Re: Help interfacing Current Transformers to ADC

Jim Thompson wrote:
On 14 Sep 2006 13:05:35 -0700, "beananimal" <beananimal@xxxxxxxxx>
wrote:

[snip]

Then using the scematic provided by Jim (If this is what you guys are
still talking about)
http://analog-innovations.com/SED/CurrentTransformer.pdf

I recommend it ;-)


What are the specs I need to be looking for in the bridge?

Any old bridge will do. You're dealing in only 10's of mA, and PIV of
no more than a few volts.


Again using the provided schematic, the output of the OP-AMP (standard
LM-324) will go to through a single 2k resistor that is tied to the
input pin of my PIC micro. To be even safer, the net between the
output resistor and the PIC should be tied to VCC and GND via reverse
biased 1N4148 or similar general purpose diodes?

Wouldn't hurt... will safely limit any fault currents flowing thru the
ESD diodes of the PIC.


This leaves me with choosing component values for (from the provided
schematic) R1, R2 and C1. Something for which I may need help. But at
this point I need to ensure that I am on the same page as those who are
helping me. If this is the case, then I can begin to look at the
circuit and how the values are derived (so that I fully understand what
is going on).

R1 <= specified value of burden resistor (transformer specs). I'd
avoid going below around 10 ohms, otherwise there may be enough
capacitance seen at the summing node of the OpAMp to cause
instability.

R2 sets Full Scale Range

C1 is chosen to minimize ripple.

I'd test it without the PIC attached and view the output with a scope
to get yourself "calibrated" to how the circuit works.


Question: Assuming that I (we) get this all worked out. Will the
input pins of the PIC need any caps or other filters to keep them from
behaving in a strange manner when not being sampled? I have read that
this can be a problem with some uC A/D topologies, as well as Analog
multiplexors (a currnet spike on all of the inputs each time the active
input is switched to the next pin).

C1 should be all you need.

...Jim Thompson

Sadly, I do not have a scope yet. I would not even know how to utilize
one. That is surely something that is hard to self learn. I keep an
eye out on eBay for cheap units but honestly don't really know what to
look for. I also play around with audio stuff as well. It does however
appear that this is a tool I am going to need to invest in to further
my knowledge and understanding of circuits such as this.

I am going to try and get this simple circuit breadboarded ASAP and see
if how well I can implement it. I just purchased a PIC book today, as
that appears to be the easiest way for me to sample the data and then
async it to the PC as needed.

I feel (and know) I am much closer to understanding this project and
being able to move forward to actually realizing a working prototype.
Thanks!

.

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