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using an smt resistor as a fuse

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Winfield Hill
Guest

Wed Jan 09, 2019 10:45 pm   



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. 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.


--
Thanks,
- Win

John Larkin
Guest

Wed Jan 09, 2019 11:45 pm   



On 9 Jan 2019 13:43:46 -0800, Winfield Hill <hill_at_rowland.harvard.edu>
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. 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.


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.

Your resistor will act like a fuse, at (guessing) several watts or so.

(Didn't somebody do some tests like that recently?)

Given good end cap heat sinking, most resistors (from 0603 to 1206)
will have about the same central hot-spot temperature at a given
power.

It's a time race between the 20 mA supply current limit (plus any
capacitive energy storage) and the SOAR of the fet. The dominant
thermal time constant of a fet is usually in the 100 ms ballpark.


Here's a 1206:

https://www.dropbox.com/s/rmgio9oagh88e90/DSC02057.JPG?dl=0

https://www.dropbox.com/s/viez4tcl6zch6in/DSC02064.JPG?dl=0

https://www.dropbox.com/s/qms6olt3zh1b5vz/DSC02053.JPG?dl=0

https://www.dropbox.com/s/sv56ee1d1p9oksh/DSC02062.JPG?dl=0


At 30 volts, 18 watts, it lasts about 60 ms with no conductive
cooling, and about 300 ms soldered to huge copper pours.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

Lasse Langwadt Christense
Guest

Wed Jan 09, 2019 11:45 pm   



onsdag den 9. januar 2019 kl. 22.44.00 UTC+1 skrev Winfield Hill:
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. 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.


https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773216%7FD%7Fpdf%7FEnglish%7FENG_DS_1773216_D.pdf%7F1879229-4

0805 fusible resistor fusing power 2.5W !

Tim Williams
Guest

Thu Jan 10, 2019 1:45 am   



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

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

"Winfield Hill" <hill_at_rowland.harvard.edu> wrote in message
news:q15pui0gsl_at_drn.newsguy.com...
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. 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.


--
Thanks,
- Win


George Herold
Guest

Thu Jan 10, 2019 2:45 am   



On Wednesday, January 9, 2019 at 4:44:00 PM UTC-5, 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.


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

Hmm well my first idea would be to try and fry some, with a bigger fet if
necessary. (It doesn't have to be fast.)
How about one of those resettable polyfuses? The ones I've tested fail
earlier (lower currents) with each fail.

George H.
*I'd most likely make a terrible reviewer of circuits, with way more
questions than 'real' errors.

Quote:
OK, I see I'm going to get roasted for this one.



--
Thanks,
- Win


John Larkin
Guest

Thu Jan 10, 2019 2:45 am   



On Wed, 9 Jan 2019 18:27:30 -0600, "Tim Williams"
<tiwill_at_seventransistorlabs.com> wrote:

Quote:
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.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

Martin Riddle
Guest

Thu Jan 10, 2019 3:45 am   



On 9 Jan 2019 13:43:46 -0800, Winfield Hill <hill_at_rowland.harvard.edu>
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. 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.


We usually use the output resistors to limit the load current eg
protect the load.
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

John Larkin
Guest

Thu Jan 10, 2019 4:45 am   



On Wed, 09 Jan 2019 21:36:58 -0500, Martin Riddle
<martin_ridd_at_verizon.net> 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 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. 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.


We usually use the output resistors to limit the load current eg
protect the load.
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


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

Winfield Hill
Guest

Thu Jan 10, 2019 4:45 am   



George Herold wrote...
Quote:

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


--
Thanks,
- Win

Piotr Wyderski
Guest

Thu Jan 10, 2019 8:45 am   



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


Guest

Thu Jan 10, 2019 9:45 am   



John Larkin wrote

Quote:
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.


Guest

Thu Jan 10, 2019 10:45 am   



On Thursday, 10 January 2019 06:48:05 UTC, Piotr Wyderski wrote:

Quote:
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


I hear you can get them in white as well as red.


Re using a jellybean resistor for fusing, there are some general issues.
Voltage rating is low for sm parts
Carbon resistors sometimes hot tunnel when overloaded. This means R drops precipitously.
General purpose small resistors can also initiate a fire.
Carbons are also prone to arcing. BTDT - suffice it to say the arc has no ability to limit the short current to a sane value.

Fusible Rs address some of these issues.


NT

Piotr Wyderski
Guest

Thu Jan 10, 2019 1:45 pm   



tabbypurr_at_gmail.com wrote:

> I hear you can get them in white as well as red.

And anything in between.

> Voltage rating is low for sm parts

There are really robust HV SMD resistors, good to 10kV (although I
prefer the VR37 THT series), but they are way more expensive than
a proper fuse, so there is no point.

> Carbons are also prone to arcing. BTDT - suffice it to say the arc has no ability to limit the short current to a sane value.

There's even a negative resistance region over a significant fraction of
the I-V curve, so the glowistor will be happy to slurp a bit more. And
more.

> Fusible Rs address some of these issues.

Fusible transistors do that too, LOL. "Im an arcist"...

Best regards, Piotr


Guest

Thu Jan 10, 2019 1:45 pm   



On Thursday, January 10, 2019 at 7:56:28 PM UTC+11, tabb...@gmail.com wrote:
Quote:
On Thursday, 10 January 2019 06:48:05 UTC, Piotr Wyderski wrote:

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

I hear you can get them in white as well as red.


Re using a jellybean resistor for fusing, there are some general issues.
Voltage rating is low for sm parts
Carbon resistors sometimes hot tunnel when overloaded. This means R drops precipitously.


Carbon resistance elements have negative temperature coefficient of resistance, so they are very likely to hot tunnel.

One of my bosses had party trick where he could persuade a 10k carbon film resistor to pass roughly an amp indefinitely, and it still measured 10k afterwards - all you could see afterwards was a dark line in the paint above where the hot channel had been. He was on the "intrinsically safe" technical review committee, and that trick had been part of the case against allowing any carbon-based resistors in intrinsically safe equipment.

Quote:
General purpose small resistors can also initiate a fire.
Carbons are also prone to arcing. BTDT - suffice it to say the arc has no ability to limit the short current to a sane value.


Nor a hot channel in a carbon composition or carbon film resistor.

> Fusible resistors address some of these issues.

But it does pay to read the data sheet carefully.

--
Bill Sloman, Sydney

whit3rd
Guest

Thu Jan 10, 2019 1:45 pm   



On Wednesday, January 9, 2019 at 1:44:00 PM UTC-8, Winfield Hill wrote:
Quote:
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).

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