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?
||
|| 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

I have here some datasheets of Beyschlag MELF-resistors (CMA0204). They take
up to 40W continuous pulses if the pulse length is 200us or shorter. So 2-3
of those should be able to absorb your pulse. They also have non-inductive
types for RF-apps.
--
ciao Ban

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?

Your question is improper- no mention of resistor type - almost any
resistor type can take the limited overload if you up the steady state
power rating. When in doubt go with bulk ceramic- these products are
well characterized for pulsed overload operation and used in RF apps.
http://www.globar.com/ec/resistor.php.html


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 will be easier because the heating time constant is longer and RF
is reflected away when it saturates- you will have to measure this
yourself- it is unlikely any manufacturer has done it.

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?

Your question is improper- no mention of resistor type - almost any
resistor type can take the limited overload if you up the steady state
power rating. When in doubt go with bulk ceramic- these products are
well characterized for pulsed overload operation and used in RF apps.
http://www.globar.com/ec/resistor.php.html


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 will be easier because the heating time constant is longer and RF
is reflected away when it saturates- you will have to measure this
yourself- it is unlikely any manufacturer has done it.

Al wrote:

In article ,
Bill Turner wrote:


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


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?

________________________________________________ _________

You really need to ask the manufacturer of the resistor. They are well
aware of the problem - if you get to the right person.

Having said that, here is a generalization: If the resistor's element
is a solid block of material, such as in a carbon composition type, it
will have very good pulse power ratings. On the other hand, if the
element is a film, it may develop tiny hot spots during pulsing and
eventually fail.



Many years ago I analyzed a problem with resistors which were
discoloring in the field. This involved two identical circuits with
identical singal inputs. In one circuit, a resistor kept turning brown
over time, in the other it did not. We were getting field returns as a
result of troubleshooting that focused on the brown resistors.

What was the difference? The resistor which did not discolor was resting
on two metal traces which ran under it, the one which discolored, was
resting on the PCB. That tiny amount of heatsinking provided by the
traces was the difference. Obviously, the circuit needed to be
redesigned, but it worked great on the breadboard when it went into
production.

The resistor should never have been run at that close to its maximum
dissipation to begin with. That's a lack of what they call a
conservative design.

Al



--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/e...s/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 at hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@
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Al wrote:

In article ,
Bill Turner wrote:


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


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?

________________________________________________ _________

You really need to ask the manufacturer of the resistor. They are well
aware of the problem - if you get to the right person.

Having said that, here is a generalization: If the resistor's element
is a solid block of material, such as in a carbon composition type, it
will have very good pulse power ratings. On the other hand, if the
element is a film, it may develop tiny hot spots during pulsing and
eventually fail.



Many years ago I analyzed a problem with resistors which were
discoloring in the field. This involved two identical circuits with
identical singal inputs. In one circuit, a resistor kept turning brown
over time, in the other it did not. We were getting field returns as a
result of troubleshooting that focused on the brown resistors.

What was the difference? The resistor which did not discolor was resting
on two metal traces which ran under it, the one which discolored, was
resting on the PCB. That tiny amount of heatsinking provided by the
traces was the difference. Obviously, the circuit needed to be
redesigned, but it worked great on the breadboard when it went into
production.

The resistor should never have been run at that close to its maximum
dissipation to begin with. That's a lack of what they call a
conservative design.

Al



--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/e...s/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 at hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@
F
o
d
d
e
r

f
o
r

s
t
u
p
i
d

n
o
t

e
n
o
u
g
h

i
n
c
l
u
d
e
d

t
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x
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m
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g

Watson A.Name \"Watt Sun - the Dark Remover\" wrote:



The resistor should never have been run at that close to its maximum
dissipation to begin with. That's a lack of what they call a
conservative design.

That's exactly right-and this story about "it worked right during
production testing" points up the fact that the price you pay for
misapplying a component is extended in-house testing designed at a
higher skill level than the usual application engineer possesses.

Watson A.Name \"Watt Sun - the Dark Remover\" wrote:



The resistor should never have been run at that close to its maximum
dissipation to begin with. That's a lack of what they call a
conservative design.

That's exactly right-and this story about "it worked right during
production testing" points up the fact that the price you pay for
misapplying a component is extended in-house testing designed at a
higher skill level than the usual application engineer possesses.