Exceeding Vgs rating

Sure, Specs are weird things, 99% of the time you just stay within limits and don't worry about it. But sometimes you'd like to explore the edges... low leakage diodes are a good example. I didn't know RF parts were good... but it figures, they'd have a small area.

//story, years ago... I was talking with the guy who designed our Earth's Field NMR. He was lamenting the leakage of some diode into the signal chain and I suggested the c-b junction of a transistor. (I think it needed 50 or 60V of Vrev.) He tried and liked it. // end story

George H.

Reply to
George Herold
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You can buy a PAD-1 for $9. It's a really crappy 1 pA diode, actually a jfet characterized as a diode. Or you can buy a BFT25A RF transistor for 30 cents, and use it as a fA diode.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

I've tested red LEDs anywhere from 20V to over 200V. Very inconsistent between sources. Green (GaP) go similarly. Blue (and white, and high intensity green and cyan, they're all InGaN) die suddenly around 20-40V.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

You're correct. I'm inexperienced at designing systems that exceed the specifications of components. I taught my engineers to derate components.

This statement gave me chills: The very low frequency gate drive may, on rare >>>> occasions, exceed the max Vgs rating of 12V by about 1V, possibly 2V.

So kinda maybe we might, on rare occasions see 1V...maybe 2V beyond the spec. But don't worry, it ain't no big thing if it fails. Why not 3V on rarer occasions? How about 10V? How rare? The OP is asking for encouragement, permission, to build in a failure mode and supporting it with kinda/maybe/possibly...

If an engineer came to me with that, I'd send him back to the drawing board. "We'll discuss it when you come back with words like absolutely, definitely, here are the options and costs..."

If you read the fine print,

usually I split out that word so you can't miss it. Curiosity isn't the only thing that can kill a cat.

specified as "guaranteed by design"... that is, it's

usually? a HUGE margin.

I once was forced to build a logic system to operate in an environment with limited power availability. We literally designed in what would fit the power budget. Problem was that it was TTL and there was a huge difference between typical and maximum power specs. If you used the max numbers, we were over the power budget by 20% or so. Prototypes ran at about 20% under the max.

The QA dept refused to sign off on the design. I did a Monte Carlo power analysis that included batch effects per part showing that the probability of excess was near zero.

QA signed off on the condition that power was measured at final test and that we supply test fixtures to the service centers and included the power measurement after every repair. I don't think we ever had a non-conforming unit.

I didn't "kinda/maybe". I did the rigorous analysis. Validation procedures were in place. The process worked.

Reply to
mike

That's all a reflection of the type of products you and many folk here design. Cheap consumer electronics is a different market.

NT

Reply to
tabbypurr

I disagree with that mindset. Start with the mindset that everything should work as advertised and be reliable. Stated another way...put on your customer hat. What do you expect from your product? When you're done, go back and try to cut cost. Weigh the savings against the loss of reliability.

If you start with slipshod design, it's hard to fix little things like overvoltaging a FET.

Thought experiment: Do two designs. Start one with a quality mindset. Start the other with a minimum cost mindset. I'd bet that if you massage each design toward the middle, you'll end up at price parity with the quality design resulting in a higher quality product. And your next design will be better for it.

If you came here for validation, you already know you shouldn't be doing it.

Reply to
mike

Also in "top-end" products, sometimes, where the part is limiting the performance that can be achieved.

--

John Devereux
Reply to
John Devereux

I think you misunderstand the basics of the process. There's nothing slipshod about it, engineers in a free market can't afford to screw up on product life expectancy. One single product that fails often during warranty can be a financial disaster.

You need to know how far you can push each type of part, to understand the tradeoffs. A substantial percentage of consumer electronics involves such tradeoffs. It's normal engineering.

NT

Reply to
tabbypurr

Now hold your horses. This was not a memo to my boss asking for permission to implement something (I have no boss).

I was not "asking for encouragement, permission,....." as you claim in your other post. I said "What do you think?", which is a request for informed opinions. How would you put a question in a more neutral tone?

The design is not for production. Nor is it to be a part of a more complex system.

How did you interpret that statement? It simply meant what it said. If the transistor fails, there's no chance of its damaging anything else. It will not cause any great inconvenience to the user (me).

Reply to
Pimpom

Exactly. It's fun and enlightening, provided that it's done under the right circumstances. It seems "mike" never does that kind of thing. Pity.

Reply to
Pimpom

Sure, I think all of that makes sense. But there are all sorts here living in different design (worlds/ niches/ markets) I'm not sure if this is exactly true, but my impression of Pimpom is he's mostly a home hobbyist, making 1- 10 of something. (That could be wrong.)

I live in a small volume/ high markup world. The last project had a temperature range of 77 to 400K. I don't think any of the parts I used as heaters, sensors were rated for that low temperature. So I torture tested and used 'em. Should I have insisted on only using 'low temp' parts? They cost ~x100 more. (if they exist)

Knowing which specs are a bit (or very) squishy is part of design. I've been noodling with a fet product idea, and was worried about Vgs (and Vgd) maybe I can find some fet's that I don't have to worry about. :^)

George H.

Reply to
George Herold

I'm always doing both at once... biggest bang for smallest buck. That is the value I deliver to my customers. (well, part of it.) And then you've got to factor your own time into the equation.

George H.

Reply to
George Herold

Very true.

You're both right and wrong. I'm both a pro and a hobbyist and I sometimes blur the line between the two. I just rarely talk about the pro side of my work here.

"mike" spoke about his engineers. Well, I don't have an army of engineers working under me. But I *have* trained several engineering graduates and most of them are in key posts in both private and government agencies. Elaborating on that might be seen by some as bragging, so I'll refrain from doing it.

Mostly correct. I've seldom produced more than a hundred or so units of my designs. The reason is a combination of two factors. One is severe restrictions imposed by my location. The other is a conscious decision made long ago not to get entangled in the management of a sizeable company.

Reply to
Pimpom

Does this source have an impedance? And you do have quiescent current loadi ng limitations? What is the range, both minimum and maximum, of your input signal? What is this low power load and how mow much voltage variation acro ss it can it take? There's not enough context to offer a fix.

Reply to
bloggs.fredbloggs.fred

Yup. My favourite technological sport is doing something amazing(*) with nearly no apparatus at all.

Folks that review my designs are sometimes surprised at the percentage of LM358s among the op amps--they work fine as bias generators, temperature controllers, and stuff like that.

Cheers

Phil Hobbs

(*) for suitably modest values of 'amazing'. ;)

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

I've been out of the business for almost 30 years, but back in the day, there was nobody to train engineers. People got moved up in the organization with zero training for their new post. They didn't have the skills, so they couldn't train those who followed them up the organization. The result was pervasive incompetence at all levels.

I was fortunate to intern under an engineer who understood the problem and taught me what I didn't learn in school. I did the same for others.

If you actually trained your engineers, you deserve some bragging rights.

Reply to
mike

Yes, that's probably why they spec it at 5V, but there are mechanisms where there is obvious degradation of the LED over a relatively short time span (1000 minutes) with moderate (40V) reverse bias.

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Something to do with hot carriers. Maybe they could design a better LED for reverse bias if anyone needed it, but they generally don't.

--sp

Reply to
Spehro Pefhany

Not enough to protect the gate if it does break down.

Not sure what you mean by that.

Normally not much above the gate threshold. The circuit is about proportional control, not hard switching. Exceeding Vgs(max) is a remote possibility under abnormal conditions.

It can take the full supply voltage continuously. > There's not enough context to offer a fix.

I'm not really looking for a fix. I can think of more than one that will cost no more than 1 US cent. The aim is for elegance with the least possible parts count.

Reply to
Pimpom

I have never understood why people are are overcome by the vapors at the use of an LM358, but not by a BJT of similar cost and worse performance in the application.

--sp

Reply to
Spehro Pefhany

I just love LM358s. I use them all the time - as amplifiers where high linearity is not essential, and as comparators when speed is not important and an open-collector output is not suitable.

Reply to
Pimpom

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