At 14:26 14/06/01, Douglas Butler wrote (in a [PIC:] thread)

>Letting the PIC be the AC source lets you choose an operating frequency
>to avoid outside interference.  Also consider synchronous rectification.
>  You can do it easily with a OP amp and a FET switch.  It will reduce
>all sorts of interference.
>
> > -----Original Message-----
> > From: Vasile Surducan [mailto:vasile@L30.ITIM-CJ.RO]
> > Sent: Thursday, June 14, 2001 6:13 AM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: Re: [PIC]: AC current
> >
> > On Thu, 14 Jun 2001, Tony Nixon wrote:
> >
> > > Hi all,
> > > Does anyone have any ideas of measuring a small AC current with the PIC
> > > A2D?
> > >
> > > A friend of mine has a hydroponic garden and wants to measure the
> > > current flow between 2 probes which are fed with 16VAC.

If you want to measure the fluid resistance with probes, then by exciting
them with AC from a high impedance source, and measuring the resulting
voltage, will allow you to calculate resistance.  One probe can be grounded
to reduce noise pickup.

Actually this is what I want to do for water detection... can anyone
suggest a scheme to generate AC with a high impedance, to measure the
resulting voltage, AND to reject any interference from power cables and
withstand static discharges. Nice and cheap too please, with up to 4
channels  :-)

John

-----------------------------------------------
John Lawton Electronics
Custom Electronics Design & Development
design@jle.co.uk   http://www.jle.co.uk
-----------------------------------------------

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