For a simpler, more reliable, and possibly even a less expensive way to do this,
take a look at a part called the VB409 from ST Micro Electronics
(http://us.st.com).

It also has the advantage of producing two output voltages, if you need them (a
5vdc 5% regulated, and a 12~15vdc unregulated).  It only requires a 35~50v 220uf
electrolytic cap, two resistors, and a rectifier to operate (maybe a small
inductor too, for more efficient operation).  Using two diodes (half rectified)
on this part will give you a -2.5dc and +2.5dc (a 5vdc bias) from "true ground",
where using a full bridge will put the circuit at 1/2 line potential (but it
still generates a 5vdc bias).  It can generate upwards of 70 or 80ma at 5vdc,
and can have peaks upwards of 400ma on the 12~15 supply up to 10ms.

The only trick to using this part, is actually finding a distributor that stocks
it as a standard off the shelf part.  Let me know if you happen to run into a
distributor like that....  Arrow has it listed at $1.23 each, but doesn't show
any in stock (i.e., special order item).

Bob

-----Original Message-----
From: Charles Craft <chucksea@MINDSPRING.COM>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Friday, March 09, 2001 3:28 PM
Subject: [EE]: Transformerless power supply for PIC


>I searched the archives and didn't come up with much.
>
>Any gotchas before diving into this type of circuit?
>Most of the other circuits I've seen on the web use a bridge rectifier instead
of just two diodes like the MicroChip circuit.
>Advantages of one over the other?
>
>thanks
>chuckc
>
>
>
>
>http://www.piclist.com/techref/microchip/power.htm
>
>
>http://www.microchip.com/download/appnote/category/pic16/91008b.pdf
>
>In most non-battery applications, the power to the
>microcontroller is normally supplied using a wall
>mounted transformer, which is then rectified, filtered
>and regulated. In most applications, this method of
>generating the regulated voltage is cost effective and
>can be justified. However, there are applications where
>the PIC12/16/17 is the main controller and low voltage
>is not required by other components except the PIC12/
>16/17. In these instances, the cost of the transformer
>becomes the sizable cost factor in the system. Transfor-merless
>power supplies, thus, have a distinct advan-tage
>in cost as well as in size. The disadvantages of
>using a transformerless power supply are: (1) low cur-rent
>supply and (2) no isolation from the AC line volt-age.
>The PIC12/16/17 microcontrollers draw a
>maximum of 10 mA, even at the highest frequency and
>voltage of operation, therefore low current availability is
>not an issue. AC line voltage isolation can be
>addressed by using MOVs or transient suppressors on
>the PIC12/16/17.
>
>--
>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
>
>
>

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