using an smt resistor as a fuse

Seems like a no brainer there. Right tool for the right job...etc...

Reply to
blocher
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Posted at 7:12 AM. I don't understand that concept "wasn't drinking coffee."

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

We've found surface-mount fuses (and surface-mount polyfuses) to be erratic.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

I built a beautiful parts-blaster board and certain unnamed parties borrowed it and arced off a bunch of my copper.

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(That's a 1000 amp mosfet)

Looks like a small part arced on the left and the arc zippered all down the gap.

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I guess the bolt-on adapters are really a better idea.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

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 to
Jon Elson

Google "time zones". ;)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

Do they fail to open? Too much of a heat sink with the pads? George H.

Reply to
George Herold

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.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

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 to
whit3rd

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.

Reply to
George Herold

It tends to be the other way round with SCRs though. :-)

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Johnny B Good
Reply to
Johnny B Good

Polyzens are very good for overvoltage. We current-limit elsewhere to protect against short circuits. (RCs in the supplies of the output amps, often.)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Transparent bolt! (The light from the junction is escaping.)

Reply to
Bill Beaty

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