Re: Thanks to all - well most...

John Larkin wrote:
On Tue, 01 Apr 2008 20:41:19 +1300, Terry Given <my_name@xxxxxxxx>
wrote:


krw wrote:

In article <1dm2v3t0g45tn45djuikmb32ttk491bdo1@xxxxxxx>, jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx says...


On Mon, 31 Mar 2008 12:20:07 -0700, Hattori Hanzo
<OutintheSnow@xxxxxxxxxxxxxxxxx> wrote:



On Mon, 31 Mar 2008 14:25:35 -0500, legg <legg@xxxxxxxxxxxxxxx> wrote:



On Mon, 31 Mar 2008 13:01:43 +0100, "Dave" <daveh@xxxxxxxxxxxx> wrote:



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.

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


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.

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."


Nice!


I have taken measurements that are almost identical. The first LED video screen I worked on had a circuit that measured the LED forward voltage. The way it was configured pumped 1uA into the LEDs, and was (barely) visible. It turned out to be easier for me to measure sub-microamp "leakage" currents with a vacuum cleaner. We had bugger all test gear (almost everything I used I was renting to the company), and the "nook & cranny cleaning" pipe from the end of the vacuum cleaner was thick black plastic that tapered down to a narrow slot; black tape made it narrower still. By placing it over a led with my eye firmly over the other end, I could see well below 1uA.


Just be careful not to suck your eyeball out.


I just used the plastic nozzle. But I'll be careful :)


the problem turned out to be easy to fix, and was inherent on the design of the voltage sensing circuit (not mine), which I duly modified. 3 years later I left, and the guy who designed the original circuit changed it back the next PCB revision (he never did like me). Funnily enough, they had a huge problem with contrast (black isnt very black when all the LEDs are glowing faintly), and had to do another PCB respin. He didnt use my exact fix, rather its dual, which ended up being more expensive. pants-wettingly funny. That firm made all their technical staff redundant this year (including him), and no longer build anything.


I've met a few really pig-headed engineers like that. They can't stand
for anybody else to be right. He's probably selling insurance or
something now.

he had a serious NIH problem, and was permanently unhappy that he was never put in charge of R&D. The first time he mentioned wanting it, *every* other engineer threatened to quit.

he was remarkable in that he could be guaranteed to find the most complex, most sensitive, most expensive, least useful solution to any problem. Asking his opininon and then specifically not doing that is a sound design strategy :)


I love that British term, "redundant."

John

Cheers
Terry
.

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