At 07:43 PM 11/5/2005 -0200, you wrote:
>Spehro Pefhany wrote:
>
> >> In addition, SCR specs are VERY unreliable, changing drmatically wit=
h
> >> heat, voltage, and load current.
> >
> > I'm with Bob on this one- I looked at one of the suggested SCRs, the
> > C103B, and the *maximum* holding current is rated at 5mA. There is no
> > minimum spec, so there is no guarantee it will drop out with 4 or 5 m=
A
> > current from a 78x05 (in fact, it's practically guaranteed that it
> > *won't*.
>
>I don't have much experience with SCRs, but I think the idea here is not
>that the SCR will drop out closely below 5 mA, the idea is that it won't
>drop out at 5 mA or more, and that it will drop out at currents in the
>range of a sleeping PIC -- in the microampere range. Is that something
>that's doable with SCRs, even with varying specs?
>
>Gerhard

Yes, the problem is that the quiescent current of a 78x05 can be as
high as 6mA at 25=B0C, so even if you unplug the PIC and throw it in the
trash the thyristor current can't fall below that.

Only large insensitive-gate SCRs have holding current that's reliably
above that level, and of course you'd have to spend significantly more
current than *that*, controlled by the PIC, to have it reliably stay *on*.

There are lower quiescent current regulators, of course, though some
bipolar LDOs have an Iq that shoots through the roof as you approach
the dropout voltage.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rew=
ard"
speff@interlog.com             Info for manufacturers: http://www.trexon.=
com
Embedded software/hardware/analog  Info for designers:  http://www.speff.=
com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZs=
peff



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