http://www.edn.com/article/512133-Use_an_optocoupler_to_make_a_simple_low_d=
ropout_regulator.php

I'm afraid I can't see the point to the circuit in most instances except,
perhaps, to save a few components, and that's a marginal gain if at all, an=
d
is obtained in exchange for far worse regulation capability than can be
obtained by more usual means.

For example, a similar arrangement can be obtained by using a TLV431
"programmable zener" as reference plus a single pnp transistor and a total
of 5 resistors - two being used to establish a voltage divider to program
the output voltage*. Vout can then be set to from about 2V to 18V and
dropout voltage can be close to zero. Using a TL431 allows Vout from about
3.2V to 36V. One more transistor and a few more resistors allows sensibly
unlimited Vout max - again at about zero dropout voltage.

* The cost and space for the additional resistors and TL431 would in most
cases be offset by their smaller combined cost wrt the opto and possibly a
lower overall footprint.

NOW I read some of the comments :-). Steve Hageman agrees :-)

      Russell

R1  Vin Q1b
R2 Q1b TL431-cathode
R3 Q1c gnd
R4 Vout TL431 ref
R5 TL431ref gnd
TL431-anode  Gnd
Q2e Vin
Q2c Vout
Q2b Q1c
E&OE
Cout as reqd
Cin as reqd




On 13 January 2011 06:17, Dr Skip <drskip@gmail.com> wrote:

> May help those in a previous thread:
>
>
>
> -Skip
>
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.