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

The amount of time and energy expended on discussing this question by far
exceeds the cost and effort of just going ahead and frying a few of them in
a "hands-on", "live fire" destuctive test.

73
Roger
ZR3RC

"Jason Hsu" wrote in message
m...
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?

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%)?

Jason Hsu, AG4DG
usenet AAAAATTTTT jasonhsu.com

The amount of time and energy expended on discussing this question by far
exceeds the cost and effort of just going ahead and frying a few of them in
a "hands-on", "live fire" destuctive test.

73
Roger
ZR3RC

"Jason Hsu" wrote in message
m...
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?

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%)?

Jason Hsu, AG4DG
usenet AAAAATTTTT jasonhsu.com

While the above statement is true in a literal sense, the time spent
discussing the issue is time well spent. Woe to any designer who learns
his craft by "live fire" testing.

Too True.

I've been a hardware/software designer for many years, don't rely on bench
tests.

Go by the manufacture specs (unless it's a one off jobby for yourself) - least
you have something to fall back on then.

Bare in mind, if any component gets pretty hot, in time the pcb will become
charcoal in the components immediate vicinity - unless heat sinking measures are
taken.

Clive

While the above statement is true in a literal sense, the time spent
discussing the issue is time well spent. Woe to any designer who learns
his craft by "live fire" testing.

Too True.

I've been a hardware/software designer for many years, don't rely on bench
tests.

Go by the manufacture specs (unless it's a one off jobby for yourself) - least
you have something to fall back on then.

Bare in mind, if any component gets pretty hot, in time the pcb will become
charcoal in the components immediate vicinity - unless heat sinking measures are
taken.

Clive

On 2003-11-26, Watson A.Name - Watt Sun, Dark Remover wrote:
I worked for a small company that was owned by a conglomerate. The
conglomerate said that they'd never use any of our equipment that were
in a blue case. Which, since almost everything was in a blue case...

Anyway, we had problems with the 7805 regulators shutting down from
overtemp. They were mounted on the PCB with a small heatsink. So we
had to ECO them by adding a few inches of wire and bolting the 7805 to
the aluminum case, which soaked up plenty of heat. After that, no
more weird behavior.

It wasn't WG Security Products, was it? Doesn't sound like it. A while
back I got a used video quad that they had made, and the 7805 was mounted
like that. Also, the 7805 was blown.

-- uns

On 2003-11-26, Watson A.Name - Watt Sun, Dark Remover wrote:
I worked for a small company that was owned by a conglomerate. The
conglomerate said that they'd never use any of our equipment that were
in a blue case. Which, since almost everything was in a blue case...

Anyway, we had problems with the 7805 regulators shutting down from
overtemp. They were mounted on the PCB with a small heatsink. So we
had to ECO them by adding a few inches of wire and bolting the 7805 to
the aluminum case, which soaked up plenty of heat. After that, no
more weird behavior.

It wasn't WG Security Products, was it? Doesn't sound like it. A while
back I got a used video quad that they had made, and the 7805 was mounted
like that. Also, the 7805 was blown.

-- uns