Gotcha.  Thank you for clarifying.


Ian McLean wrote:

> 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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