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Over Voltage Protection Crowbar Circuit

L

legg

Jan 1, 1970
0
#3 is the only option I ever consider. Personally I like to have the SCR
right at the circuit side of the fuse. If the fuse were at the input and
the SCR were at the output you'd have the upper FET and the inductor in
the path. This will slow down the fuse tripping and there is a chance
that the FET grenades before the fuse trips, something that is generally
not desired.

Applying the crowbar to the output is the most effective way of
reducing potential damage to the load - which is it's intended
function. The end user would usually prefer that your power supply
turn itself into rubble, before any harm can come to the load. Best to
test the effectiveness of any protection method, as this has priority.
A grenading fet would only pass single-fault abnormals in an enclosure
(if isolation barriers are not compromised in the process) and is best
avoided by design.

In many cases (though not in this simple buck example) the presence of
an isolation barrier will alter design considerations for ovp
protection.

RL
 
L

legg

Jan 1, 1970
0
That was my point, ideally you don't want to have the inductor in the path.

Presence of the inductor will not reduce the effectiveness of ovp
crowbar protection that is located on the output terminals. Neither
will a grenading fet.

RL
 
P

P E Schoen

Jan 1, 1970
0
"Jim Thompson" wrote in message
On Fri, 30 Nov 2012 13:54:33 -0800 (PST), "[email protected]"
Yep. I always like my own circuits >:-}
The fuse blower design was done before MOSFET's were common.

I see some problems with that circuit. The IRLM6401 is only rated at 12V and
the gate only +/-8V. Of course this is apparently for a low voltage battery,
probably 6V, considering the 3.3 ohm load. M1 seems to be connected so that
its internal diode normally conducts, although the 0V biased gate would turn
it on. I think there should be a resistor on the gate. The shunt regulator
DZ1 is set to 5.2V, so I see this may be for a 5V logic supply. I see that a
voltage over about 5.8V will start to shunt the Vgs of M2 and turn it off
when it becomes less than about 0.6 to 0.95 V gate threshold. But it still
does not seem to provide any voltage regulation to the load, and with 0.05
ohms or each MOSET at 1.5A loses 150 mV. If the voltage is only marginally
high, or if the load increased, there could be about 2A and 1 or 2 volts on
the MOSFET which would exceed its power dissipation of 1.3W.

I don't see what is so great about this circuit. It does not have voltage
regulation or current limiting and it does not have any hysteresis to keep
it from entering a destructive linear range or possibly oscillate. The SCR
crowbar with a fuse (or maybe a polyswitch device) is more robust and far
simpler.

Maybe you or someone can explain the merits of your circuit.

Paul
 
P

P E Schoen

Jan 1, 1970
0
"legg" wrote in message
There's no reason why a commercial bench power supply should
smoke on application of external voltage, within it's normal
output compliance, save the naivete of it's designer, or the
complaisance of it's purchaser.

There is not that much difference between charging a battery with a bench
supply, or charging a large capacitor in a circuit. I have a large capacitor
that I put across my bench supply so that the circuit I'm using it on can
draw current surges much higher than the current limit. I haven't had a
problem if I leave the capacitor connected and turn off the supply.

Paul
 
M

mike

Jan 1, 1970
0
I don't follow your reasoning.

You're sounding a bit snippy, but let's see if we can wrap our
heads around it.
Is it the battery back-up,
I never mentioned backup.
overvoltage protection method, or the power
supply design you're questioning?
Overvoltage protection IS part of the power supply design.
The subject is crowbar ovp methods.
You're point was the ovp condition occurring in a battery charging
situation. I think was addressed.
My point was that people use power supplies in ways that don't show
up on a spec sheet.
Having a stiff voltage supply hooked across the output is a proxy for
one type of potential application.
It's important for designers to consider what conditions might occur
and do whatever is reasonable to prevent failure of the supply
or destruction of the DUT. Transient conditions
inside the power supply are part of the analysis.

In particular, it's easy to design a circuit that crowbars the output when
it reaches 28V.
What's not so obvious is when the output is held at 24V by a battery or
a big bank of caps, or back EMF from a motor when the input power glitches.
Anything that triggers the SCR while the internal power supplies are
dropping out can cause problems.
Power supply compatibility in parallel redundant or backup
applications is a subject in itself. Misapplication of products, or
their immunity to such conditions can also be an interesting
discussion, along with the sad state of the equipment and the
amazement or frustration of the misapplicator.

Well, misapplicators don't realize that they're misapplicators.
It's the designer's job to protect them from themselves.
If you do nothing but meet the numbers on the spec sheet, you're
not doing enough.
Rest assured that you won't see a product that wasn't immune to the
application of external voltages occupy or originate in my work area.
This is, in fact, a crude test method to confirm the function of OVP
circuitry, though alternative methods exist that may perform the same
function automatically and with reduced hardware, in both design and
in production test.

Not clear exactly what you mean. If you are also testing shutdown
transients due to power line fluctuations with external voltage
applied, I'd agree.
I do not, however, assume this capacity in any equipment that is
unfamiliar to me, until proven otherwise.

It's not about you. It's about designing robust products.
Users are very creative with unintended applications.
My experience from design reviews is that designers tend to have
tunnel vision. A skilled designer with ability to see the big
picture is rare.

I used to build test equipment for a living. In many a design review,
I witnessed a circuit that probably met spec, but would have been
a failure in the hands of a typical user.
Hanging a SCR across the output without serious consideration of all
the ways it could get triggered would qualify in that respect.
The crowbar method of protection isn't always used, and it's external
application in any black box situation should probably be evaluated.

Your battery charging or back-up situation isn't exactly a black box,
so you should be able to get your mind around it, with a little
effort.
Again, it's not about me or my application. It's a caution that an SCR
crowbar can be very effective or a ticking time bomb if you're not
paying attention. Designers sometimes don't pay attention.

We all like to think that we're the world's greatest engineer.
But the sad fact is that half of us are below average. The cutoff
point for incompetent is well above average.
There's no reason why a commercial bench power supply should smoke on
application of external voltage, within it's normal output compliance,
save the naivete of it's designer, or the complaisance of it's
purchaser.
As sentences with no content go, that's a good one. I'd have to agree.
Power supplies are invulnerable unless they aren't.
 
J

John S

Jan 1, 1970
0
On Sat, 01 Dec 2012 22:14:05 -0800, John Larkin


Sure, but how is it suppose to tell the difference between a depleted
but chargeable battery and a battery with a shorted cell? Without
some manner of ESR measuring algorithm, they look the same to a
commodity charger. The problem is that putting current into a shorted
cell is the same as a room heater.

Putting current into a short produces no heat in the short.
 
"Jim Thompson"  wrote in message



I see some problems with that circuit. The IRLM6401 is only rated at 12V and
the gate only +/-8V. Of course this is apparently for a low voltage battery,
probably 6V, considering the 3.3 ohm load. M1 seems to be connected so that
its internal diode normally conducts, although the 0V biased gate would turn
it on. I think there should be a resistor on the gate. The shunt regulator
DZ1 is set to 5.2V, so I see this may be for a 5V logic supply. I see that a
voltage over about 5.8V will start to shunt the Vgs of M2 and turn it off
when it becomes less than about 0.6 to 0.95 V gate threshold. But it still
does not seem to provide any voltage regulation to the load, and with 0.05
ohms or each MOSET at 1.5A loses 150 mV. If the voltage is only marginally
high, or if the load increased, there could be about 2A and 1 or 2 volts on
the MOSFET which would exceed its power dissipation of 1.3W.

I don't see what is so great about this circuit. It does not have voltage
regulation or current limiting and it does not have any hysteresis to keep
it from entering a destructive linear range or possibly oscillate. The SCR
crowbar with a fuse (or maybe a polyswitch device) is more robust and far
simpler.

Maybe you or someone can explain the merits of your circuit.

Paul

look at it as a concept, of course you have to pick fet that can
handle the
voltage and current and you need something to limit the Vgs if you are
running
more than ~12V
and if you intend to run at marginally high voltage you need something
else
or make it snap on

M1 works as a diode to provide reverse voltage protection if you only
need
overvoltage you don't need it

see it as something that will prevent a supply with the wrong polarity
or
say 24V instead of 12V from frying stuff

-Lasse
 
L

legg

Jan 1, 1970
0
I used to build test equipment for a living. In many a design review,
I witnessed a circuit that probably met spec, but would have been
a failure in the hands of a typical user.
Hanging a SCR across the output without serious consideration of all
the ways it could get triggered would qualify in that respect.
Again, it's not about me or my application. It's a caution that an SCR
crowbar can be very effective or a ticking time bomb if you're not
paying attention. Designers sometimes don't pay attention.

It's just that; with a battery applied, an overvoltage condition is
highly unlikely to occurr, due to the clamping action of the battery,
itself. A failure in the regulator doesn't result in overvoltage, it
results in overcharging.

To protect against overcharging due to regulator failure, you'd be
counting on the input fuse, as the battery looks just like a high
capacity TVS.

OVP thresholds of the crude cowbar circuit are set higher than the
circuit function can normally achieve. This would be a voltage higher
than any battery you might attempt to charge with this normal output.
As sentences with no content go, that's a good one. I'd have to agree.
Power supplies are invulnerable unless they aren't.

Rephrased - you gets what you pays for, and if you insist on buying
crap, then crap ye shall get.

RL
 
J

Joerg

Jan 1, 1970
0
legg said:
Presence of the inductor will not reduce the effectiveness of ovp
crowbar protection that is located on the output terminals. Neither
will a grenading fet.

A grenading FET or inductor will, however, increase the chance of
consequential damage and is thus best avoided.
 
J

Joerg

Jan 1, 1970
0
legg said:
Applying the crowbar to the output is the most effective way of
reducing potential damage to the load - which is it's intended
function. The end user would usually prefer that your power supply
turn itself into rubble, before any harm can come to the load. Best to
test the effectiveness of any protection method, as this has priority.
A grenading fet would only pass single-fault abnormals in an enclosure
(if isolation barriers are not compromised in the process) and is best
avoided by design.

In most of my fields any sort of grenading has serious consequences.
Like Federales waltzing in for a throrough investigation. That is not
something the user wants to have happen.

In many cases (though not in this simple buck example) the presence of
an isolation barrier will alter design considerations for ovp
protection.


Could be, it depends. Usually on the amount of reservoir capacitance
available downstream. If there is a cap downstream that holds enough
energy to keep pushing an overvoltage for a while you need to crowwbar
there. But then there should also be a fuse at that spot. Creating a
dead short across a cap with a big SCR is a bit like Russian roulette.

In most cases the iso barrier only means one has to ferry the trigger
signal across. This adds cost but sometimes that is well-invested money.
 
L

legg

Jan 1, 1970
0
Presence of an inductor slows the rise of current in the fuse AND the
SCR, adding more risk to the I^2*t margin.

...Jim Thompson

Actually, the I^2t term is agravated by peakiness in the current
waveform, for a constant transfer of stored charge. If fusing is the
limit, then keeping I^2t stress of the fuse and downstream components
within the same ballpark makes coordination more predictable, ensuring
that the fuse will blow first.

RL
 
J

Joerg

Jan 1, 1970
0
Jim said:
Presence of an inductor slows the rise of current in the fuse AND the
SCR, adding more risk to the I^2*t margin.

But the *PHUT* sound will be more pronounced :)
 
On Friday, November 30, 2012 4:17:59 PM UTC-5, panfilero wrote:

You need a combination of everything. Ckt below is based on data for 5A Fast Blo http://www1.cooperbussmann.com/pdf/6d98543c-8f41-4e37-9ba5-0349eff27a02.pdf and will clear ciruit in 10ms maximum. Check your blocking diode surge ratings.

Please view in a fixed-width font such as Courier.

..
..
.. SD
.. .------|<|-------------.
.. | |
.. FUSE | -------- + |
.. 100VDC>--/\/\-----+---+----+-|>|->>| 24VDC |-->>-+----->> TO LOAD
.. 5A F | | | | |
.. | | | SUPPLY| |
.. | R | -------- |
.. | [1R] | |
.. | | - - |
.. C | | /// |
.. +| | |
.. 7500uF === | |
.. -| | __ -------
.. | |A _| TRIG| OVP |
.. | --- ---+----------------| DETECT|
.. | \ //G | | CKT |
.. | --- | -------
.. | |K [470] |
.. | | | |
.. >----------+---+------+--------------------+----->
.. | |
.. --- GND ---
.. /// ///
..
..
.. 2
.. AMP SECONDS CALC:
..
.. |
.. |
.. IPK| *
.. | * *
.. | * *
.. | * *
.. | *
.. | * *
.. | * *
.. |
.. ----+-------------------------
.. |
.. TIME ->
..
..
..
.. T 2
.. / 2 IPK
.. E = | (IPK exp(-t/RC)) dt= --- RC (1- exp(-2T/RC))
.. / 2
.. 0
..
.. T usually taken to be 50% current amplitude point T50
..
.. T50= RC LN(2)
..
.. T50 must exceed fuse rated maximum melt time at IPK
.. which is 10ms for F-rated fuse
..
.. 2
.. IPK 2
.. E = --- RC X 0.75 or C= 2 x E /(IPK x R x 0.75)
.. 2
..
.. Using Bussman TR1/MCRW5A Fast Blow with 300A interrupting
..
.. rating at 125 VDC, E= 3 A^2 x s
..
..
.. Then select R= 1R so IPK is 50A minimum with allowance for other
..
.. limiting effects such as ESR of C of addional 1R.
..
..
.. 2
.. making C the larger of C= 2x3/(50 x 2 x 0.75)= 1600uF
..
..
.. and T50/(RLN(2))= 10ms/(2 x 0.7)= 7500uF
..
..
 
W

whit3rd

Jan 1, 1970
0
Please remember to put some ferrites in series with the SCR gate
drive.

Maybe not in the gate, but at the anode? SCRs have a dI/dt
limit to be observed, and this a capacitor-being-discharged
event. It'd be a shame to blow both the fuse and the
protective SCR in each event (but the SCR is likely to fail
short, which is at least safe). Some surge protect schemes
expend fuse and SCR together.
 
I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.



Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output



1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate



2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application



3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop



Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now



much thanks!

Would help to discharge the C thru the fuse:
Please view in a fixed-width font such as Courier.

..
..
.. SD
.. .------|<|-------------.
.. | |
.. FUSE | -------- + |
.. 100VDC>----+---/\/\---+----+-|>|->>| 24VDC |-->>-+----->> TO LOAD
.. | 5A F | | | |
.. | | | SUPPLY| |
.. | R | -------- |
.. | [1R] | |
.. | | - - |
.. C | | /// |
.. +| | |
.. 7500uF === | |
.. -| | __ -------
.. | |A _| TRIG| OVP |
.. | --- ---+----------------| DETECT|
.. | \ //G | | CKT |
.. | --- | -------
.. | |K [470] |
.. | | | |
.. >---+------+---+------+--------------------+----->
.. | |
.. --- GND ---
.. /// ///
..
..
.. 2
.. AMP SECONDS CALC:
..
.. |
.. |
.. IPK| *
.. | * *
.. | * *
.. | * *
.. | *
.. | * *
.. | * *
.. |
.. ----+-------------------------
.. |
.. TIME ->
..
..
..
.. T 2
.. / 2 IPK
.. E = | (IPK exp(-t/RC)) dt= --- RC (1- exp(-2T/RC))
.. / 2
.. 0
..
.. T usually taken to be 50% current amplitude point T50
..
.. T50= RC LN(2)
..
.. T50 must exceed fuse rated maximum melt time at IPK
.. which is 10ms for F-rated fuse
..
.. 2
.. IPK 2
.. E = --- RC X 0.75 or C= 2 x E /(IPK x R x 0.75)
.. 2
..
.. Using Bussman TR1/MCRW5A Fast Blow with 300A interrupting
..
.. rating at 125 VDC, E= 3 A^2 x s
..
..
.. Then select R= 1R so IPK is 50A minimum with allowance for other
..
.. limiting effects such as ESR of C of addional 1R.
..
..
.. 2
.. making C the larger of C= 2x3/(50 x 2 x 0.75)= 1500uF
..
..
.. and T50/(RLN(2))= 10ms/(2 x 0.7)= 7500uF
..
..
..
 
On a sunny day (Sun, 2 Dec 2012 10:33:46 -0800 (PST)) it happened

[email protected] wrote in

<[email protected]>:


You need a combination of everything. Ckt below is based on data for 5A Fast Blo
http://www1.cooperbussmann.com/pdf/6d98543c-8f41-4e37-9ba5-0349eff27a02.pdf and will clear ciruit in 10ms maximum. Check your blocking diode surge ratings.

Please view in a fixed-width font such as Courier.



. SD
. .------|<|-------------.
. FUSE | -------- + |
. 100VDC>--/\/\-----+---+----+-|>|->>| 24VDC |-->>-+----->> TO LOAD
. 5A F | | | | |
. | | | SUPPLY| |
. | R | -------- |
. | [1R] | |
. | | - - |
. C | | /// |
. 7500uF === | |
. -| | __ -------
. | |A _| TRIG| OVP |
. | --- ---+----------------| DETECT|
. | \ //G | | CKT |
. | --- | -------
. | |K [470] |
. | | | |
. >----------+---+------+--------------------+----->
. --- GND ---
. /// ///


. 2
. AMP SECONDS CALC:

. |
. ----+-------------------------
. TIME ->



. T 2
. / 2 IPK
. E = | (IPK exp(-t/RC)) dt= --- RC (1- exp(-2T/RC))
. 0

. T usually taken to be 50% current amplitude point T50

. T50= RC LN(2)

. T50 must exceed fuse rated maximum melt time at IPK
. which is 10ms for F-rated fuse

. 2
. E = --- RC X 0.75 or C= 2 x E /(IPK x R x 0.75)
. 2

. Using Bussman TR1/MCRW5A Fast Blow with 300A interrupting

. rating at 125 VDC, E= 3 A^2 x s


. Then select R= 1R so IPK is 50A minimum with allowance for other

. limiting effects such as ESR of C of addional 1R.


. 2
. making C the larger of C= 2x3/(50 x 2 x 0.75)= 1600uF


. and T50/(RLN(2))= 10ms/(2 x 0.7)= 7500uF



Nice

I deleted that post on Google- see follow-up with capacitor on other side of fuse - <smacks self in head >
 
M

mike

Jan 1, 1970
0
OVP thresholds of the crude cowbar circuit are set higher than the
circuit function can normally achieve. This would be a voltage higher
than any battery you might attempt to charge with this normal output.

You're exhibiting symptoms of the pervasive disease I seek to
eradicate/prevent.
It's known by various names...
Tunnel vision.
Not invented here.
I'm too smart for my own good syndrome.
Pervasive incompetence.

Infected engineers get defensive and tell you why you're wrong
according to their limited view of the COMPONENT.

The guys you want doing your design reviews listen to what's being
said in SYSTEM context and figure out how broadening their
view might make their
designs better. They look for learning opportunities, not arguments.

Power supplies are frequently subjected to conditions not
spelled out in the spec.
The spec may not say much about AC line transients. Inexperienced
engineers may not pay attention to that at all. Same for load transients.

Pick a power supply. Grab the schematic and the designer.
Ask, "what happens at this node when I rip the line cord out of the wall?"
Most won't have even considered the possibility.

I'm not saying that it's hard to design a crowbar circuit that
doesn't have the problem. I'm saying that, if you're not
paying attention, it's easy to let a bad design get through.
Crowbar misbehavior is not an isolated case as evidenced by other
inputs to this thread.

You'll recall that I asked a question about the application.
For a dedicated application, a less-than-optimal crowbar may not be
an issue.

Here's the backstory that gets me so excited about power supplies.

I inherited a hardware group designing a computer workstation.
One component was a custom power supply designed by a local
power supply house.
We'd seen prototypes.
Our engineer went down the spec sheet running tests and signed
off on the design.
But when they got put into computers, we had random failures.
The design firm denied responsibility because our engineer
couldn't reproduce the symptom. This went on for months.

I finally gave up an took one home over the weekend.
I returned on Monday with a test fixture.
I invited the designer and his boss in for a demo.
They brought their latest revision.
I put a current probe on the transformer primary
and a transient load synchronized to the switcher.
By moving the load transient across the timing cycle, I could
change the duty factor off 50% and walk the drain current
right up the saturation curve.
I gave 'em safety glasses and asked them to put their fingers
in their ears as I embedded pieces of FET in the ceiling.
I then asked how many more they wanted me to destroy before they
got the message.
The problem wasn't on the spec sheet.
The problem wasn't on the schematic.
It was on the layout.
After all that, they still couldn't get it right.

I expect that if I'd suggested here that someone build
a current transient tester capable of 10 amps in 10ns, I'd have
been told I was an idiot.
All I can say is that it's often quicker, easier, cheaper to run
the test than to argue why it won't work. But I digress...
Back to the story.

Purchasing fired 'em and contracted with a firm 500 miles away.
Long story short, the first look at the new schematic showed
it to be identical to the old schematic.
I asked a few questions and discovered that the new firm didn't
have the manpower for our design, so they hired someone.
And who did they hire but the guy who was laid off from the
first local firm. Didn't see that coming.

I threw a hissy-fit, so they hired a consultant "fixer" to clean it up.
I met with the guy. Took me 15 minutes to decide that he exuded
competence.

The new design came in on schedule, on budget, it worked and it flew
through EMC testing with margin to spare.

The design was almost identical to the first one. But it was executed
by someone competent and paying attention to the system details.

I wanted to hire him, but he wouldn't work that cheap. ;-)

Good times...
 
J

Joerg

Jan 1, 1970
0
whit3rd said:
Maybe not in the gate, but at the anode? SCRs have a dI/dt
limit to be observed, and this a capacitor-being-discharged
event. It'd be a shame to blow both the fuse and the
protective SCR in each event (but the SCR is likely to fail
short, which is at least safe). Some surge protect schemes
expend fuse and SCR together.


That can be ok because the whole unit has become a service case anyhow.
Something on there must have catastrophically failed to cause this
tripping in the first place.
 
R

Ralph Barone

Jan 1, 1970
0
mike said:
You're exhibiting symptoms of the pervasive disease I seek to eradicate/prevent.
It's known by various names...
Tunnel vision.
Not invented here.
I'm too smart for my own good syndrome.
Pervasive incompetence.

Infected engineers get defensive and tell you why you're wrong
according to their limited view of the COMPONENT.

The guys you want doing your design reviews listen to what's being
said in SYSTEM context and figure out how broadening their
view might make their
designs better. They look for learning opportunities, not arguments.

Power supplies are frequently subjected to conditions not
spelled out in the spec.
The spec may not say much about AC line transients. Inexperienced
engineers may not pay attention to that at all. Same for load transients.

Pick a power supply. Grab the schematic and the designer.
Ask, "what happens at this node when I rip the line cord out of the wall?"
Most won't have even considered the possibility.

I'm not saying that it's hard to design a crowbar circuit that
doesn't have the problem. I'm saying that, if you're not
paying attention, it's easy to let a bad design get through.
Crowbar misbehavior is not an isolated case as evidenced by other
inputs to this thread.

You'll recall that I asked a question about the application.
For a dedicated application, a less-than-optimal crowbar may not be
an issue.

Here's the backstory that gets me so excited about power supplies.

I inherited a hardware group designing a computer workstation.
One component was a custom power supply designed by a local
power supply house.
We'd seen prototypes.
Our engineer went down the spec sheet running tests and signed
off on the design.
But when they got put into computers, we had random failures.
The design firm denied responsibility because our engineer
couldn't reproduce the symptom. This went on for months.

I finally gave up an took one home over the weekend.
I returned on Monday with a test fixture.
I invited the designer and his boss in for a demo.
They brought their latest revision.
I put a current probe on the transformer primary
and a transient load synchronized to the switcher.
By moving the load transient across the timing cycle, I could
change the duty factor off 50% and walk the drain current
right up the saturation curve.
I gave 'em safety glasses and asked them to put their fingers
in their ears as I embedded pieces of FET in the ceiling.
I then asked how many more they wanted me to destroy before they
got the message.
The problem wasn't on the spec sheet.
The problem wasn't on the schematic.
It was on the layout.
After all that, they still couldn't get it right.

I expect that if I'd suggested here that someone build
a current transient tester capable of 10 amps in 10ns, I'd have
been told I was an idiot.
All I can say is that it's often quicker, easier, cheaper to run
the test than to argue why it won't work. But I digress...
Back to the story.

Purchasing fired 'em and contracted with a firm 500 miles away.
Long story short, the first look at the new schematic showed
it to be identical to the old schematic.
I asked a few questions and discovered that the new firm didn't
have the manpower for our design, so they hired someone.
And who did they hire but the guy who was laid off from the
first local firm. Didn't see that coming.

I threw a hissy-fit, so they hired a consultant "fixer" to clean it up.
I met with the guy. Took me 15 minutes to decide that he exuded
competence.

The new design came in on schedule, on budget, it worked and it flew
through EMC testing with margin to spare.

The design was almost identical to the first one. But it was executed
by someone competent and paying attention to the system details.

I wanted to hire him, but he wouldn't work that cheap. ;-)

Good times...

Great story Mike. As someone who has worked in both the forensics and
design departments in a similar field, I can say that it's a lot easier to
cultivate the right paranoid attitude when doing post-failure root cause
analysis. Insulating the design department from the people handling product
failures doesn't help either.
 
P

P E Schoen

Jan 1, 1970
0
"Jim Thompson" wrote in message
Perhaps study it more carefully, and observe the 2nd page.
And I did say...
"Yep. I always like my own circuits >:-}
The fuse blower design was done before MOSFET's were common."
Did you miss the fuse blower discussion?

OK, it seems to work well with an LTSpice simulation:
http://enginuitysystems.com/pix/OVR_Protect.png

I had missed the second page of your pdf. But I still think the circuit
should be made more robust.

I read the fuse blower discussion, and agree that it's best, as you also
concurred.

But I don't really see the need for your circuit, and for all that
complexity it might as well include regulation and overcurrent protection.

Paul
 
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