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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 |
Make a parallell/ series combination to equal 51 ohm. Immerse in oil,
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Make a parallell/ series combination to equal 51 ohm. Immerse in oil,
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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@@ |
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....... |
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....... |
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? || || 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. |
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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 e x t m s g |
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 e x t m s 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. |
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In article , Bill Turner
writes: 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. Absolutely a good, pragmatic answer, Bill. MANY components can survive a very short power overload. Carbon Comps do. A "sacrifice" test can be done with a DC power supply, an electrolytic capacitor, and a switch. Charge up the electrolytic, then dump the charge through the resistor. It should be a trivial thing to calculate the "pulse" of the initial charge-dump through the resistor for any under- or post-grad engineering type...for a particular resistance, capacitor, and voltage. A rough time-constant of t = R x C yields the "pulse" time. Figure the "pulse" power as half the capacitor charge voltage across the resistor under test. Not precision, true, but pragmatic and quick. If it doesn't survive, well, too bad. [sacrifice] If it survives, try it several more times, noting appearance, resistance before and after each "pulse" test. ...or, everyone else can spend a lot of time at "intellectual arguments" in here without ever trying it out on the bench... :-) Len Anderson retired (from regular hours) electronic engineer person |
In article , Bill Turner
writes: 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. Absolutely a good, pragmatic answer, Bill. MANY components can survive a very short power overload. Carbon Comps do. A "sacrifice" test can be done with a DC power supply, an electrolytic capacitor, and a switch. Charge up the electrolytic, then dump the charge through the resistor. It should be a trivial thing to calculate the "pulse" of the initial charge-dump through the resistor for any under- or post-grad engineering type...for a particular resistance, capacitor, and voltage. A rough time-constant of t = R x C yields the "pulse" time. Figure the "pulse" power as half the capacitor charge voltage across the resistor under test. Not precision, true, but pragmatic and quick. If it doesn't survive, well, too bad. [sacrifice] If it survives, try it several more times, noting appearance, resistance before and after each "pulse" test. ...or, everyone else can spend a lot of time at "intellectual arguments" in here without ever trying it out on the bench... :-) Len Anderson retired (from regular hours) electronic engineer person |
Your question is improper- no mention of resistor type
I thought there were no improper questions, just improper answers: -). You can substitute stupid or whatever for improper. When a beginner asks a question (not sure Jason is a beginner) it doesn't help them much to tell it is improper. Just makes them feel bad. 73 Gary N4AST |
Your question is improper- no mention of resistor type
I thought there were no improper questions, just improper answers: -). You can substitute stupid or whatever for improper. When a beginner asks a question (not sure Jason is a beginner) it doesn't help them much to tell it is improper. Just makes them feel bad. 73 Gary N4AST |
In article ,
mentioned... 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. 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. -- @@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@@ |
In article ,
mentioned... 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. 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. -- @@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@@ |
"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 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 |
"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 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 |
In article ,
Bill Turner wrote: [...] 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. 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. -- -- forging knowledge |
In article ,
Bill Turner wrote: [...] 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. 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. -- -- forging knowledge |
Watson A.Name - "Watt Sun, Dark Remover" wrote in message om...
But what happens if the relay fails to close in 7 mSec? OOPS! Or fails to close at all? BIG OOPS! I think the resistor becomes a fuse. |
Watson A.Name - "Watt Sun, Dark Remover" wrote in message om...
But what happens if the relay fails to close in 7 mSec? OOPS! Or fails to close at all? BIG OOPS! 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 :-#)# On 24 Nov 2003 22:50:40 -0800, (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 (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." |
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) 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 (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. On 24 Nov 2003 22:50:40 -0800, (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? |
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) 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? |
In article ,
"Roger Gt" 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) 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? |
In article ,
"Roger Gt" 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) 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? |
"John Robertson" wrote in message ... 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) 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 |
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