Need Op Amp for design

I found "over the top" op-amps such as LT1639 that draws only 16 uA with power removed and 20 volts applied. Its output is some 690 mV under such conditions, and costs about $6 each.

There is also the LT1496 and LT1694 which draws only 235 pA under the same conditions. It costs about $7 each.

Also look at LT1079, and perhaps FET input op-amps like the TL084.

You could easily make a test circuit and see how the LM324 and other op-amps behave. Simulations sometimes don't match real world results, especially in unusual conditions like this. For example the LT1674 shows an output of -301 mV under such conditions, with input current of 235 pA.

You don't show how your voltage monitor works. That may dictate the best approach to the power-off problem. It seems that you are off-handedly dismissing most of the ideas presented without fully understanding and analyzing the cost/performance benefits. You seem to want us to provide free advice to save you a few dollars on your flashlights, while our engineering efforts have taken maybe an hour from each of us, who would normally charge $50-$100 per hour. Your attitude is not very conducive to additional help.

Paul

Right, wire up some LM324's and let us know*. A dip over copper clad with rosin solder (Kester '44 or similar) is good to a nA...at least. Well that's what I'd do.

George H.

*I was thinking JL and others may be in the know.

Those are the prices that came up from Newark, Mouser, and DigiKey.

No, I did not say they would not work, I asked if they would, and if not, f or some alternates. I know that some Op Amps will be destroyed if either i nput exceeds the Vcc rail. Some can be destroyed even if Vcc is floating. Some survive intact one or both ways.

I found some dual FETs. I did not find any with 4 or more.

In business, a fair bit. This is not a business venture, and I am not a pr ofessional EE. I am doing this for the joy of creating something unique

wn.

Any Op Amp with PNP or FET inputs will not, as long as the breakdown voltag e or greater is applied to the input. The question is not, "Hw much current will it draw?", but rather, "Will it survive the experience without damage . Yes, of course if one does exceed the breakdown potential, then the unit will draw a lot of current, but the point is moot.

The prices that come up on my computer are different:

As I mentioned, for hobby purposes you can get them for a few pennies via EBay.

Which ones would survive and not pull excessive input current when shut down? I could imagine there being some sort of boutique amp that does this but those tend to be expensive.

The datasheets usually answer that in the fine print such as here on page 4:

Quote "The magnitude of the input voltage must never exceed the magnitude of the supply voltage or 15 volts, whichever is less". The "whichever is less" would be key in your case.

While a CMOS opamp where a parasitic diode path is coming on or a BJT opamp where a BE junction begins to zener (they usually do above 5-7V) such as the LM324 may not be destroyed if you provide a large enough resistor up front the current that will flow can throw your idea out the window. Unless there is some current that is acceptable to you and still provides sufficient measuring precision (that can be tough). Nanoamps, not really going to happen. One question to ask here though is why nanoamps. Is the battery so small that its self discharge is not orders of magnitude higher?

Here is a totally different idea but it might take the fun out of the project: How about a ready-to-roll Li-Ion stack supervisor IC? Those usually can be set into a very low power "ship mode", a mode that is used when battery packs with these devices connected are spending a long time in containers, in warehouses or on store shelves. Example:

[...]

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Regards, Joerg 

http://www.analogconsultants.com/

I also thought of PNP-input single-supply op-amps as a good solution, but the O.P. seems not to notice.

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 Thanks, 
    - Win

The datasheet says "Don't do that" though. Looking at the innards on page 4 it seems you'd hit a diode path to V+:

Quote page 4, footnote (1) "Neither of the input voltages must exceed the magnitude of (VCC+) or (VCC-)".

Even if there wasn't a diode path, wouldn't the then reversed BE junction zener somewhere around 6V?

--
Regards, Joerg 

http://www.analogconsultants.com/

That's ok, Win. These people are probably too young to know who you are, or to have read AOE.

Not to mention the Radiotron Designer's Handbook, the Radiation Lab Series, Terman, AT&T, and so on.

This will probably happen more often as time goes on. Never mind. Just keep posting your gems as always. Some will listen.

For those who are interested, here are some links:

Electrons And Holes In Semiconductors by William Shockley 1950

Radiotron Designers Handbook 1954 Edition

Radiation Lab Series

Reference Data For Engineers

Terman, 1943

- url's verified on Tue, Nov 21, 2017

- all links show a preview so you can see where they will take you

That's ST's LM324A, their non-A part doesn't have the diodes.

These old silicon IC pnp trannies don't suffer from a low Veb breakdown. The abs max spec of most company's LM324 says you can go 28/ 32V above the neg rail: e.g., NSC, ON Semi, NXP, UTC, TI, Fairchild, Bay Linear, etc.

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 Thanks, 
    - Win

The original National data sheet specifically allows +32 irrespective of V+. Can't trust those eye-talian chips. [1]

But DO NOT allow an input to go below V-, operating. All sorts of weird stuff will happen.

PNPs tend to have higher zener voltage. The 324 must have some special, old-fashioned diffusion.

[1] And why do we sell them good wheat for a few dollars a bushel, and let them add water, take out the water, and sell it back to us for $8 a pound?

--

John Larkin   Highland Technology, Inc   trk 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Two more for completeness

Jim Williams Lab Notes

Bob Pease Lab Notes

For a hobby design that may be ok but not some much for industrial. It's like "You can only use parts from the days when Frank still worked there" :-)

--
Regards, Joerg 

http://www.analogconsultants.com/

BTW, even with well-vetted parts that can happen. One was a FET where production was humming just fine. One day the phone rang. "None of the boards since Thursday work and it's this one FET that always burns up". Turns out the manufacturer had "ESD-improved" the design by incorporating back-to-back gate to source zeners at 8V. The previous abs max Vgs was 20V and we had 12V on there. The abs max in the next Rev datasheet was 8V ... tzzzt ... PHUT.

--
Regards, Joerg 

http://www.analogconsultants.com/

Thanks for all those Steve!

George h.

Also, let's not forget the ARRL Handbook. That got me my start into the world of serious electronics, for very little money.

--
Regards, Joerg 

http://www.analogconsultants.com/

Yes. I also got started with Radiotron and ARRL. Very important. Many issues are now available on the web.

ARRL QEX is also on the web. They give a free sample article with each issue:

Many other books and magazines are also on the web. You just have to search a bit to find them. Sometimes the link will disappear, then reappear at a different location. So you have to verify the url before posting it. And download it before it disappears again!

Not at all in this case. It's the ST "A" part that's completely out of line, not a single other supplier of this true jelly-bean commodity part has played that game.

--
 Thanks, 
    - Win

That can work if there is explicit warning in the company parts database that the ST A version is not ECO-released and never to be purchased as substitute. Without that it'll be risky especially if contract assemblers are used (got some stories there).

What tops that is the BSS83 which comes in a P-channel and an N-channel flavor. One has a P behind it but that can be overlooked. Both born in Europe. Beats me how that could happen.

--
Regards, Joerg 

http://www.analogconsultants.com/

Techically it will, 0 is nore than -60uA (or whatever the bias current is).

:)

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use the TI part.

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