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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@@ |
"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 |
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