What can kill an SCR......

[Dimiter_Popoff]

I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm.
The third part I unsoldered turned out to be it.
An SCR, a C106. Its job is to short the input power (past the
fuse) if any of the other 4 similar protections for the lower
voltages (5, 3.3, 2.5 and 1.5) got triggered.
Once I replaced the shorted SCR everything worked, no other victims
No artifacts on the lower voltages, these never trigger except during
development when I short something with the probe or something
(I am quite impatient so they have come to rescue more than once).

Any idea what could have caused this SCR to get short? After nearly
5 years of service that one. I have never had this and we have
netMCA-s (standard, this one is repurposed somewhat but not in a
way relevant to what is discussed) and they typically run 24/7
for years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the
unit (I had two units sabotaged at this customer some 2 years ago,
but it was mechanical, someone had put some aspirin or something
on the heaters (260C) and they got corroded - with the remnants of
the stuff on them for me to find).
So foul play is my first thought. But could it be something else?
Part defect showing after 5 years...?
I am just wondering, whatever the conclusion my course of action
will remain the same (give them the repaired unit, that is;
anything else could be reputation damaging).

------------------------------------------------------
Dimiter Popoff, TGI http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/

 

[Phil Hobbs]

On 2023-03-09 17:26, Dimiter_Popoff wrote:

I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm.
The third part I unsoldered turned out to be it.
An SCR, a C106. Its job is to short the input power (past the
fuse) if any of the other 4 similar protections for the lower
voltages (5, 3.3, 2.5 and 1.5) got triggered.
Once I replaced the shorted SCR everything worked, no other victims
No artifacts on the lower voltages, these never trigger except during
development when I short something with the probe or something
(I am quite impatient so they have come to rescue more than once).

Any idea what could have caused this SCR to get short? After nearly
5 years of service that one. I have never had this and we have
netMCA-s (standard, this one is repurposed somewhat but not in a
way relevant to what is discussed) and they typically run 24/7
for years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the
unit (I had two units sabotaged at this customer some 2 years ago,
but it was mechanical, someone had put some aspirin or something
on the heaters (260C) and they got corroded - with the remnants of
the stuff on them for me to find).
So foul play is my first thought. But could it be something else?
Part defect showing after 5 years...?
I am just wondering, whatever the conclusion my course of action
will remain the same (give them the repaired unit, that is;
anything else could be reputation damaging).

------------------------------------------------------
Dimiter Popoff, TGI             http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/

The usual reason is inadequate gate drive, which reduces the dI/dt
capability.

When an SCR fires, the conducting region starts at the gate contact and
spreads out across the die. In a crowbar application, there\'s a race
between the expansion of the conducting area and the inductive rising
current waveform: Will the SCR have enough conducting area that it
doesn\'t melt?

The often-seen zener + resistor + SCR crowbar circuit is a disaster for
this--the SCR cannibalizes its own gate drive as it turns on.

You really want nice beefy gate drive with a sharp trigger edge to get
the most out of your crowbar SCR.

Back when linear supplies were more common, there used to be dedicated
crowbar controllers for this job, such as the MC3423,
<https://www.onsemi.com/pdf/datasheet/mc3423-d.pdf>.

There\'s a discussion of this problem on pages 5 and 6.

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

 

[Dimiter_Popoff]

On 3/10/2023 0:57, Phil Hobbs wrote:

On 2023-03-09 17:26, Dimiter_Popoff wrote:
I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm.
The third part I unsoldered turned out to be it.
An SCR, a C106. Its job is to short the input power (past the
fuse) if any of the other 4 similar protections for the lower
voltages (5, 3.3, 2.5 and 1.5) got triggered.
Once I replaced the shorted SCR everything worked, no other victims
No artifacts on the lower voltages, these never trigger except during
development when I short something with the probe or something
(I am quite impatient so they have come to rescue more than once).

Any idea what could have caused this SCR to get short? After nearly
5 years of service that one. I have never had this and we have
netMCA-s (standard, this one is repurposed somewhat but not in a
way relevant to what is discussed) and they typically run 24/7
for years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the
unit (I had two units sabotaged at this customer some 2 years ago,
but it was mechanical, someone had put some aspirin or something
on the heaters (260C) and they got corroded - with the remnants of
the stuff on them for me to find).
So foul play is my first thought. But could it be something else?
Part defect showing after 5 years...?
I am just wondering, whatever the conclusion my course of action
will remain the same (give them the repaired unit, that is;
anything else could be reputation damaging).

------------------------------------------------------
Dimiter Popoff, TGI             http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/


The usual reason is inadequate gate drive, which reduces the dI/dt
capability.

When an SCR fires, the conducting region starts at the gate contact and
spreads out across the die.  In a crowbar application, there\'s a race
between the expansion of the conducting area and the inductive rising
current waveform: Will the SCR have enough conducting area that it
doesn\'t melt?

The often-seen zener + resistor + SCR crowbar circuit is a disaster for
this--the SCR cannibalizes its own gate drive as it turns on.

You really want nice beefy gate drive with a sharp trigger edge to get
the most out of your crowbar SCR.

Back when linear supplies were more common, there used to be dedicated
crowbar controllers for this job, such as the MC3423,
https://www.onsemi.com/pdf/datasheet/mc3423-d.pdf>.

There\'s a discussion of this problem on pages 5 and 6.

Cheers

Phil Hobbs

The gate drive of the dead SCR is not via a zener,it is a 2907,
emitter to +13V, collector to the SCR gate via some 100 Ohm;
OK, instead of torturing you and myself with explanations here
it is:
http://tgi-sci.com/misc/nmcps2.gif

The dead SCR is SR205; I cannot say if any of the rest have
been triggered at all, no reason for that I can think of for this
to happen on its own. I looked at the voltages, clean. The delayed/soft
start - as it has to be. No signs on the case of the C106 from
being fried (hence it was my third choice to unsolder, a .... zener
that is not a zener, one of these overvoltage things came first, then
an electrolythic cap (pathetic attempt, it was visibly good).
Then the 12V the unit gets are not so powerful ( a stepdown from 18V
in a separate case with other stuff), I really have no idea what
may have happened. Even including tampering, what do you do to
cause that?! I would expect the stepdown to die, not the shorting
SCR... Then except for a fuse there is a current limiting coin
thingie, \"recoverable fuse\" or whatever they call them, it\'s been
a while since I did it, its resistance becomes pretty huge
above a current threshold.

 

[Phil Hobbs]

On 2023-03-09 18:44, Dimiter_Popoff wrote:

On 3/10/2023 0:57, Phil Hobbs wrote:
On 2023-03-09 17:26, Dimiter_Popoff wrote:
I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm. The third part I
unsoldered turned out to be it. An SCR, a C106. Its job is to
short the input power (past the fuse) if any of the other 4
similar protections for the lower voltages (5, 3.3, 2.5 and 1.5)
got triggered. Once I replaced the shorted SCR everything worked,
no other victims No artifacts on the lower voltages, these never
trigger except during development when I short something with the
probe or something (I am quite impatient so they have come to
rescue more than once).

Any idea what could have caused this SCR to get short? After
nearly 5 years of service that one. I have never had this and we
have netMCA-s (standard, this one is repurposed somewhat but not
in a way relevant to what is discussed) and they typically run
24/7 for years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the
unit (I had two units sabotaged at this customer some 2 years
ago, but it was mechanical, someone had put some aspirin or
something on the heaters (260C) and they got corroded - with the
remnants of the stuff on them for me to find). So foul play is my
first thought. But could it be something else? Part defect
showing after 5 years...? I am just wondering, whatever the
conclusion my course of action will remain the same (give them
the repaired unit, that is; anything else could be reputation
damaging).

------------------------------------------------------ Dimiter
Popoff, TGI http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/


The usual reason is inadequate gate drive, which reduces the dI/dt
capability.

When an SCR fires, the conducting region starts at the gate
contact and spreads out across the die. In a crowbar application,
there\'s a race between the expansion of the conducting area and
the inductive rising current waveform: Will the SCR have enough
conducting area that it doesn\'t melt?

The often-seen zener + resistor + SCR crowbar circuit is a
disaster for this--the SCR cannibalizes its own gate drive as it
turns on.

You really want nice beefy gate drive with a sharp trigger edge to
get the most out of your crowbar SCR.

Back when linear supplies were more common, there used to be
dedicated crowbar controllers for this job, such as the MC3423,
https://www.onsemi.com/pdf/datasheet/mc3423-d.pdf>.

There\'s a discussion of this problem on pages 5 and 6.

The gate drive of the dead SCR is not via a zener,it is a 2907,
emitter to +13V, collector to the SCR gate via some 100 Ohm; OK,
instead of torturing you and myself with explanations here it is:
http://tgi-sci.com/misc/nmcps2.gif

The dead SCR is SR205; I cannot say if any of the rest have been
triggered at all, no reason for that I can think of for this to
happen on its own.

The 2907 is powered off the same supply that the SCR is trying to
crowbar, with no local reservoir cap. That\'s not the ideal, but since
the rail is 12V, it might well work fine. Provided nothing steals the
2907\'s base drive, the 12V rail has to come down to 2-3 volts before
it\'ll turn off. There won\'t be much energy left in the caps at that point.

I looked at the voltages, clean. The delayed/soft start - as it has
to be. No signs on the case of the C106 from being fried (hence it
was my third choice to unsolder, a .... zener that is not a zener,
one of these overvoltage things came first, then an electrolytic
cap (pathetic attempt, it was visibly good).

Then the 12V the unit gets are not so powerful ( a stepdown from 18V
in a separate case with other stuff), I really have no idea what may
have happened. Even including tampering, what do you do to cause
that?!

I would expect the stepdown to die, not the shorting SCR...

Your average switching regulator has cycle-by-cycle current limiting, so
it won\'t fight the SCR very hard. If the SCR died the way that I
suspect, it\'s the reservoir caps that are the killers.

Then except for a fuse there is a current limiting coin thingie,
\"recoverable fuse\" or whatever they call them, it\'s been a while
since I did it, its resistance becomes pretty huge above a current
threshold.

Probably a polyswitch. Those typically take a few hundred milliseconds
to switch even with a big overload.

Of course, it could just be a bad SCR, or else the dI/dt or I**2t
capacity could be marginal.

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

 

[Dimiter_Popoff]

On 3/10/2023 2:49, Phil Hobbs wrote:

On 2023-03-09 18:44, Dimiter_Popoff wrote:
On 3/10/2023 0:57, Phil Hobbs wrote:
On 2023-03-09 17:26, Dimiter_Popoff wrote:
I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm. The third part I
unsoldered turned out to be it. An SCR, a C106. Its job is to short
the input power (past the fuse) if any of the other 4 similar
protections for the lower voltages (5, 3.3, 2.5 and 1.5) got
triggered. Once I replaced the shorted SCR everything worked,
no other victims No artifacts on the lower voltages, these never
trigger except during development when I short something with the
probe or something (I am quite impatient so they have come to
rescue more than once).

Any idea what could have caused this SCR to get short? After nearly
5 years of service that one. I have never had this and we have
netMCA-s (standard, this one is repurposed somewhat but not in a way
relevant to what is discussed) and they typically run 24/7 for
years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the unit
(I had two units sabotaged at this customer some 2 years ago, but it
was mechanical, someone had put some aspirin or something on the
heaters (260C) and they got corroded - with the remnants of the
stuff on them for me to find). So foul play is my
first thought. But could it be something else? Part defect showing
after 5 years...? I am just wondering, whatever the conclusion my
course of action will remain the same (give them the repaired unit,
that is; anything else could be reputation damaging).

------------------------------------------------------ Dimiter
Popoff, TGI             http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/


The usual reason is inadequate gate drive, which reduces the dI/dt
capability.

When an SCR fires, the conducting region starts at the gate
contact and spreads out across the die.  In a crowbar application,
there\'s a race between the expansion of the conducting area and
the inductive rising current waveform: Will the SCR have enough
conducting area that it doesn\'t melt?

The often-seen zener + resistor + SCR crowbar circuit is a
disaster for this--the SCR cannibalizes its own gate drive as it
turns on.

You really want nice beefy gate drive with a sharp trigger edge to
get the most out of your crowbar SCR.

Back when linear supplies were more common, there used to be
dedicated crowbar controllers for this job, such as the MC3423,
https://www.onsemi.com/pdf/datasheet/mc3423-d.pdf>.

There\'s a discussion of this problem on pages 5 and 6.

The gate drive of the dead SCR is not via a zener,it is a 2907,
emitter to +13V, collector to the SCR gate via some 100 Ohm; OK,
instead of torturing you and myself with explanations here it is:
http://tgi-sci.com/misc/nmcps2.gif

The dead SCR is SR205; I cannot say if any of the rest have been
triggered at all, no reason for that I can think of for this to happen
on its own.

The 2907 is powered off the same supply that the SCR is trying to
crowbar, with no local reservoir cap.  That\'s not the ideal, but since
the rail is 12V, it might well work fine.  Provided nothing steals the
2907\'s base drive, the 12V rail has to come down to 2-3 volts before
it\'ll turn off.  There won\'t be much energy left in the caps at that point.

Of course it does work, hey. I know what I am doing, you know .
The +12 has at least 1000uF (on the board where the SCR is, which
consumes about 0.5A off the 12V).

I looked at the voltages, clean. The delayed/soft start - as it has to
be. No signs on the case of the C106 from being fried (hence it was my
third choice to unsolder, a .... zener that is not a zener, one of
these overvoltage things came first, then an electrolytic
cap (pathetic attempt, it was visibly good).

Then the 12V the unit gets are not so powerful ( a stepdown from 18V
in a separate case with other stuff), I really have no idea what may
have happened. Even including tampering, what do you do to cause
that?!

I would expect the stepdown to die, not the shorting SCR...

Your average switching regulator has cycle-by-cycle current limiting, so
it won\'t fight the SCR very hard.  If the SCR died the way that I
suspect, it\'s the reservoir caps that are the killers.

My average stepdown is very similar to these you see on the posted
circuit. I rarely use off the shelf stepdowns, they invariably have
had stability surprises for me etc. so I prefer to have things under
control so the first board iteration has things work as I expect.
Not much current limiting though the MOSFET resistance provides
some; the stepdown in question had not died, not sure it was the
one powering the board when the SCR was killed... If they have toyed
with it - I know they have opened the case with the stepdown for
no valid reason (from my point of view).
Who knows what voltage the+12 input has seen.

Then except for a fuse there is a current limiting coin thingie,
\"recoverable fuse\" or whatever they call them, it\'s been a while
since I did it, its resistance becomes pretty huge above a current
threshold.

Probably a polyswitch.  Those typically take a few hundred milliseconds
to switch even with a big overload.

Oddly I still can\'t remember what it was called. Though I know at which
shop to order them.... (it is a local one, comet.bg)
Most likely exactly what you say, I think something like that starts
to come out of the fog...

Of course, it could just be a bad SCR, or else the dI/dt or I**2t
capacity could be marginal.

Can\'t imagine the SCR would become bad after nearly 5 years of
service (that unit was delivered 2018 and it has had its online
support on most of the time so I know it has been in use a lot).
Could not the SCR be triggered by dV/dt? (Not likely
with >1000uF on the rail of course).
Most likely they have powered the unit off something else, probably
at much higher voltage than 12. I can imagine some of the 2907-s
turning on because of that - and the power being applied just
long enough to kill the SCR by overcurrent without the case
showing signs of it.. (that polyswitch thingie?) (??? - not an easy
postmortem anyway .

Cheers

Phil Hobbs

 

[Phil Hobbs]

On 2023-03-10 02:10, Dimiter_Popoff wrote:

On 3/10/2023 2:49, Phil Hobbs wrote:
On 2023-03-09 18:44, Dimiter_Popoff wrote:
On 3/10/2023 0:57, Phil Hobbs wrote:
On 2023-03-09 17:26, Dimiter_Popoff wrote:
I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm. The third part I
unsoldered turned out to be it. An SCR, a C106. Its job is to short
the input power (past the fuse) if any of the other 4 similar
protections for the lower voltages (5, 3.3, 2.5 and 1.5) got
triggered. Once I replaced the shorted SCR everything worked,
no other victims No artifacts on the lower voltages, these never
trigger except during development when I short something with the
probe or something (I am quite impatient so they have come to
rescue more than once).

Any idea what could have caused this SCR to get short? After nearly
5 years of service that one. I have never had this and we have
netMCA-s (standard, this one is repurposed somewhat but not in a
way relevant to what is discussed) and they typically run 24/7 for
years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the
unit (I had two units sabotaged at this customer some 2 years ago,
but it was mechanical, someone had put some aspirin or something on
the heaters (260C) and they got corroded - with the remnants of the
stuff on them for me to find). So foul play is my
first thought. But could it be something else? Part defect showing
after 5 years...? I am just wondering, whatever the conclusion my
course of action will remain the same (give them the repaired unit,
that is; anything else could be reputation damaging).

------------------------------------------------------ Dimiter
Popoff, TGI             http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/


The usual reason is inadequate gate drive, which reduces the dI/dt
capability.

When an SCR fires, the conducting region starts at the gate
contact and spreads out across the die.  In a crowbar application,
there\'s a race between the expansion of the conducting area and
the inductive rising current waveform: Will the SCR have enough
conducting area that it doesn\'t melt?

The often-seen zener + resistor + SCR crowbar circuit is a
disaster for this--the SCR cannibalizes its own gate drive as it
turns on.

You really want nice beefy gate drive with a sharp trigger edge to
get the most out of your crowbar SCR.

Back when linear supplies were more common, there used to be
dedicated crowbar controllers for this job, such as the MC3423,
https://www.onsemi.com/pdf/datasheet/mc3423-d.pdf>.

There\'s a discussion of this problem on pages 5 and 6.

The gate drive of the dead SCR is not via a zener,it is a 2907,
emitter to +13V, collector to the SCR gate via some 100 Ohm; OK,
instead of torturing you and myself with explanations here it is:
http://tgi-sci.com/misc/nmcps2.gif

The dead SCR is SR205; I cannot say if any of the rest have been
triggered at all, no reason for that I can think of for this to
happen on its own.

The 2907 is powered off the same supply that the SCR is trying to
crowbar, with no local reservoir cap.  That\'s not the ideal, but since
the rail is 12V, it might well work fine.  Provided nothing steals the
2907\'s base drive, the 12V rail has to come down to 2-3 volts before
it\'ll turn off.  There won\'t be much energy left in the caps at that
point.

Of course it does work, hey. I know what I am doing, you know .
The +12 has at least 1000uF (on the board where the SCR is, which
consumes about 0.5A off the 12V).


I looked at the voltages, clean. The delayed/soft start - as it has
to be. No signs on the case of the C106 from being fried (hence it
was my third choice to unsolder, a .... zener that is not a zener,
one of these overvoltage things came first, then an electrolytic
cap (pathetic attempt, it was visibly good).

Then the 12V the unit gets are not so powerful ( a stepdown from 18V
in a separate case with other stuff), I really have no idea what may
have happened. Even including tampering, what do you do to cause
that?!

I would expect the stepdown to die, not the shorting SCR...

Your average switching regulator has cycle-by-cycle current limiting,
so it won\'t fight the SCR very hard.  If the SCR died the way that I
suspect, it\'s the reservoir caps that are the killers.

My average stepdown is very similar to these you see on the posted
circuit. I rarely use off the shelf stepdowns, they invariably have
had stability surprises for me etc. so I prefer to have things under
control so the first board iteration has things work as I expect.
Not much current limiting though the MOSFET resistance provides
some; the stepdown in question had not died, not sure it was the
one powering the board when the SCR was killed... If they have toyed
with it - I know they have opened the case with the stepdown for
no valid reason (from my point of view).
Who knows what voltage the+12 input has seen.


Then except for a fuse there is a current limiting coin thingie,
\"recoverable fuse\" or whatever they call them, it\'s been a while
since I did it, its resistance becomes pretty huge above a current
threshold.

Probably a polyswitch.  Those typically take a few hundred
milliseconds to switch even with a big overload.

Oddly I still can\'t remember what it was called. Though I know at which
shop to order them.... (it is a local one, comet.bg)
Most likely exactly what you say, I think something like that starts
to come out of the fog...

Of course, it could just be a bad SCR, or else the dI/dt or I**2t
capacity could be marginal.

Can\'t imagine the SCR would become bad after nearly 5 years of
service (that unit was delivered 2018 and it has had its online
support on most of the time so I know it has been in use a lot).
Could not the SCR be triggered by dV/dt? (Not likely
with >1000uF on the rail of course).
Most likely they have powered the unit off something else, probably
at much higher voltage than 12. I can imagine some of the 2907-s
turning on because of that - and the power being applied just
long enough to kill the SCR by overcurrent without the case
showing signs of it.. (that polyswitch thingie?) (??? - not an easy
postmortem anyway .

Your customers are obviously very creative folks.

The polyswitch could easily protect the box and still allow the SCR to
turn to lava.

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

 

[Dimiter_Popoff]

On 3/10/2023 9:35, Phil Hobbs wrote:

On 2023-03-10 02:10, Dimiter_Popoff wrote:
On 3/10/2023 2:49, Phil Hobbs wrote:
On 2023-03-09 18:44, Dimiter_Popoff wrote:
On 3/10/2023 0:57, Phil Hobbs wrote:
On 2023-03-09 17:26, Dimiter_Popoff wrote:
I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm. The third part I
unsoldered turned out to be it. An SCR, a C106. Its job is to
short the input power (past the fuse) if any of the other 4
similar protections for the lower voltages (5, 3.3, 2.5 and 1.5)
got triggered. Once I replaced the shorted SCR everything worked,
no other victims No artifacts on the lower voltages, these never
trigger except during development when I short something with the
probe or something (I am quite impatient so they have come to
rescue more than once).

Any idea what could have caused this SCR to get short? After
nearly 5 years of service that one. I have never had this and we
have netMCA-s (standard, this one is repurposed somewhat but not
in a way relevant to what is discussed) and they typically run
24/7 for years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the
unit (I had two units sabotaged at this customer some 2 years ago,
but it was mechanical, someone had put some aspirin or something
on the heaters (260C) and they got corroded - with the remnants of
the stuff on them for me to find). So foul play is my
first thought. But could it be something else? Part defect showing
after 5 years...? I am just wondering, whatever the conclusion my
course of action will remain the same (give them the repaired
unit, that is; anything else could be reputation damaging).

------------------------------------------------------ Dimiter
Popoff, TGI             http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/


The usual reason is inadequate gate drive, which reduces the dI/dt
capability.

When an SCR fires, the conducting region starts at the gate
contact and spreads out across the die.  In a crowbar application,
there\'s a race between the expansion of the conducting area and
the inductive rising current waveform: Will the SCR have enough
conducting area that it doesn\'t melt?

The often-seen zener + resistor + SCR crowbar circuit is a
disaster for this--the SCR cannibalizes its own gate drive as it
turns on.

You really want nice beefy gate drive with a sharp trigger edge to
get the most out of your crowbar SCR.

Back when linear supplies were more common, there used to be
dedicated crowbar controllers for this job, such as the MC3423,
https://www.onsemi.com/pdf/datasheet/mc3423-d.pdf>.

There\'s a discussion of this problem on pages 5 and 6.

The gate drive of the dead SCR is not via a zener,it is a 2907,
emitter to +13V, collector to the SCR gate via some 100 Ohm; OK,
instead of torturing you and myself with explanations here it is:
http://tgi-sci.com/misc/nmcps2.gif

The dead SCR is SR205; I cannot say if any of the rest have been
triggered at all, no reason for that I can think of for this to
happen on its own.

The 2907 is powered off the same supply that the SCR is trying to
crowbar, with no local reservoir cap.  That\'s not the ideal, but
since the rail is 12V, it might well work fine.  Provided nothing
steals the 2907\'s base drive, the 12V rail has to come down to 2-3
volts before it\'ll turn off.  There won\'t be much energy left in the
caps at that point.

Of course it does work, hey. I know what I am doing, you know .
The +12 has at least 1000uF (on the board where the SCR is, which
consumes about 0.5A off the 12V).


I looked at the voltages, clean. The delayed/soft start - as it has
to be. No signs on the case of the C106 from being fried (hence it
was my third choice to unsolder, a .... zener that is not a zener,
one of these overvoltage things came first, then an electrolytic
cap (pathetic attempt, it was visibly good).

Then the 12V the unit gets are not so powerful ( a stepdown from 18V
in a separate case with other stuff), I really have no idea what may
have happened. Even including tampering, what do you do to cause
that?!

I would expect the stepdown to die, not the shorting SCR...

Your average switching regulator has cycle-by-cycle current limiting,
so it won\'t fight the SCR very hard.  If the SCR died the way that I
suspect, it\'s the reservoir caps that are the killers.

My average stepdown is very similar to these you see on the posted
circuit. I rarely use off the shelf stepdowns, they invariably have
had stability surprises for me etc. so I prefer to have things under
control so the first board iteration has things work as I expect.
Not much current limiting though the MOSFET resistance provides
some; the stepdown in question had not died, not sure it was the
one powering the board when the SCR was killed... If they have toyed
with it - I know they have opened the case with the stepdown for
no valid reason (from my point of view).
Who knows what voltage the+12 input has seen.


Then except for a fuse there is a current limiting coin thingie,
\"recoverable fuse\" or whatever they call them, it\'s been a while
since I did it, its resistance becomes pretty huge above a current
threshold.

Probably a polyswitch.  Those typically take a few hundred
milliseconds to switch even with a big overload.

Oddly I still can\'t remember what it was called. Though I know at which
shop to order them.... (it is a local one, comet.bg)
Most likely exactly what you say, I think something like that starts
to come out of the fog...

Of course, it could just be a bad SCR, or else the dI/dt or I**2t
capacity could be marginal.

Can\'t imagine the SCR would become bad after nearly 5 years of
service (that unit was delivered 2018 and it has had its online
support on most of the time so I know it has been in use a lot).
Could not the SCR be triggered by dV/dt? (Not likely
with >1000uF on the rail of course).
Most likely they have powered the unit off something else, probably
at much higher voltage than 12. I can imagine some of the 2907-s
turning on because of that - and the power being applied just
long enough to kill the SCR by overcurrent without the case
showing signs of it.. (that polyswitch thingie?) (??? - not an easy
postmortem anyway .

Your customers are obviously very creative folks.

Hah, some of them are indeed. Most are normal folks just using their
netMCA-s to measure spectra; but I have had so far 3 customers trying
to clone it (mission impossible *for me* without documentation).
These do use their units to measure with but apparently they are
trying to be the fourth ones...

The polyswitch could easily protect the box and still allow the SCR to
turn to lava.

This seems to be the likeliest scenario, thanks for the insight, I do
appreciate it.

 

[Fred Bloggs]

On Thursday, March 9, 2023 at 5:58:04 PM UTC-5, Phil Hobbs wrote:

On 2023-03-09 17:26, Dimiter_Popoff wrote:
I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm.
The third part I unsoldered turned out to be it.
An SCR, a C106. Its job is to short the input power (past the
fuse) if any of the other 4 similar protections for the lower
voltages (5, 3.3, 2.5 and 1.5) got triggered.
Once I replaced the shorted SCR everything worked, no other victims
No artifacts on the lower voltages, these never trigger except during
development when I short something with the probe or something
(I am quite impatient so they have come to rescue more than once).

Any idea what could have caused this SCR to get short? After nearly
5 years of service that one. I have never had this and we have
netMCA-s (standard, this one is repurposed somewhat but not in a
way relevant to what is discussed) and they typically run 24/7
for years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the
unit (I had two units sabotaged at this customer some 2 years ago,
but it was mechanical, someone had put some aspirin or something
on the heaters (260C) and they got corroded - with the remnants of
the stuff on them for me to find).
So foul play is my first thought. But could it be something else?
Part defect showing after 5 years...?
I am just wondering, whatever the conclusion my course of action
will remain the same (give them the repaired unit, that is;
anything else could be reputation damaging).

------------------------------------------------------
Dimiter Popoff, TGI http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/

The usual reason is inadequate gate drive, which reduces the dI/dt
capability.

When an SCR fires, the conducting region starts at the gate contact and
spreads out across the die. In a crowbar application, there\'s a race
between the expansion of the conducting area and the inductive rising
current waveform: Will the SCR have enough conducting area that it
doesn\'t melt?

The often-seen zener + resistor + SCR crowbar circuit is a disaster for
this--the SCR cannibalizes its own gate drive as it turns on.

You really want nice beefy gate drive with a sharp trigger edge to get
the most out of your crowbar SCR.

Not for this one, it\'s a sensitive gate:

IGM= 0.2 A PW=1.0us TC=80oC
( TC might be T case )

And PG(AV)= 0.1W

The schematic shows a gate drive of ~ 130mA ( AT LEAST) and PG~0.07W continuous for practical purposes relative to this limit.

The trigger circuit blows the gate out.

The fix will be to replace R219 (100R) with a 47kR, and then place a series 100R + 0.015u in parallel with it.

And he needs to verify his fuse is <0.7 x 1.65 A^2t

Then the 40A/us limit makes for PWR/L(uH) limit, or L> PWR/40 uH, usually, should be enough protection against damaging hot spot development.

https://www.littelfuse.com/products/power-semiconductors/discrete-thyristors/scr/c106.aspx#:~:text=Datasheet,Electrical%20Characteristics

There\'s a bunch of C106 out there, the OP is part number is incomplete, but close enough.

Not going to check all his parts, but it would behoove anyone using sensitive gate SCR to put some delay capacitance across those gates to avoid false triggering due to transients.

Back when linear supplies were more common, there used to be dedicated
crowbar controllers for this job, such as the MC3423,
https://www.onsemi.com/pdf/datasheet/mc3423-d.pdf>.

I used that one. It gets complicated for bipolar supplies when they both need to run down to zero at the same time.

There\'s a discussion of this problem on pages 5 and 6.

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

 

[Phil Hobbs]

On 2023-03-10 12:51, Fred Bloggs wrote:

On Thursday, March 9, 2023 at 5:58:04 PM UTC-5, Phil Hobbs wrote:
On 2023-03-09 17:26, Dimiter_Popoff wrote:
I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm. The third part I
unsoldered turned out to be it. An SCR, a C106. Its job is to
short the input power (past the fuse) if any of the other 4
similar protections for the lower voltages (5, 3.3, 2.5 and 1.5)
got triggered. Once I replaced the shorted SCR everything
worked, no other victims No artifacts on the lower voltages,
these never trigger except during development when I short
something with the probe or something (I am quite impatient so
they have come to rescue more than once).

Any idea what could have caused this SCR to get short? After
nearly 5 years of service that one. I have never had this and we
have netMCA-s (standard, this one is repurposed somewhat but not
in a way relevant to what is discussed) and they typically run
24/7 for years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the
unit (I had two units sabotaged at this customer some 2 years
ago, but it was mechanical, someone had put some aspirin or
something on the heaters (260C) and they got corroded - with the
remnants of the stuff on them for me to find). So foul play is
my first thought. But could it be something else? Part defect
showing after 5 years...? I am just wondering, whatever the
conclusion my course of action will remain the same (give them
the repaired unit, that is; anything else could be reputation
damaging).

------------------------------------------------------ Dimiter
Popoff, TGI http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/

The usual reason is inadequate gate drive, which reduces the dI/dt
capability.

When an SCR fires, the conducting region starts at the gate
contact and spreads out across the die. In a crowbar application,
there\'s a race between the expansion of the conducting area and the
inductive rising current waveform: Will the SCR have enough
conducting area that it doesn\'t melt?

The often-seen zener + resistor + SCR crowbar circuit is a
disaster for this--the SCR cannibalizes its own gate drive as it
turns on.

You really want nice beefy gate drive with a sharp trigger edge to
get the most out of your crowbar SCR.

Not for this one, it\'s a sensitive gate:

IGM= 0.2 A PW=1.0us TC=80oC ( TC might be T case )

And PG(AV)= 0.1W


The schematic shows a gate drive of ~ 130mA ( AT LEAST) and PG~0.07W
continuous for practical purposes relative to this limit.

The trigger circuit blows the gate out.

The SCR collapses the supply that the gate drive comes from, so that
shouldn\'t be a big problem, I wouldn\'t think.

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

 

[Fred Bloggs]

On Friday, March 10, 2023 at 7:47:04 PM UTC-5, Phil Hobbs wrote:

On 2023-03-10 12:51, Fred Bloggs wrote:
On Thursday, March 9, 2023 at 5:58:04 PM UTC-5, Phil Hobbs wrote:
On 2023-03-09 17:26, Dimiter_Popoff wrote:
I just got a unit (a netMCA spinoff) which was basically short
circuit at its 12V power input, 0.1 to 0.2 Ohm. The third part I
unsoldered turned out to be it. An SCR, a C106. Its job is to
short the input power (past the fuse) if any of the other 4
similar protections for the lower voltages (5, 3.3, 2.5 and 1.5)
got triggered. Once I replaced the shorted SCR everything
worked, no other victims No artifacts on the lower voltages,
these never trigger except during development when I short
something with the probe or something (I am quite impatient so
they have come to rescue more than once).

Any idea what could have caused this SCR to get short? After
nearly 5 years of service that one. I have never had this and we
have netMCA-s (standard, this one is repurposed somewhat but not
in a way relevant to what is discussed) and they typically run
24/7 for years, the first ones for over a decade now.

I can\'t think of another reason than someone tampering with the
unit (I had two units sabotaged at this customer some 2 years
ago, but it was mechanical, someone had put some aspirin or
something on the heaters (260C) and they got corroded - with the
remnants of the stuff on them for me to find). So foul play is
my first thought. But could it be something else? Part defect
showing after 5 years...? I am just wondering, whatever the
conclusion my course of action will remain the same (give them
the repaired unit, that is; anything else could be reputation
damaging).

------------------------------------------------------ Dimiter
Popoff, TGI http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/

The usual reason is inadequate gate drive, which reduces the dI/dt
capability.

When an SCR fires, the conducting region starts at the gate
contact and spreads out across the die. In a crowbar application,
there\'s a race between the expansion of the conducting area and the
inductive rising current waveform: Will the SCR have enough
conducting area that it doesn\'t melt?

The often-seen zener + resistor + SCR crowbar circuit is a
disaster for this--the SCR cannibalizes its own gate drive as it
turns on.

You really want nice beefy gate drive with a sharp trigger edge to
get the most out of your crowbar SCR.

Not for this one, it\'s a sensitive gate:

IGM= 0.2 A PW=1.0us TC=80oC ( TC might be T case )

And PG(AV)= 0.1W


The schematic shows a gate drive of ~ 130mA ( AT LEAST) and PG~0.07W
continuous for practical purposes relative to this limit.

The trigger circuit blows the gate out.
The SCR collapses the supply that the gate drive comes from, so that
shouldn\'t be a big problem, I wouldn\'t think.

Then it has to be the PWR crowbar is exceeding I^2t, 1.65 A^2. If someone changed something, they probably replaced the fuse with a different type. People just look for Amps-Volts, fast, slow or nothing, and don\'t usually look into the manufacturer I2t rating.

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