Help interfacing Current Transformers to ADC

In message , dated Thu, 14 Sep 2006, Jim Thompson writes

When you are washing vegetables, you put your hand in the water with spread fingers and move it about to trap the veg as the water drains away.

Thanks, That gives me someplace to start. I am in Pittsburgh, PA :)

Are any of the "sound card" or similar PC based scopes worth the trouble?

I have replaced the Diodes with the Schottkys. I chose common 1n5 type. I have decided to use the Amveco 1000:1 CT for the reasons you have stated (PCB connection) The docs specify a 100 ohm burden to get .1 Volts per Amp (and something about a Ratio Correction Factor of 1.02)

Yes, I am still having a hard time with the current source as opposed to the voltage source, esp when it comes to the op-amp. R2 is still bothering me and I am not sure how to determine it's value. I suppose I need to revisit some of the better OP-AMP and active filter websites and start from scratch with regards to what I understand.

Here is the updated schematic:

Bill

"Kikusui COS6100M 100MHz 5 Channel Oscilloscope"

Wow, there are a lot of these $100 100MHz scopes (I looked around a bit and some folks are saying they are every bit as good as the Tek 465? I think I do understand that there are really only 2 full featured channels.

This may be just what I want! Anybody have any opinions on them?

I'd do the 1000:1, since the OpAmp has to source the current delivered by the CT (see below).

Sort of, I'd use about 1/2 of the specified burden resistor.

No. R2 is selected such that the current from the CT, filtered, times R2 is your desired full-scale voltage.

Just think currents

Yes

Your drawing is correct.

Just use the 1N4148... speed isn't an issue here.

Build it in a metal box ?:-)

One last caution, and I hope it doesn't confuse you. It's possible that the LM324 won't be able to source output current sufficiently to support the CT current, if your full scale is over 10 Amps. In that case add an NPN emitter follower to the output of the LM324 and drive the feedback network with the emitter.

...Jim Thompson

Umm, err, I think someone has already mentioned this as a potential problem. Not to mention the fact that ct accuracy heads for the pits as burden voltage increases.

John

I've already said, several times now, use a resistor somewhat SMALLER than the specified burden resistor ;-)

I've actually done this without the rectifier, and R1=0, with a fast OpAmp, for current limit on-the-fly (cycle-by-cycle) applications., but I was reluctant to have the OP do R1=0, for fear that a cheap OpAmp will sing, and the OP is inexperienced.

...Jim Thompson

That just means that it looses 2% of its turns accuracy when driving a

All the current transformer current has to pass through R2 and into the opamp output. The input pins are very high resistance, so they respond to applied voltage(amplifying the difference voltage between + and - inputs) without drawing any current. Since the + input is at zero volts, and the input current from the CT is negative (because it comes from the - end of the bridge) the output has to go positive to suck that current through R2. But the voltage to drive that current through R2 comes from the opamp output, rather than from the CT, so its accuracy is kept high. If you want the output to be 5 V when the average of the absolute line current is 1 A, calculate what R2 value will drop 5 volts when 1/1000 A passes through it. That would be 5 k.

The capacitor across R2 is what converts the reading from instantaneous absolute current to average of the absolute current, if R2*C is longer than a cycle.

Since they are as cheap as free, why not?

Yes. Often the sellers don't know what they are either. More than two real channels is not common.

...Or, better, use R1=10 ohms between the bridge output and ground, and Rin=100 ohms between the bridge output and the op amp input. You get both a good, low resistance for the CT and low input current for the op amp. You would do well to use a precision or auto-zero op amp.

Also, for input protection I like four hefty LEDs, series pairs, each pair back-to-back for CT output protection. You get insignificant shunt current, a pretty sharp limiting voltage, and a bonus is that if you can see them, you failure indication. Just have to be sure the selected LEDs can handle the temporary inrush currents.

I've seen an easy-to-use schematic module that would put out pdf's, but I can't find it right now. I'd like to illustrate some of my comments on abse. Any pointers?

John Perry

That works. Allows scaling of the CT current that is applied to the OpAmp, but, as you point out, aggravates VOS effects.

I'm basically using the original (non-crispy :-) version of MicroSim Schematics as a front-end for PSpice.

Surfing around you can probably find it on the web, and it doesn't require licensing. Unlike OrCAD Capture, it's VERY easy to use, VERY intuitive, and it EASILY supports hierarchical drawings.

As for outputting PDF's, all you need is a printer driver for PDF... PDFWriter (Adobe, $) or CutePDF Writer (free).

...Jim Thompson

Every time I think I this worked out, you guys improve the design. I honestly have to say I feel somewhat helpless. Though I am learning, this is one of the hardest projects I have had to wrap my head around (code makes sense to me, current sources and analog stuff is a struggle). This has gone a long way to expose my gaps in knowledge and basic understanding of some the basic fundamentals of electronics and the building blocks of circuits and how they interact.

John, I would love to see your ideas (as it is becoming more and more obvious that my grasp of all of this is just glimmers of understanding that I am trying to sort out from post to post).

With regards to R2, I will be using a PIC with a 5V supply rail. From what I can decipher from the datasheet, the MAX input voltage to the A/D is the rail voltage +0.3V

I would also gather that I want to scale the voltage to cover as much of that range (0-5V) as possible to garner the most resolution I can.

If I were to decide that 20A was the max input to the CT, it would follow that the 1000:1 ration would give me 20ma at the output and 2 volts on the bridge? It would follow that to scale the 2V to 5V I would need a 250 Ohm resistor for R2?

In similar, If I decided to use the 10A CT and wanted full scale (5V) at 10A, then R2 would be 500 Ohm?

With regards to the emitter follower. I understand the basic concept but again lose some of the details of the exact method to connect the transistor. It's not something that is intuitive to me and looking at the emitter follower circuits I have seen in the past and in my "book" the emitter is usually tied to ground via a resistor. I would assume that would hold true in this case. The output (between the emitter and ground) would feed the network and the A/D. The base is connected to the output of the opamp and the collector to Vcc?

I am not sure if the (4) LEDs are to replace D1 (but would assume so)

Again, here is my interpretation of the circuit with the values I have calculated (er guessed at).

Would the LM318 be considered suitable (precision) or maybe one of the Maxim MXL1014 types?

You can also use a PDF printer like "win2pdf" as a print driver. Most of the free PDF printers do put a visible watermark or add a page to the PDF.

Bill

Well... I've only been a tinkerer for 50 years... it takes time to get good at this stuff. ;-)

What supply do you have available for the OpAmp?

Not correct. Pin 1 of rectifier goes to ground. 10 ohms goes from pin 3 to ground.

If VCC for OpAmp is 5V, you will NOT be able to output 5V to the ADC. A minimum of 8V for the OpAmp is needed.

...Jim Thompson

I can provide the OpAmps whatever they need. I think the max on the PIC was 5.5V supply. I suppose I will need to power the PIC with a different supply rail than the filter. I had counted on supplying the filter section with the supply rail from the the 15V rail that powers the other electronics in the project.

I knew this part (at least!) but somehow neglected to remember it as my brain tried consume all of the new information.

fixed:

Bill

But now you have a scale factor issue ;-)

Suppose you have 20A RMS or 20A*rt(2) peak, AVERAGE current into summing node is...

So feedback resistor needs to be ~3K for a 5V full-scale

Of course, since you used the 10 ohm, the OpAmp is no longer supplying wads of current, so you don't need the follower ;-)

...Jim Thompson

I'd probably go back to R2=787, and change R6=53.6

...Jim Thompson

Have a look at abse. I found out how to get a pdf from Eagle :-)

John Perry