Shawn,

I am measuring the output of an AC variable autotransformer ("Variac").
These work by varying inductance, not resistance, to vary the output
voltage.  I cannot calculate voltage only knowing the current draw using
standard Ohm's Law.

Rgs
Ian.

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Shawn Wilton
> Sent: Thursday, 15 April 2004 5:48 am
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE:] AC Voltage measurement
>
>
> Nah, just figured you would know the resistance of the
> circuit and would
> be able to back calculate voltage after measuring the current.
>
>
>
> Ian McLean wrote:
>
> >Yer thanks Shawn, but my question is regarding sensing AC
> voltage, not
> >current.  Did you misread this, or am I missing something?
> >
> >I have looked at many options for sensing AC current,
> including the hall
> >effect sensor approach as used by these Allegro chips.  In the end, I
> >decided for the power levels I am measuring (up to about 50A
> AC, or approx.
> >12kW), the easiest solution was using a 50:5 CT, with a TVS
> diode and burden
> >resistor on the secondary output of the CT.  This works
> great for monitoring
> >average AC current.  Thanks to those on this list just
> recently for their
> >advice concerning this.
> >
> >Rgs
> >Ian.
> >
> >
> >
> >>-----Original Message-----
> >>From: pic microcontroller discussion list
> >>[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Shawn Wilton
> >>Sent: Thursday, 15 April 2004 1:51 am
> >>To: PICLIST@MITVMA.MIT.EDU
> >>Subject: Re: [EE:] AC Voltage measurement
> >>
> >>
> >>Yeah, go to Allegromicro.com and pick up some of their
> current sensing
> >>chips.
> >>
> >>
> >>Shawn Wilton
> >>Junior in CpE
> >>MicroBiologist
> >>
> >>Phone: (503) 881-2707
> >>Email: shawn@black9.net
> >>
> >>http://black9.net
> >>
> >>
> >>Ian McLean wrote:
> >>
> >>
> >>>Hi all,
> >>>
> >>>Could someone point me somewhere or give some advice on
> >>>
> >>>
> >>monitoring 240VAC
> >>
> >>
> >>>50Hz mains voltage using PIC A/D.
> >>>
> >>>The reason I wish to do this is to display the output of a
> >>>
> >>>
> >>variac on an LCD
> >>
> >>
> >>>panel.
> >>>
> >>>Immediate methods that come to mind are:
> >>>a) Step down the mains with a step down transformer,
> >>>
> >>>
> >>rectify the output,
> >>
> >>
> >>>filter, and resistive voltage divider.
> >>>b) Leave the step down transformer out of the equation, and
> >>>
> >>>
> >>use higher
> >>
> >>
> >>>voltage rated filter caps, and a bigger resistor divider.
> >>>
> >>>Both the above methods could empoy a level of PIC
> >>>
> >>>
> >>protection by using an
> >>
> >>
> >>>op-amp configured as a voltage follower with no gain to
> >>>
> >>>
> >>clip the output to
> >>
> >>
> >>>the AD 5V rail limit.  I figure both methods should give a
> >>>
> >>>
> >>reasonably enough
> >>
> >>
> >>>linear output.
> >>>
> >>>I can see problems with both of these methods however,
> >>>
> >>>
> >>particularly in
> >>
> >>
> >>>regards to ground rail impedance caused by the high voltage
> >>>
> >>>
> >>divider, esp. in
> >>
> >>
> >>>option(b), and causing major headaches for the low voltage
> >>>
> >>>
> >>ground reference
> >>
> >>
> >>>side for the digital electronics.  Can anyone suggest a
> better way ?
> >>>
> >>>Hmmm.  Maybe I should have just stuck with the simple
> >>>
> >>>
> >>moving iron meters ;)
> >>
> >>
> >>>Regards,
> >>>Ian
> >>>
> >>>--
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