It's very possible if the 5V supply is for very low current.  For any
decent current, you will need the transformer.

Basically what you do is build a low pass filter, and set it up so that
the drop across the non-polarized HV capacitor is ~5VAC.  Then run this
into a diode, then an electrolytic capacitor, and maybe a small 78L05
or a zener/resistor combination.

The problem is that your series resistor in the low pass will have to
be rated for huge dissipation.  For instance if you wanted 50mA:

P=IV, P=(220-5)/0.05 = 10.75W   Thus I would use a 20W resistor.

Unless you need a heater in your design, and feel you can ignore the
previous isolation warnings, this method is not recommended.

Craig

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Bob Ammerman
> Sent: Friday, September 15, 2000 8:57 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: 5V DC from 220V AC supply without a transformer?
>
>
> You might want to check the PICLIST archives on this. I'm pretty
> sure there
> was a similar thread a month or three ago.
>
> Bob Ammerman
> RAm Systems
> (contract development of high performance, high function, low-level
> software)
>
> ----- Original Message -----
> From: Byron A Jeff <byron@CC.GATECH.EDU>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Friday, September 15, 2000 10:43 AM
> Subject: Re: [EE]: 5V DC from 220V AC supply without a transformer?
>
>
> > >
> > > Someone told me a while ago that I can do without the expensive =
> > > transformer, diode bridge and regulator to get  5V power from
> a 220 - =
> > > 250V AC line to drive a PIC with some other components like a relay, =
> > > triac, leds etc.=20
> > >
> > > I remember them talk about caps and zeners..=20
> > >
> > > (I am use to the XFormer and the well known 78L05 method)
> > >
> > > Can sombody help me with a plan
> >
> > Microchip has a application note on a transformless power
> supply. The PDF
> can
> > be found here:
> >
> > http://search1.microchip.com/ie/appnote/category/pic16/91008a.pdf
> >
> > Now here's my stearn warning in all caps:
> >
> > NOTE THAT THIS DESIGN AS WITH ALL TRANSFORMERLESS DESIGNS HAVE A DIRECT
> > CONNECTION TO THE HOT AC LINE. CARELESS HANDLING OF SUCH A CIRCUIT CAN
> RESULT
> > IN SERIOUS INJURY OR DEATH! TREAT ALL PARTS OF THE CIRCUIT AND
> ANY DEVICES
> > CONNECTED TO IT AS HIGH VOLTAGE AC!
> >
> > Note you'll have to pay attention to the reactance of the cap
> because the
> > voltage is doubled.
> >
> > Good Luck.
> >
> > BAJ
> >
> > --
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