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using an smt resistor as a fuse

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Tom Gardner
Guest

Thu Jan 10, 2019 2:45 pm   



On 09/01/19 21:43, Winfield Hill wrote:
Quote:
I've placed 22-ohm 0805 resistors in series with the HV supply lines in the
latest version of my easy-to-build AMP-62 high-voltage amplifier board. This
version is often made with aggressive output current-limit values, e.g., 150mA.
But it's intended that such currents last only a short time, e.g., to rapidly
charge a capacitive load (during the rapid output slewing, the 22-ohms only
drops 3.3 volts). By comparison, the original version of this board, as
described in our x-Chapter book, has a wimpy 5mA current limit. But we assume
that users of this amplifier version who need its extreme slewing capability
understand the risks, when they select the 150mA current-limiting resistor
values.

If the output were to be shorted, and draw 150mA from the supply, firstly it's
hoped the supply voltage will soon fall dramatically, due to its own say 20mA
current limit.


Could you /ensure/ that happens?


Quote:
Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 watts, or
4x its rated power, and hopefully after a while it would fail open. But if it
doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 watts,
might short. At that point the current would increase further, and 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.


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

Klaus Kragelund
Guest

Thu Jan 10, 2019 2:45 pm   



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

Tim Williams
Guest

Thu Jan 10, 2019 3:45 pm   



"John Larkin" <jjlarkin_at_highland_snip_technology.com> wrote in message
news:u18d3ep9k2k51emo173eeeii060tcr8c85_at_4ax.com...
Quote:

"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.


Sure, it pops...

How many hundreds of times was the transistor dead before it cleared?

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

Phil Hobbs
Guest

Thu Jan 10, 2019 4:45 pm   



On 1/10/19 1:47 AM, Piotr Wyderski wrote:
Quote:
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

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

George Herold
Guest

Thu Jan 10, 2019 5:45 pm   



On Wednesday, January 9, 2019 at 9:53:56 PM UTC-5, Winfield Hill wrote:
Quote:
George Herold wrote...

OK no more x-chapter teases, unless I get to proof early copies*,
or it's being published soon.

I'm happy to send a draft of the HV amplifier section to anyone
who wants to look at it. It's a good example of the advanced
material in the x-Chapters.

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.

>(We're aiming to be out this year.)
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.

Quote:


--
Thanks,
- Win



Guest

Thu Jan 10, 2019 6:45 pm   



On Thursday, January 10, 2019 at 7:42:05 AM UTC-5, whit3rd wrote:

Quote:

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...


John Larkin
Guest

Thu Jan 10, 2019 7:45 pm   



On Thu, 10 Jan 2019 10:12:04 -0500, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
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."



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

John Larkin
Guest

Thu Jan 10, 2019 7:45 pm   



On Thu, 10 Jan 2019 04:42:00 -0800 (PST), whit3rd <whit3rd_at_gmail.com>
wrote:

Quote:
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

John Larkin
Guest

Thu Jan 10, 2019 7:45 pm   



On Thu, 10 Jan 2019 07:48:02 GMT, <698839253X6D445TD_at_nospam.org>
wrote:

Quote:
John Larkin wrote

On Wed, 9 Jan 2019 18:27:30 -0600, "Tim Williams"
tiwill_at_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

Jon Elson
Guest

Thu Jan 10, 2019 9:45 pm   



On Wed, 09 Jan 2019 14:27:25 -0800, John Larkin wrote:

Quote:
On 9 Jan 2019 13:43:46 -0800, Winfield Hill <hill_at_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

Phil Hobbs
Guest

Thu Jan 10, 2019 11:45 pm   



On 1/10/19 12:57 PM, John Larkin wrote:
Quote:
On Thu, 10 Jan 2019 10:12:04 -0500, Phil Hobbs
pcdhSpamMeSenseless_at_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". Wink


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

George Herold
Guest

Fri Jan 11, 2019 12:45 am   



On Thursday, January 10, 2019 at 1:01:25 PM UTC-5, John Larkin wrote:
Quote:
On Thu, 10 Jan 2019 04:42:00 -0800 (PST), whit3rd <whit3rd_at_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.
Quote:


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


John Larkin
Guest

Fri Jan 11, 2019 12:45 am   



On Thu, 10 Jan 2019 15:18:22 -0800 (PST), George Herold
<gherold_at_teachspin.com> wrote:

Quote:
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_at_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

whit3rd
Guest

Fri Jan 11, 2019 4:45 am   



On Thursday, January 10, 2019 at 3:18:27 PM UTC-8, George Herold wrote:
Quote:
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.

George Herold
Guest

Fri Jan 11, 2019 4:45 am   



On Thursday, January 10, 2019 at 6:33:37 PM UTC-5, John Larkin wrote:
Quote:
On Thu, 10 Jan 2019 15:18:22 -0800 (PST), George Herold
gherold_at_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_at_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.
Quote:

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


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