a processor and several customer chips inside a tightly
closed plastic box. All was O.K. at power-up. I was called for service
after two days. The device was hot and really did not work. I let it
cool down and all was O.K. again - for next two days. I was forced to
remove the top cover of the box forever. The device has been working
for several years now.
Proper cooling seems to be a must.

BR from Ivan OK1SIP