Re: How can I convert ac current to near linear dc voltage

From: Robert Baer (robertbaer_at_earthlink.net)
Date: 07/27/04 

Date: Tue, 27 Jul 2004 16:00:06 GMT

"." wrote:
>
> I want to purchase or construct 3 clip on current transformers that
> will produce a small current in a parallel connected resister. This
> voltage will be rectified using full wave rectifier, and then passed
> to the input of a voltage monitor card that will used to monitor line
> current in the cables that are running through the transformers.
>
> First of all will this work? I am wondering if there will be a
> linearity problem due to the voltage drop in the diodes or maybe
> something else. The input resistance of the monitor card is 1 Mohm
> and maximum 30 Vdc.
>
> Can anyone see other problems with this design. Accuracy needs to
> be within 2 or 3 %.
>
> Chris
>
>
> To send me e-mail remove the sevens
> from my address.
>
> Chrisd

  First off, you do not state the current range that you wish to
monitor.
  Secondly, you do not state of you would like 2% or 3% over the whole
range (ie: from zero) or just full scale accuracy.
  Therefore.
  Yes, a current transformer will allow monitoring in a fashion you
suggest, and can allow isolation of the monitor from the line voltage.
  The ideal case is to have the secondary shorted, and monitor that
current; a low resistance can be used and be a very good and useable
approximation; especially if a large turns ratio is used.
  Say the turns ratio is 1,000 and a toroid is used; the input (primary)
is a wire going thru the hole once.
  In this case, every amp in the primary would induce one milliamp in a
shorted secondary; you can see that use of a "large" resistance load in
the secondary (to develop a voltage for monitoring) can and will reduce
that current.
  But as long as the load resistance is fairly low, and linear, the
device can be calibrated for use.
  If the currents are low, then use of an opamp to act as a short to the
secondary and convert that current to a decent AC voltage would be of
great help.
  As long as the frequencies involved are reasonable, an opamp ideal
rectifier (FW or HW) could be used to "remove" the nonlinearity of the
diode forward loss.
  The net result of this, is a possible good linearity and accuracy
better than your 2% target.

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