Need Op Amp for design

There is also a lower power version of the lm324 / 358 from ti. It's imaginatively called an LP324 and LP358

That could be a good contender if it doesn't have any brethens with the same name but less votage tolerance in shut-down. The question in this case though is what the leakage current of the highly stretched BE junction will be.

NEVER allow a CM to substitute parts!

Yes, quite, I am amazed that Jeorg allows this to happen actually given his previous postings here. (Regarding documentation control, certifications, procedures, etc etc).

Also it seems rather unfair on the original manufacturer of the part. As soon as someone releases a clone, only hobbyists should then use the original?

My client explicitly didn't allow that. Certainly not for this cap because there was a thorough warning about it in the module spec I always write during designs (whether a client wants one or not).

Yet the way things sometimes go in far-away lands ...

As I wrote I didn't allow it and neither did my client. The CM folks took it upon themselves to "cost-optimize". Which is why they had to eat the cost for an expedited re-run and fast air-shipment.

In this case it isn't a clone and if one knows about it then IMO one needs to point out the risk.

The other question in this case is that even for the versions that allow an unusually high reverse Vbe there is no spec in the datasheets about the maximum leakage current that will flow, AFAIK. Li-Ion protection is a pretty serious matter, not something to take lightly.

Weren't those the original phase inversion cheapos? Meaning you could end up with anything powering them up with a live input.

le difficulty in the concept stage.

f. A few nanoamps is fine. The device uses a 6S LiPo battery that must be monitored during operation to make sure the battery is not discharged beyo nd critical limits. To this end, the battery is connected to a 6 input dig ital battery monitor which constantly monitors the health of each cell. Du ring operation, the battery monitor consumes an insignificant amount of pow er, but it is in the milliamp range, making it too large to be able to rema in connected when the device is stored for moderately extended period of ti me.

onitor, energizing the relays when the main power is switched on. This wor ks perfectly, but there is an issue. Reed relays are comparatively large, and the resulting design cannot be shrunk down to much less than 3" x 3" af ter adding the additional components.

ional amplifiers. A pair of TL074CDR op amps is much smaller than the red relays, and mechanically I should be able to reduce the board to a much sma ller footprint. The problem is when the mains power is shut off, the op am ps will still be attached to ground, while their inputs will still be attac hed to various voltages up to 25V with no Vcc. I am thinking this may dest roy the JFET inputs of the devices. Looking at the logical design, I'm not sure, however. Both input sources are attached to the collector of a PNP transistor, which most certainly could carry current to its base, but the b ase is attached to the collector of an NPN transistor which should exhibit a very high impedance when the power is off. The base is also attached to the bases of a pair of PNP transistors, but those junctions should also off er a very high impedance to ground. The drains of both JFETs are attached to the bases of NPN transistors which offer a fairly low impedance to groun d. Given that, I am thinking the JFETs would be destroyed by an input volt age greater than 15V when the power is shut off.

op amp that can withstand up to 25V when power is shut off?

You should buy a battery management IC, some of them handle lots of cells, and this is especially true of Li batteries. There are physics based consid erations that dwarf the triviality of the electronics interconnection topol ogy.

I don't know of any problems with the original LM324.

Except the ghastly crossover distortion.

And the pitiful slew rate.

And the wimpy drive.

And the horrible stuff that happens if any of the inputs go below ground.

The LM709 had front-end zener quirks.

I don't know why you say that. I agreed it was the best solution, given th e space and cost parameters in my reply to John Larkin. Ever since then it has come up the winner in every comparison. The only real competitor is t he TLP175, and it loses on both cost and space.

I use a fair number of LM358s these days, but not for things requiring speed or great fidelity.

The original FET-input amps (e.g. TL081, LF356, MC34081 and their relatives) had the so-called "phase inversion" problem. It wasn't really phase inversion, because what happens is that when the negative CM range is exceeded the output goes high irrespective of which input is higher or lower. It's most commonly a problem with noninverting stages, of course, which is why it looks like phase inversion.

The old Motorola bipolar amps such as the MC33078 (which I also still use) were also bad for this because their output swing was wider than their input CM range, so the second stage of a noninverting cascade was liable to misbehave.

It's one of those minor warts that you get used to. As Jim Williams used to say, "Always invert, except when you can't."

Cheers

Phil Hobbs

By "these people", are you referring to me? I'm 58.

None of which matter in this situation, which is one reason why your original suggestion is definitely the front runner. I've ordered a couple of Op Amps, including a 324, from Mouser for testing. They should arrive Monday.

e op amp that can withstand up to 25V when power is shut off?

, and this is especially true of Li batteries. There are physics based cons iderations that dwarf the triviality of the electronics interconnection top ology.

I am using an off-the shelf LiPo battery monitor specifically designed to m onitor LiPo batteries during use. These are extremely common in the Radio Control hobby sector, and are readily available at very low cost. Below is an example. The only problem is these are designed to be manually disconn ected when not in use. The reed relays work perfectly, but are too large t o be able to shrink the design down to a desirable size for a lower power v ersion of the light. The 100W version uses a battery that is nearly 4" x 4 " x 12", so a smaller board is not of any value, but I would like to create a few 20W lights with a smaller footprint and less weight.

One other quirk: if one of the four opamps rails, it can mess up the other three. Shared current sources. I don't remember just how bad that is.

The good ones don't. Read up about "ship mode". I brought you an example in another post.

Have it your way...

The issues discussed there don't apply to the LM324, when used with input voltages between Vee - 0.3V and Vee +32V.