At 17:31 02/26/99 -0500, Steve Smith wrote:
>I have a minor problem !
>I am using the power supply transformer on a circuit which dosent have a
>constant load to measure the mains supply voltage on a '73.
>Problem, transformer is only 6 VA load regulation is less than brillant
>consequently measurements are not consistant depandant on the state of the
>load.
>I can provide an external voltage ref (TL431) and if a current sensing
>resistor is placed in the -ve supply after the DC cap a current value can be
>sensed if the TL431 ref is taken to the supply side then a load proportinal
>voltage ref can be generated for the A-D
>I need to measure a 3% supply window in the renge 207 to 277 v AC and a load
>current varation of 100mA to 500mA at 12v this provides about a 12% secondary
>voltage varation which needs corecting before measurements are taken.
>Any body had this problem before ?.... Got a better idea ?........

if i understand you correctly, you want to measure the primary side -- and
in that case i'd do exactly that. if you don't need the isolation, you can
use resistors. otherwise, maybe a linear opto coupler, or some kind of
simple pwm circuit, powered directly from the mains voltage, driving an
opto. or something the like.

ge