Re: Current Transformer to 0-10 Volts

Thanks for the info. I still can't justify the overpriced
transducers. I was looking at analog.com at some of their power
metering IC's like the ADE7757. Are there other products like this to
simplify getting the info into some form analog signal or
communication protocol like RS232?

On Feb 5, 5:42 pm, "Tom Bruhns" <k...@xxxxxxx> wrote:
On Feb 5, 12:01 pm, "Tom" <thickerson...@xxxxxxxxx> wrote:





I want to take a current measurement of 0 to 20 Amps AC using a
current transfomer, and then convert the current siginal to input into
a Analog input of a PLC (programmable logic controller). I know they
sell current transducers that do this alreay, but I want to make about
20 of these.

The PLC will take 0-10V DC, or 4-20mA

I was thinking about using a 50:5 Current Transformer, and a 5 ohm
resistor.

20 Amps ----> 50:5 ----> 2 Amps

Voltage = 5 * 2 = 10 V

Am I on the right track or does the current transformer output a
sinusoidal value.

Note that 10V at 2A is 20 watts you'll be dissipating in the load
resistor! I'd suggest that you go to a much higher turns ratio.
100:1 (500:5) might be reasonable. Then the output is 0.2A. You could
use an even higher ratio. If the transformers are easy to get, I'd
look at even 1000:1, though you may have some trouble finding those.
If you used your original 10:1 ratio, the ten volt drop would reflect
back as a 1 volt drop in your line, which is way more than you need to
allow. A ten volt output at 1000:1 reflects only ten millivolts drop
along the monitored line--actually somewhat more because of less than
perfect coupling, but still not a lot. A ten volt output at 1000:1 or
even 100:1 is also much more likely to be a reasonable burden for the
transformer. At 1000:1, a ten volt output with 20A in the primary is
20mA secondary current and 200mW dissipation.

You'll need to rectify the output; an op-amp precision rectifier is
appropriate. Then you need to convert the output to whatever the PLC
wants. The precision recitifer can easily be made to put out 0-10
volts, even if the input is only 1 volt instead of 10, but then you
need power to run the op amps. If you _know_ that your circuit will
always have some minimum current in it and the ratio between min and
max isn't too great, you could probably arrange to run the op amps on
the current transformer output, but that's not a wonderful idea from
the standpoint of precision, since the amplifier power will appear as
additional transformer load. A better idea would be to use the 4-20mA
loops, especially if these sensors will be some distance from the
PLC. You can find example 4-20mA circuits in op amp manufacturers'
data sheets. I believe that Linear Technology is one good source for
such circuits. Try their ap notes, too.

Too bad you didn't have this need a year or so ago. Marlin P. Jones
had some nice split-core 4-20mA AC current transducers with a jumper
for, um 10A, 20A and 50A full scale as I recall, for about $10 each.
But even if you have to pay $100 or more each for them, you'll
probably be better off buying them. I have a feeling from the last
sentence of your posting that you'll be in trouble trying to build
them.

Cheers,
Tom- Hide quoted text -

- Show quoted text -


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