using an smt resistor as a fuse

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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

Re: using an smt resistor as a fuse
onsdag den 9. januar 2019 kl. 22.44.00 UTC+1 skrev Winfield Hill:
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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 !




Re: using an smt resistor as a fuse
wrote:

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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  

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Re: using an smt resistor as a fuse
On Wed, 09 Jan 2019 14:27:25 -0800, John Larkin wrote:

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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

Re: using an smt resistor as a fuse
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
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Re: using an smt resistor as a fuse
On Wed, 9 Jan 2019 18:27:30 -0600, "Tim Williams"

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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  

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Re: using an smt resistor as a fuse

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DC is different and sustains the arc.



Re: using an smt resistor as a fuse
wrote:

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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  

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Re: using an smt resistor as a fuse
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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
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Re: using an smt resistor as a fuse
Tim Williams wrote:

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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

Re: using an smt resistor as a fuse
On Thursday, 10 January 2019 06:48:05 UTC, Piotr Wyderski  wrote:

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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

Re: using an smt resistor as a fuse
snipped-for-privacy@gmail.com wrote:

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And anything in between.

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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.

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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.

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

    Best regards, Piotr


Re: using an smt resistor as a fuse
:
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 precipitously.

Carbon resistance elements have negative temperature coefficient of resista
nce, 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 aft
erwards - all you could see afterwards was a dark line in the paint above w
here the hot channel had been. He was on the "intrinsically safe" technical
 review committee, and that trick had been part of the case against allowin
g any carbon-based resistors in intrinsically safe equipment.

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 ability to limit the short current to a sane value.

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

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But it does pay to read the data sheet carefully.

--  
Bill Sloman, Sydney


Re: using an smt resistor as a fuse
On 1/10/19 1:47 AM, Piotr Wyderski wrote:
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Good thing I wasn't drinking coffee!

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: using an smt resistor as a fuse
On Thu, 10 Jan 2019 10:12:04 -0500, Phil Hobbs

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



--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: using an smt resistor as a fuse
On 1/10/19 12:57 PM, John Larkin wrote:
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Google "time zones". ;)

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: using an smt resistor as a fuse
On Wednesday, January 9, 2019 at 10:48:05 PM UTC-8, Piotr Wyderski wrote:

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

Re: using an smt resistor as a fuse
On Wed, 09 Jan 2019 18:27:30 -0600, Tim Williams wrote:

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

--  
Johnny B Good

Re: using an smt resistor as a fuse
On Wednesday, January 9, 2019 at 4:44:00 PM UTC-5, Winfield Hill wrote:
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OK no more x-chapter teases, unless I get to proof early copies*,
or it's being published soon.  
 But we assume
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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.  

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Re: using an smt resistor as a fuse
George Herold wrote...
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 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

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