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

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

 

_________________________________________________________

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.

 

Make a parallell/ series combination to equal 51 ohm. Immerse in oil,

 

Hi. I've always read upto 10x the rated diss is workable. 50x is
really pushing it I would think. My experience with this is limited
though - but I can tell you from experience that 1500x rated power
doesnt work so well


Regards, NT

 

But what happens if the relay fails to close in 7 mSec? OOPS!

Or fails to close at all? BIG OOPS!

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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.

Al

 

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.

 

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

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.

 

Depends on the resistor's construction. It is a rating that will be
published in the mfr's data sheets. Cement-coated resistors have a low
peak stress rating. Enamelled and welded wirewound, composition and
ceramic resistors can have high pulse power handling capabilities in a
single surge event that is characterised as being less than a half
cycle of the AC line frequency ( a common surge stress rating).

Toroids are basically thermally limited to their material construction
class. If wire doesn't fuse, the em force applied doesn't cause
mechanical damage and you don't care if volt-seconds are exceeded
(saturation), a 7mSec overwattage isn't going to hurt the part
permanently.

RL

 

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.

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Don't be ripped off by the big book dealers. Go to the URL
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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
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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.

 

Resistor pulse ratings can be difficult to obtain from manufacturers.
As stated by another poster, peak power ratings on datasheets are
often about 10x continuous, sometimes even lower. For high value
resistors, the voltage across the resistor may be the limiting factor.
Anyhow, I've destructively tested lots of low value resistors. For
some intermediate range of pulse durations, their failure point tends
to follow the famous 'I-squared-t equals K' curve, which describes the
ideal tradeoff between time and power level. At some short pulse
duration, the failure threshold becomes roughly constant. The
resistor reaches a power density that causes rapid destruction of the
conductor. For wirewound and metal oxide resistors, this power level
can be 100s of times the average power rating. For carbon composition
resistors, 50x may or may not be destructive. Carbon film and thick
film resistors have failure points that vary all over the map. This
is partly due to the way that they are trimmed: scribing or laser
cutting produces a narrow area in the resistive element, which
concentrates power in a small zone. Failure is usually due to
cracking of the element as it expands more rapidly than the substrate.
This is why pulse ratings for these type resistors are usually very
low multiples of their continuous rating. So, it depends.
Paul Mathews

 

_________________________________________________________

The only problem with do-it-yourself pulse testing, as opposed to using
the manufacturer's specs, is the manufacturer might change their
processes over time, resulting in a resistor which no longer behaves the
same way. If the mfr doesn't spec pulse ratings, they will be justified
in changing things with no notice to the users, at least justified in
their own minds. It happens.

I'll say it again: Use the manufacturer's pulse rating specs. If they
don't provide such specs, you'd be better off finding one who does.

 

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.

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Don't be ripped off by the big book dealers. Go to the URL
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Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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Kamaya Ohm Resistors. I love those guys. Check it out, 1W SMD 500W @ 7ms.

http://www.kamaya.co.jp/us/image/catalog-46.pdf

enjoy
harry

 

Also:

If the maker's specification says "2.5 times overload for 3 seconds",
don't figure that 3 times overload for 2.5 seconds is ok too.

Watch out for the voltage limits as well as power limits. With high value
resistors it is sometimes the voltage that limits you.

With SMT parts, all limits only apply if the part in mounted the way the
maker expected.

 

I think the resistor becomes a fuse.

 

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 :-#)#

(Please post followups or tech enquires to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

 

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.

 

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?