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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." |
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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