what load current do you have and from what supply  ?
Example: in  5V, out 3.3V supply current 100mA.
Answer: yes  R = ( 5 - 3.3 )/ 0.1 = 17 ohm, choose R=15ohm/0.5W
dissipation on zenner without load : 0.3W choose a 1W zenner
don't forget 10uF(tantal) or 10uF (paper dielectric) + 100nF parallel with
zenner, probably you have to choose a zenner to have the right 3.3V value
( read data spec about absolute supply values for your chip )
Vasile

On Mon, 12 Nov 2001, Chetan Bhargava wrote:

> I just need to power an EZ-USB chip that uses 3.3v. Would the zener approach
> be ok?
>
>
> ----- Original Message -----
> From: "Vasile Surducan" <vasile@L30.ITIM-CJ.RO>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Sunday, November 11, 2001 11:42 PM
> Subject: Re: [EE]: ~ 3.3V from 7805
>
>
> > Well, if you don't want only 50mA why don't you use a simple 3.3V zenner ?
> > regards, Vasile
> >
> >
> >
> > On Sat, 10 Nov 2001, Richard Sloan wrote:
> >
> > > Unless you really need the 5V!
> > >
> > > I just did exactly this on a project and for me it worked out fine.
> > >
> > > My 3.3V is only at 50ma
> > >
> > > R.
> > >
> > >  >>  On Fri, Nov 09, 2001 at 02:59:15PM -0800, Chetan Bhargava wrote:
> > >  >>  > Hello list,
> > >  >>  >
> > >  >>  > Is it possible to achieve near about 3.3 volts out of a 7805 plus
> a
> > >  >>  couple
> > >  >>  > of IN4007 (1 volt drop) on the output pin?
> > >
> > >  >>  It'll have very poor regulation characteristics dependant on the
> diodes,
> > >  >>  temperature, and current draw.
> > >
> > >  >>  A much better idea is to replace the 7805 with a LM317 and a couple
> of
> > >  >>  precision resistors.
> > >
> > >  >>  BAJ
> > >
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