Hello,

I think it is worth to mention the inverse function of the LEDs esp. at
such a low current; I mean light generates a remarkable high voltage on
idle LEDs, so even if u have a black enclosure to set the working point is
an issue. Once I measured a ready-made PCB and it was a big question for
me for a while how it is possible the PCB is position-sensitive so I
measure different voltage at the SAME points but different side. Then the
LED and the operating desk lamp let it resolved. I told only *&$#@!

Regards,
Imre

On Thu, 18 Mar 2004, Clint Turner wrote:

> Years ago (nearly a decade) I needed to use a PIC in a product that,
> when off, had to consume under 100 uA.
>
> The "power control" processor was a PIC (A 'C54, as I recall) running at
> 32 KHz - so it pulled only a few 10s' of microamps - but as you guess,
> about any inexpensive linear regulator at the time pulled far more than
> that.
>
> Using a standard Zener was suggested - but its problem is that at very
> low currents (10's of microamps) a 5.6 volt zener may "regulate" to
> anywhere between a volt and 4 volts, depending on the zener,
> temperature, and phase of the moon.  (We tried it just for laughs.)
>
> The workaround was to use 3  cheap red LEDs in series - yielding a bit
> over 5 volts.  This voltage changed acceptably little (15-20% for the
> LEDs that we used) over a range of a few 10's of microamps to 10's of
> milliamps and it didn't change a lot with temperature.  The LEDs were
> biased at about 30 uA, as I recall, and an emitter-follower was used
> (plus a few caps, of course...) and the problem was solved.
>
> This didn't provide particularly good regulation, but that wasn't
> important in this application and the voltage was well within the 4-6
> volt range of the of the PIC.  When the "main" supply came on (being
> turned on by the PIC) it was diode-ORed and took over.  Several
> thousands of these things were built, subject to extreme environments
> (voltage, temperature, etc.) and nary a one ever had a problem with this
> circuit.  As an unexpected side-effect, the 3 LEDs provided a valuable
> diagnostic tool, being extremely dim (but visible in low light) when
> "idle" and the flashing sequence (when the LEDs were given more current
> as part of the start-up sequence) gave clues as to what might be going
> on with main computer.
>
> Nowadays there are certainly regulators with very low quiescent current,
> but in a pinch, you could always resort to the "cheap LEDs + 2N3904" method.
>
> CT
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics
>

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.