On Thursday, January 10, 2019 at 7:42:05 AM UTC-5, whit3rd wrote:> > OK, I'll say just use a fuse <https://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/466%20Series%20Drawing.pdf> > because they're available, in a variety of specifications. >Seems like a no brainer there. Right tool for the right job...etc...
using an smt resistor as a fuse
Started by ●January 9, 2019
Reply by ●January 10, 20192019-01-10
Reply by ●January 10, 20192019-01-10
On Thu, 10 Jan 2019 10:12:04 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 1/10/19 1:47 AM, Piotr Wyderski wrote: >> Tim Williams wrote: >> >>> The transistor protects the fuse, not the other way around!! >> >> If the transistor is not properly selected for fusing, one may also try >> to augment it with the Soviet LED technique: >> >> https://imgur.com/gallery/YegLM >> >> ����Best regards, Piotr > >Good thing I wasn't drinking coffee! > >Cheers > >Phil HobbsPosted at 7:12 AM. I don't understand that concept "wasn't drinking coffee." -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●January 10, 20192019-01-10
On Thu, 10 Jan 2019 04:42:00 -0800 (PST), whit3rd <whit3rd@gmail.com> wrote:>On Wednesday, January 9, 2019 at 1:44:00 PM UTC-8, Winfield Hill wrote: >> I've placed 22-ohm 0805 resistors in series with the HV supply lines ... > >> If the output were to be shorted, and draw 150mA from the supply,... the 22-ohms >> would certainly fail, stopping the high power dissipation and removing the load >> from the supply. >> >> OK, I see I'm going to get roasted for this one. > >OK, I'll say just use a fuse <https://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/466%20Series%20Drawing.pdf> >because they're available, in a variety of specifications. > >Three things the fuse has to do: carry the current, break the connection on overcurrent, >and recover from the fault afterward. A metal-film resistor will get hot enough in places to kill its >metal film, or ceramic core, or endcaps, when it fuses, and THAT can damage >the PCB underneath it. Using a non-fuse-rated component risks the heat >damage (potentially, fire) that a fuse was intended to prevent. > >It's that third part, repairing the damage and returning the gizmo to service, that >is the question: is it OK to throwaway the board on an overload (potentially, just >a stray wire) event, or would you want to repair/rebuild? And, is heat damage >limited to the resistor/fuse, or does it scorch the printed wiring? > >I've never greatly admired surface-mount fuses, but you can also get fuseholders. >I HAVE repaired boards with failed SMD fuses a few times, and found surface-mount to >be only slightly annoying (tweezer-type irons help).We've found surface-mount fuses (and surface-mount polyfuses) to be erratic. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●January 10, 20192019-01-10
On Thu, 10 Jan 2019 07:48:02 GMT, <698839253X6D445TD@nospam.org> wrote:>John Larkin wrote > >>On Wed, 9 Jan 2019 18:27:30 -0600, "Tim Williams" >><tiwill@seventransistorlabs.com> wrote: >> >>>What are you fusing? To what end? >>> >>>"High voltage" is particularly onerous because said fuse will continue >>>arcing the whole time the supply is draining down. >>> >>>The transistor protects the fuse, not the other way around!! >>> >>>Tim >> >>I've connected 0805 resistors across the AC line. Try it. >> >>They usually just go PIP. > >DC is different and sustains the arc. >I built a beautiful parts-blaster board and certain unnamed parties borrowed it and arced off a bunch of my copper. https://www.dropbox.com/s/721wuheto3ti105/Z420_C1.JPG?dl=0 (That's a 1000 amp mosfet) Looks like a small part arced on the left and the arc zippered all down the gap. https://www.dropbox.com/s/yst9rux520nyziu/plasma-etch.jpg?dl=0 https://www.dropbox.com/s/gpc169hj1ilp6xa/photo01x_exploded_resistors.jpg?dl=0 https://www.dropbox.com/s/clu7re99pitk5mb/Resistor_Failures_1.jpg?dl=0 https://www.dropbox.com/s/hdson19yrdxu15n/mezz_02.jpg?dl=0 I guess the bolt-on adapters are really a better idea. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●January 10, 20192019-01-10
On Wed, 09 Jan 2019 14:27:25 -0800, John Larkin wrote:> On 9 Jan 2019 13:43:46 -0800, Winfield Hill <hill@rowland.harvard.edu> > wrote: > >>I've placed 22-ohm 0805 resistors in series with the HV supply lines in> An 0805 will probably survive 0.5 watts for a long time. If the end caps > are heat sunk to pretty big pads or traces, it will usually survive > forever. >Yup, I used some 1210 resistors in a half bridge snubber, and the resistors ended up frying the traces on the PC board. Eventually, after a lot of cooking, the pads lifted off the substrate, the thermal conduction was lost, and only THEN, finally, did the resistors fail. I had to replace these with the next size (or two) up resistors with a 2W rating, to stop the cooking. At the worst case, I'm guessing those 1210 resistors were dissipating something awful like 6 W for many months before finally giving up the ghost. Jon
Reply by ●January 10, 20192019-01-10
On 1/10/19 12:57 PM, John Larkin wrote:> On Thu, 10 Jan 2019 10:12:04 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 1/10/19 1:47 AM, Piotr Wyderski wrote: >>> Tim Williams wrote: >>> >>>> The transistor protects the fuse, not the other way around!! >>> >>> If the transistor is not properly selected for fusing, one may also try >>> to augment it with the Soviet LED technique: >>> >>> https://imgur.com/gallery/YegLM >>> >>> Best regards, Piotr >> >> Good thing I wasn't drinking coffee! >> >> Cheers >> >> Phil Hobbs > > Posted at 7:12 AM. I don't understand that concept "wasn't drinking > coffee." > > >Google "time zones". ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●January 10, 20192019-01-10
On Thursday, January 10, 2019 at 1:01:25 PM UTC-5, John Larkin wrote:> On Thu, 10 Jan 2019 04:42:00 -0800 (PST), whit3rd <whit3rd@gmail.com> > wrote: > > >On Wednesday, January 9, 2019 at 1:44:00 PM UTC-8, Winfield Hill wrote: > >> I've placed 22-ohm 0805 resistors in series with the HV supply lines ... > > > >> If the output were to be shorted, and draw 150mA from the supply,... the 22-ohms > >> would certainly fail, stopping the high power dissipation and removing the load > >> from the supply. > >> > >> OK, I see I'm going to get roasted for this one. > > > >OK, I'll say just use a fuse <https://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/466%20Series%20Drawing.pdf> > >because they're available, in a variety of specifications. > > > >Three things the fuse has to do: carry the current, break the connection on overcurrent, > >and recover from the fault afterward. A metal-film resistor will get hot enough in places to kill its > >metal film, or ceramic core, or endcaps, when it fuses, and THAT can damage > >the PCB underneath it. Using a non-fuse-rated component risks the heat > >damage (potentially, fire) that a fuse was intended to prevent. > > > >It's that third part, repairing the damage and returning the gizmo to service, that > >is the question: is it OK to throwaway the board on an overload (potentially, just > >a stray wire) event, or would you want to repair/rebuild? And, is heat damage > >limited to the resistor/fuse, or does it scorch the printed wiring? > > > >I've never greatly admired surface-mount fuses, but you can also get fuseholders. > >I HAVE repaired boards with failed SMD fuses a few times, and found surface-mount to > >be only slightly annoying (tweezer-type irons help). > > We've found surface-mount fuses (and surface-mount polyfuses) to be > erratic.Do they fail to open? Too much of a heat sink with the pads? George H.> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●January 10, 20192019-01-10
On Thu, 10 Jan 2019 15:18:22 -0800 (PST), George Herold <gherold@teachspin.com> wrote:>On Thursday, January 10, 2019 at 1:01:25 PM UTC-5, John Larkin wrote: >> On Thu, 10 Jan 2019 04:42:00 -0800 (PST), whit3rd <whit3rd@gmail.com> >> wrote: >> >> >On Wednesday, January 9, 2019 at 1:44:00 PM UTC-8, Winfield Hill wrote: >> >> I've placed 22-ohm 0805 resistors in series with the HV supply lines ... >> > >> >> If the output were to be shorted, and draw 150mA from the supply,... the 22-ohms >> >> would certainly fail, stopping the high power dissipation and removing the load >> >> from the supply. >> >> >> >> OK, I see I'm going to get roasted for this one. >> > >> >OK, I'll say just use a fuse <https://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/466%20Series%20Drawing.pdf> >> >because they're available, in a variety of specifications. >> > >> >Three things the fuse has to do: carry the current, break the connection on overcurrent, >> >and recover from the fault afterward. A metal-film resistor will get hot enough in places to kill its >> >metal film, or ceramic core, or endcaps, when it fuses, and THAT can damage >> >the PCB underneath it. Using a non-fuse-rated component risks the heat >> >damage (potentially, fire) that a fuse was intended to prevent. >> > >> >It's that third part, repairing the damage and returning the gizmo to service, that >> >is the question: is it OK to throwaway the board on an overload (potentially, just >> >a stray wire) event, or would you want to repair/rebuild? And, is heat damage >> >limited to the resistor/fuse, or does it scorch the printed wiring? >> > >> >I've never greatly admired surface-mount fuses, but you can also get fuseholders. >> >I HAVE repaired boards with failed SMD fuses a few times, and found surface-mount to >> >be only slightly annoying (tweezer-type irons help). >> >> We've found surface-mount fuses (and surface-mount polyfuses) to be >> erratic. >Do they fail to open? Too much of a heat sink with the pads? >George H.Some Littlefuse 1206 fuses failed long-term, running below their trip currents. Surface-mount polyfuses have wildly varying actual trip points and can be damaged by overloads. Probably depends on PCB layout for cooling. We use radial leaded polyfuses, which seem to be OK. I use fuses to basically prevent blowing traces off boards, which means that I size them for that, not really to limit operating currents. Some people here like the TI eFuses, but I don't. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●January 10, 20192019-01-10
On Thursday, January 10, 2019 at 3:18:27 PM UTC-8, George Herold wrote:> On Thursday, January 10, 2019 at 1:01:25 PM UTC-5, John Larkin wrote:> > We've found surface-mount fuses (and surface-mount polyfuses) to be > > erratic. > Do they fail to open? Too much of a heat sink with the pads?Polyfuses were a BIG improvement in the 1990s on SCSI busses. SCSI termination resistors required one or more of the attached devices to supply bus power, up to 800mA or so, with two terminators; but, there was no obvious way of COUNTING terminators-some users had up to four or five terminators installed... When the last lonely fuse in the SCSI chain that was intact went open, EVERY device had to be checked, repaired, and counseling on wise apportionment of termination applied. Polyfuses for surface mount were likely, alas, to fail mechaically (they were large and less flexible than the PCBs), but we didn't see any open polyfuses that came off in one piece.
Reply by ●January 10, 20192019-01-10
On Thursday, January 10, 2019 at 6:33:37 PM UTC-5, John Larkin wrote:> On Thu, 10 Jan 2019 15:18:22 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > > >On Thursday, January 10, 2019 at 1:01:25 PM UTC-5, John Larkin wrote: > >> On Thu, 10 Jan 2019 04:42:00 -0800 (PST), whit3rd <whit3rd@gmail.com> > >> wrote: > >> > >> >On Wednesday, January 9, 2019 at 1:44:00 PM UTC-8, Winfield Hill wrote: > >> >> I've placed 22-ohm 0805 resistors in series with the HV supply lines ... > >> > > >> >> If the output were to be shorted, and draw 150mA from the supply,... the 22-ohms > >> >> would certainly fail, stopping the high power dissipation and removing the load > >> >> from the supply. > >> >> > >> >> OK, I see I'm going to get roasted for this one. > >> > > >> >OK, I'll say just use a fuse <https://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/466%20Series%20Drawing.pdf> > >> >because they're available, in a variety of specifications. > >> > > >> >Three things the fuse has to do: carry the current, break the connection on overcurrent, > >> >and recover from the fault afterward. A metal-film resistor will get hot enough in places to kill its > >> >metal film, or ceramic core, or endcaps, when it fuses, and THAT can damage > >> >the PCB underneath it. Using a non-fuse-rated component risks the heat > >> >damage (potentially, fire) that a fuse was intended to prevent. > >> > > >> >It's that third part, repairing the damage and returning the gizmo to service, that > >> >is the question: is it OK to throwaway the board on an overload (potentially, just > >> >a stray wire) event, or would you want to repair/rebuild? And, is heat damage > >> >limited to the resistor/fuse, or does it scorch the printed wiring? > >> > > >> >I've never greatly admired surface-mount fuses, but you can also get fuseholders. > >> >I HAVE repaired boards with failed SMD fuses a few times, and found surface-mount to > >> >be only slightly annoying (tweezer-type irons help). > >> > >> We've found surface-mount fuses (and surface-mount polyfuses) to be > >> erratic. > >Do they fail to open? Too much of a heat sink with the pads? > >George H. > > Some Littlefuse 1206 fuses failed long-term, running below their trip > currents. > > Surface-mount polyfuses have wildly varying actual trip points and can > be damaged by overloads. Probably depends on PCB layout for cooling.I've only used the Littlefuse leaded poly fuses, but one could imagine a surface mount would be different tacked right to ground or a wide power trace, versus through a narrow 'thermal' trace. I looked on a few spec sheets for mounting or pad layouts, but found nothing. (Besides the obvious stuff.) George H.> > We use radial leaded polyfuses, which seem to be OK. > > I use fuses to basically prevent blowing traces off boards, which > means that I size them for that, not really to limit operating > currents. > > Some people here like the TI eFuses, but I don't. > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
