# using an smt resistor as a fuse - Page 2

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Re: using an smt resistor as a fuse
On Wednesday, January 9, 2019 at 9:53:56 PM UTC-5, Winfield Hill wrote:

OK, I'd be happy to read it.  (gherold-at-teachspin.com)  The technical
book i'm reading now is Feynman's gravity text... kinda hard slogging,
so something a little lighter would be welcome.

Oh dear, well now you've done it, don't you know that predicting how long
something will take immediately causes God to multiply that time by pi. :^)

George H.

Re: using an smt resistor as a fuse
wrote:

We usually use the output resistors to limit the load current eg
I would put current protection on the MOSFET to protect the MOSFETs.
Why let the driver burn up if the resistor fails closed, which there
is good chance it arc's over and turns into a carbon blob.

But thats just me.

Cheers

Re: using an smt resistor as a fuse
On Wed, 09 Jan 2019 21:36:58 -0500, Martin Riddle

I like to compute fet power dissipation, rather than current limiting.
You can safely push a fet a lot harder that way.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

Re: using an smt resistor as a fuse
On Wednesday, January 9, 2019 at 1:44:00 PM UTC-8, Winfield Hill 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.

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).

Re: using an smt resistor as a fuse
If the fuse or fuse able resistor is connected to the mains it needs UL approval

It will then be listed as a Critical Component

Cheers

Klaus

Re: using an smt resistor as a fuse
On Thursday, January 10, 2019 at 7:42:05 AM UTC-5, whit3rd wrote:

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

Re: using an smt resistor as a fuse
wrote:

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

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

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Re: using an smt resistor as a fuse
On Thursday, January 10, 2019 at 1:01:25 PM UTC-5, John Larkin wrote:

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

Re: using an smt resistor as a fuse
On Thu, 10 Jan 2019 15:18:22 -0800 (PST), 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.

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

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Re: using an smt resistor as a fuse
On Thursday, January 10, 2019 at 6:33:37 PM UTC-5, John Larkin wrote:

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.

Re: using an smt resistor as a fuse
On Thursday, January 10, 2019 at 3:18:27 PM UTC-8, George Herold wrote:

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.

Re: using an smt resistor as a fuse
On 1/10/19 1:01 PM, John Larkin wrote:

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

--
Dr Philip C D Hobbs
Principal Consultant
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Re: using an smt resistor as a fuse
On 09/01/19 21:43, Winfield Hill wrote:

Could you /ensure/ that happens?

What's the *benefit* of not using a "proper" fuse or equivalent?