Lots of interesting stuff here thanks. Just to clarify I have a 2 x AAA cell supply and want to drive an LED at
150uA which will be around 2V. The AA supply will obviously not be completely exhausted by the time there is insufficient voltage to light the LED but the intensity needs to be reasonably constant until then. By the way, why do trolls despise themselves so much as indicated by the intense anger? Dave
Not if you use a really-really slow op-amp. The real part of the Zout of the op amp will be high enough to prevent the 2N3904 from tuning up and the 2N3904 is so fast compared to the op-amp that it adds no noticable phase shift.
If you are using a non-really slow op-amp: Modified version:
I've found that on some of the micro power ones it is.
The modern offsets are low enough to make something like this work too: View in a fixed-width font such as Courier.
. . . . BATT---+-------------+-----------. . | | | . | | | . [3.3k] | [330] . | | | . +--[330]-+----|---[3.3k]--+ . | | | | . | | |\\ | | . | '-|-\\| |< . | | >---[33]--| 2N3904 . +----------|+/| |\\ . | |/ | | . | | | . | | Iout . | | . [166k] | . | | . | | . COM---+-------------+------- . . .
You should have stated that in the beginning. Most of you newbies do a poor job of abstracting your requirements, next time, state what you're trying to do upfront. There are plenty of micropower OA that fill this bill and take up extremely small board space and battery current. View in a fixed-width font such as Courier.
. . . . - OR - . . BATT BATT . | | . .----------+ .------+----+ . | | | | | . [166k] | [3.3k] [330] | . | |\\| | | |\\| . +--------|+\\ ~~ +---------|+\\ ~~ . | | >--|>|-. | | | >--|
Which is an IC and is excluded by your "needs to be discrete" requirement.
Geo
That post is a forgery. The circuit is miswired and even when correctly wired will never stay within +/-20% for the same variation in power supply regardless of matching...
I think you have earned full status of nonexistence.
PLONK.
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |
No you didn't.
You simply didn't think it through...then you tried to justify yourself - what an IDIOT!
Why is it that top guys like Spehro, Bill S and others can provide helpful excellent replies and in a generous non-confrontational manner and you wade in with your beligerant nonsense? Maybe it is because you are a deeply unhappy and bitter IDIOT who is hiding a dark secret?
Are Beech and Raytheon aware of the sort of malicious noxious abuse you dish out and the sort of dumb mistakes you make? Are their shareholders aware?
Well if they weren't, they soon will be...
Dave
"Fred Bloggs" schreef in bericht news: snipped-for-privacy@nospam.com...
Sorry Fred. It isn't a forgery - I usually post via google groups, but that isn't working at the moment so I'm use the Astraweb login that Frank Bemelman passed on to me when he stopped needing it - but you are right about the miswiring (which has been corrected above).
The circuit isn't stable against changes in the battery voltage, but then again, neither is yours, though yours will suffer somewhat less because the asymmetry in this version of the asymmetric current mirror does decrease as the battery voltage drops.
-- Bill Sloman, Nijmegen
Hey John,
I think this would actually work even better than it might seem at first. The response of the eye is not particularly sensitive to absolute brightness levels, especially when they change slowly. All a constant current circuit would achieve is to hasten the hasten the end of battery life - because of the increased current *and* the inevitable voltage drop accross the regulator.
At least he doesn't hide behind an anonymous email address, "Dave".
Or is hothouse.com really your ISP?
-- John Devereux
Sorry, but nobody said anything about the eye or visible light, but thanks anyway.
No, not hiding. Just a convenient spam harvester trap which inconveniences the owners of such sites as well. Are you objecting to the second use John? Dave
He is a total retard. His very first statement is ambiguous, at best. He states "150uA which will be around 2V". I don't think this dope knows ANY electronics to be that far off the mark.
LEDs typically top out at 150mA, not micro-amps.
This LED should probably run at 20mA.
150µA probably won't illuminate it at all.
This used to be up the LM10(L)'s alley, and it still is if parts larger than S08 are tolerated - supply current is still in the 300uA range.
Getting a low voltage low power op amp is no trouble (tlv2760 and the like), but including a reference (~1V2) to keep things constant at low currents isn't easy. A TLVH431 may operate at as low as 50uA, but is not characterized to do so. A crude reference can be obtained using a simple diode.
120uA doesn't seem like much current to keep an LED visibly lit. Have you tried it?RL
What, not IDIOT2. Try it IDIOT2 Then you can shut up. Dave
What the hell is "hand balling?" Or do we really want to know that...
Apparently, "Handballing is a true expression of boy-love"
:)
-- John Devereux
As a rule, yes.
Also it will have the effect of increasing visibility of the site, pushing up its ranking in google etc.
-- John Devereux
Now add a ferrite bead in the collector. That doesn't affect the oscillation situation much, but it does improve the constantness-of-the-current at higher frequencies.
And bypass the 3.9K, for Pete's sake.
John
A couple of weeks ago, we characterized a bunch of neato Osram surface-mount right-angle leds. "Bright" was in the ballpark of 5-10 mA, depending on color, and "dim" [1] was 0.5 to 1 mA. One of them was just barely visible, in office light, at 2 uA.
John
[1] surely you're familiar with the term "dim."ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.