I don't see anything warranting arrest. Consequences likely result in civil suits.
The OP asked for input. I gave input.
Most of the rest of this thread is about contributors overreacting and implying that they often practice incompetent engineering.
Let me hasten to add that I don't believe that the problem is nearly as serious as implied by the very general nature of their statements. The answer is always, "it depends..." The implication that you can arbitrarily disregard device specifications that conflict with your needs is irresponsible. People come here for learned advice. That's what they should get. The DSP story on this page is an example of learned, relevant advice.
Doesn't matter at all to me, unless I unwittingly purchase the result of one of their designs and it causes me harm.
There's nothing incompetent about testing and understanding parts. There are some opamps that I wouldn't run anywhere close to the data sheet abs max supply voltage, and there are caps that I run at 1/4 spec sheet voltage, and others at twice.
I tested some polymer aluminum caps at 4x rated voltage, and at (totally unspecified) reverse voltage... cycled a bunch of them for over a month. Now I know.
RF parts are woefully unspecified for time-domain applications. So I test them and derate my own measurements.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
Manufactures have to put in safety factor. The probability that a mos rated for 12V will blow at 14 is very low.
For example, a typical IC process for a "5.5V" rated Vgs (a nominal 5V part at 10% tol) will typically data sheet spec a typical breakdown of around
14V, with 7.5V as a minimum.
Even the ESDs devices have to operate at a nominal voltage higher than the rated voltage of the part, otherwise they might activate when the system is sitting at its rated voltage due to the tolerance of the ESD protector.
Split the line so you couldn't miss it testing and understanding parts. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Never said anything about testing/qualifying parts and taking calculated risk that your samples will continue to perform beyond their specification for the life of the product (or the vendor of said product).
I suggested that it was irresponsible to encourage someone to IGNORE a Vgs spec on some unspecified part in some vague application that may or may not exceed the spec by one, maybe two, whoknowswhatthehellthenumbermightbe volts.
There's that word again... tested
some polymer aluminum caps at 4x rated voltage, and at
See a trend here?
them and Oooo, be still my heart... derate
my own measurements.
We appear to be in complete agreement. Pissing contest not required.
Now that we've established that testing is required, we can look at the cost/benefit ratio.
YOU seem to work at or beyond the state of the art. Testing and creative application of components can yield significantly better performance that's worth the risk.
If the OP can buy a better FET, or a protection diode for a penny or three, how much testing can he afford? How much risk should he take?
I submit that the cost of engineering resources wasted on this thread might have bought a lifetime supply of diodes.
We know nothing about the system that started this thread. A holistic approach can often yield significant benefit.
I once attended a design review of two subsystems designed by different engineers. Took about 10 seconds to ask the question, "What happens if the assembler gets distracted and fails to plug in that cable?" Took another ten minutes to get them to appreciate that it stressed a component and another ten minutes for them to appreciate that I wasn't gonna allow release to manufacturing with that design defect. I knew how to fix it, so I insisted that they work together to achieve a solution. They were pissed, but eventually realized that all they had to do was to move a component from one board to the other. And they became better engineers for the exercise.
Your holistic vision becomes more acute when you can expect your manager to come around with clip leads and short a power supply to ground. Or if there was a plug that could reach two sockets of the same format, I was gonna plug it onto the wrong one and hit the power switch. Cascading failures become less common. ;-) I almost never got the call from manufacturing to send some engineers over to clean up their mess.
I've always wondered if manufacturers intend you to select TVSs based on the max rating and that's that, or using a more responsible minmax constraint.
Common sense supposes that the average user will take the simpler option, wrong though it may be...
Doesn't matter much for today's logic, what with TVS diodes being useless under 5V. I suppose they would've worked just fine back in the day, a "5V" TVS protecting TTL or HC CMOS (or a 12V TVS protecting CD4000, but probably not a 15 or 18V TVS!).
That's good information, but I'd still verify by using the actual parts.
Sounds like you have a good mindset for an engineer. I'm bitching about general statements implying that you can ignore specs without testing on the ACTUAL parts you're gonna use.
Nobody has mentioned binning. Many manufacturing processes have great variability. And the parts that test better can be sold at a higher price as a different part number, or selected from the same part number. What happens when the vendor decides to obsolete a process and produce the part on a different machine that still meets the specs? Or someone else special orders high spec parts. Or my favorite...some purchasing agent decides to save half a cent.
Back in the day, when the ink on my diploma was still wet, I needed a 1% resistor for a test. I had a few dozen carbon resistors in the drawer and expected I'd be able to select one close enough for the test. Well, there weren't any. So, I went to the engineering stockroom and borrowed the whole supply of them. There were far fewer than would be suggested by a bell-shaped curve. So, I took a few minutes to bin a bunch. There was a deep hole in the distribution around 5%. "So that's where 5% resistors come from." That was my first realization that expecting parts to be better than spec was risky. Browsing the approved parts catalog confirmed that we had quite a few parts under different internal part numbers selected by the vendor for specific characteristics. And the same hole in the distribution on the generic ones.
What would you find if you ordered a bunch of 2N3904 transistors from different sources? I'd bet that most of them exceed specification, but in very different ways.
If you're lucky, the parts you get tomorrow will still work in the design you did yesterday. The probability of that goes way up when you stay within ALL the specifications. Don't worry, there will still be plenty of unspecified issues to keep you busy.
I believe that calculated risks are a critical component of a business strategy, but only if you actually do the calculations.
Nice stories, all the ways things can f'up should be talked about more. Plugging everything into every other similar plug, is how I discovered that you can fry an lm395 with a big inductor (8" OD Helmholtz coils, ~10 ohms, 100 mT at 3A.. I think.) Well, students at college physics labs discovered it. Fortunately, our customers are mostly happy to do field repairs and updates.
I'm sure John, and his customers, are tickled pink that you approve of his mindset.
But you just said that John had a good mindset for an engineer (or was that a pejorative?). That's exactly what he does. He doesn't test every device that goes on the board, rather will test a sample of parts and if they exceed the specs, he'll use that information in the design. He doesn't use *those* parts (they're probably toast) for product.
How would a "purchasing agent" change the specs on a part?
They didn't teach you much in school. Of *course* parts are binned. I bet you didn't take the temperature coefficient into account in your design.
But John would test a bunch from one supplier and figure out where they exceeded the spec and use that information.
John makes high value, low volume products that push the performance envelope. His needs are different that, apparently, yours. I'm on the other end of that spectrum, too. We worry about making a million widgets a year for five years, or more. It's a very different world.
Most resistors are laser trimmed nowadays, on a super-high speed production line; they are not generally binned, and the average 1% resistor (we only buy 1%s) is a lot better than 1%.
Some ICs may be binned. Mosfets possibly. But nobody is going to bin for max gate voltage, because measuring Vgs-max destroys the part.
I don't always believe the data sheets. We have one fast DAC that we buy from TI. We asked them about some timing ambiguities on the data sheet, and they said "That's an old Burr-Brown part; nobody knows anything about it."
Probably some junior test engineers come up with the data sheet max limits anyhow. What did some famous person say, "Trust but verify."
And it's fun to blow up parts.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
The "purchasing agent" sources a part that MEETS the specification, period! They have a whole engineering team that verifies specs. Cost reduction is a huge deal for them. If your requirement exceeds the specification, you may have a problem.
I had a production line shut down. After a lot of head scratching, we determined that one of the components wasn't doing what was expected. Funny, it didn't even look the same.
Corporate had placed a new demand on purchasing: All components will have multiple sources. Purchasing complied. They wanted the quantity price break, so we had a year's supply of parts that didn't work. Stock was rotated, so there weren't any working parts.
I went to purchasing and had them pull their copy of the corporate specification. Right on the first page, in BIG letters, it said, "DO NOT APPROVE OTHER VENDORS FOR THIS PART WITHOUT CONSULTING PRODUCT ENGINEERING."
They pulled out the memo and said something equivalent to, "Suck it. Come back with a waiver from corporate." Of course, it got resolved with only a few bruised knuckles on each side...including sales who couldn't meet deliveries... and production who had people sitting around with partial assemblies then later had to pay them overtime to catch up... and finance who wanted the cash flow we didn't get and and and ...
It was a cascading fubar that wasn't really anybody's fault. But the root CAUSE was using a part beyond its specification.
Bottom line is that shit happens no matter how smart you think you are or how explicit your instructions that violate the norm.
Let me try to anticipate your next salvo... If you change the spec to what you need and the vendor refuses to test a part to those specs at a price you're willing to pay, that's a clue that you shouldn't be doing that.
Calculated risk is great in the aggregate. In the specific instance of failure, it's a bitch. Nobody appreciates your successes. But they sure do complain about your failures.
This thread has brought back a lot of old memories... Good times...
I once tested a 5V logic device with higher power supply voltages. It lasted only a few minutes at 6.5V. I can't remember exactly what it was, or even why I was doing it, but it was most likely a GAL programmable logic device. John
Once had a 3.3V PIC overvolted by a mis-spec'd LDO, it was simmering about
3.8-4.2V and 100-200mA. Not a bad LV zener, I suppose. It was still "thinking", mostly, but USB wasn't behaving -- probably because of substrate current zapping the analog bits (PLL?) or something.
74LVC is technically rated for 5V, but running it that hot gives me a bit of the creeps.
On that thought, standard LVC logic has 5.5V recommended supply voltage, and 6.5V absolute max
Page 6 of this app note gives some insight:
formatting link
So, really any function is not guaranteed at max abs limits, just no destruction
But, why then specify it. Maybe some funny application where sometimes a pulse comes along and the VDD ramps up to abs max, but then the function of that application is not guaranteed. I cannot really think of a function in which I would use it like that
I suspect it's specmanchip. The device probably works up to abs max, but the fine print specification is no longer valid (leakage, voltage levels etc)
As Mr. Larkin said, not in these words, doing the testing yourself instead of the mfr doing it is just fine. It seems everyone now agrees on that. How ever what you criticise there is also done and also fine for some products
- and not for some others. If I were doing mil spec hardware, absolutely no t.
Snip a lot for brevity. I'm sure we all know really what the uspides & down sides of using parts past their specs are. That processes can change etc is only news to students.
Precisely!
Have you bought any consumer goods lately? And noticed how well they last? You really think it's all running within mfr spec in there?
Some things you need to test. Some you really don't, for some products. I'v e used transistors that don't even come with any spec. They're just 'transi stors.' The numbers on them I don't get any useful hits online for. I don't care, any random small low cost transistor can handle 6v 10mA, and for the low cost goods I design for the downside to that is fine.
Yes there are 4v rated trs out there, but not at that price.
I recently tested some TinyLogic gates and flops. They are rated 4.6V abs max, 3.6 recommended opearting max, and do weird non-destructive things at 7 volts. I think they have ESD zeners inside, and those are pretty tightly controlled.
I may run them at 5 volts. Or just have a 4.6 volt "5 volt" rail.
I may have a Vcc ladder, logic powered from 3.3, next stage 4.2, final stage 5.0, to keep up the speed but avoid shoot-through current.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
That's complete bullshit! "Purchasing agents" wouldn't know a spec if it bit their ass completely off. They buy what the system tells them to buy, when it needs to be bought. No more. No less. If you can't tell them what to buy (and *only* what to buy) then your company is completely broken.
...and you blame a lowly purchasing agent FOR YOUR MISTAKE? Amazing.
With no input from engineering? Sell your stock. That one's going under.
Find a new employer. Your paycheck is about to bounce.
And who didn't follow orders?
No, the bottom line is that your company is completely out of control and will probably go under.
Wrong all around.
Good time? Working for a shit company? I don't _think_ so.
for a corporation to ever be buying parts that doesn't have multiple source s is a clear hazard. It may be acceptable if there's no other way to get yo ur bleeding edge gear done, but normally it's a big no go. So why was this not implemented from the start? And am I understanding right that it was implemented part way through a pro duction run, thereby stopping that production, causing the problem it was i ntended to prevent?
It totally reminds me of my time in bigcorp. Incompetents making key decisi ons, no-one noticing there was any kind of problem. And anyone that noticed and said anything got the blame for anything they can think up. No surpris e that the mortality rate of big successful businesses is so high.
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