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

[Ralph & Diane Barone]

In article <Z3gxb.1274$rQ2.106@newssvr27.news.prodigy.com>,
"Roger Gt" <Xenot@pacbell.net> wrote:

Check for wire wound resistors, they are most likely to be capable of a high
pulse of current. But check for the manufacturers specification on what
they have been tested to tolerate.
There used to be several sources of these, but I haven't used any for years.
Of course a insulated bobbin with copper or iron wire could be used, if size
isn't critical. Wind it as a single or double layer to get the heat out.

On 24 Nov 2003 22:50:40 -0800, jason_hsu@my-deja.com (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?

http://www.globar.com/ec/resistor.php.html shows some bulk ceramic
resistors. One of their data sheets shows their 1.5 W resistor having a
peak energy rating of 75 Joules. The key here is having a lot of physical
mass directly in the current path (something that metal film resistors
don't do well).

PS: What happens when your relay fails?

 

[normanstrong]

"John Robertson" <spam@flippers.com> wrote in message
news:711bsv8ohlo696802lkon2cvr4ur67bts3@4ax.com...

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@my-deja.com (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

 

[Dana Myers]

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]

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" <k6jq@arrl.net> wrote in message
news:3FC643D6.4000908@arrl.net...

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

 

[OK1SIP]

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

 

[Roger Conroy]

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" <jason_hsu@my-deja.com> wrote in message
news:f7d9a152.0311242250.6353d9c@posting.google.com...

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

 

[Bill Turner]

On Fri, 28 Nov 2003 10:04:12 +0200, "Roger Conroy"
<rogerconroy@nospam.hotmail.com> wrote:

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.

_________________________________________________________

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.

--
Bill, W6WRT

 

[Uns Lider]

On 2003-11-26, Watson A.Name - Watt Sun, Dark Remover <alondra101@hotmail.com> 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