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#1
November 25th 03, 06:50 AM
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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 |
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#2
November 25th 03, 10:53 AM
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Make a parallell/ series combination to equal 51 ohm. Immerse in oil,
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#3
November 25th 03, 10:53 AM
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Make a parallell/ series combination to equal 51 ohm. Immerse in oil,
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#4
November 25th 03, 11:33 AM
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#7
November 25th 03, 01:04 PM
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In article ,
mentioned... 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 But what happens if the relay fails to close in 7 mSec? OOPS! Or fails to close at all? BIG OOPS! -- @@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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#8
November 25th 03, 02:02 PM
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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. Al -- There's never enough time to do it right the first time....... |
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#9
November 25th 03, 02:02 PM
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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. Al -- There's never enough time to do it right the first time....... |
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#10
November 25th 03, 02:40 PM
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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 |
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