Re: Help interfacing Current Transformers to ADC

Hello Jim,


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.


I'd add a resistor between C and the PIC. Low enough to make sure the ADC in the PIC works fine (I don't know their input circuitry) and high enough to avoid driving the subtrate diode of the PIC on that input pin past the abs max limit. Probably a few kohm.

You never know which supply comes up first and driving a PIC via a substrate diode might not be a good thing. Although I have seen a schematic where a guy deliberately did that.

--
Regards, Joerg

http://www.analogconsultants.com
.

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