Help interfacing Current Transformers to ADC

Either your posting client or the way you created the PDF is non-standard.

Your PDF came up as "application/pdf"

Standard PDF's show as "application/octet-stream"

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

"application/octet-stream" is a catch-all for "I don't know what this is", and is generated by the web server, not the file itself.

PDFs *should* be application/pdf so that you can have your browser open up the right kind of viewer.

Most web servers look at the filename suffix (.pdf) or the magic numbers in the file to automatically determine the type of the file; it has nothing to do with how the file is itself created (other than to create a pdf vs some other type of file).

Maybe it's a quirk of Agent and Acrobat combined.

John's PDF would only open if I added application/pdf as a file type to Agent.

I did a trial post of a PDF made by Acrobat v7. It shows (and opens) as application/octet-stream in Agent.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Hmmm... when a file is attached to a mail or posting, the user agent doing the posting needs to do the same checks a web server would do - look at the suffix and/or magic numbers and determine a file type. Any time you end up with application/octet-stream, it means whoever is doing these checks doesn't know anything about the file in question, which almost always means it's "wrong" if you intend for people to be able to click on the file and have it automatically come up in the right viewer.

Since application/octet-stream is the "I don't know" catch-all, if you tell your reader/browser to use acroread to read such files, I could post a chunk of pseudorandom numbers (which *should* be application/octet-stream, since that's all it is - a stream of octets), and you would try to view it in acroread. That would be bad.

So it sounds like your Agent is misconfigured; it should post PDFs as application/pdf and it should never try to automatically view files of type application/octet-stream. Otherwise, next time someone with a naive Agent tries to post a spreadsheet, you're going to have an unhappy acroread on your hands.

You're right... makes no sense. I'm going to delete "octet-stream" and see what happens.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Deleted. Still tried to open as "application/octet-stream".

Deleted the post. Re-posted, now works as "application/pdf".

Somehow "application/octet-stream" got tagged as PDF.

I suspect it happened when I tried to install Acrobat v7 and still keep Acrobat v4. Had so many quirks I had to move Acrobat v4 to another machine.

There are times when I could "go postal" at Adobe headquarters ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

...Was that a simulation of my circuit? Thanks, if I'm understanding it correctly. Apparently I should have kept your big (100uf?) capacitor.

I guess you found a model for the AD8551? Looks like it really did go rail-to-rail. If I'm completely off the wall, please be kind :-).

I wish the bastards at hp would get my replacement hard drive to me so I can run my own simulations and such. They've been jerking me around now for over a month. I would've bought a drive, only they keep giving me a sure shipping date. I'd rather hear a truthful "we can't yet" than a "september 15th" lie. I really like my hp printers, and thought I was in good shape with this damn Pavilion laptop.

jp

I bought an hp computer ONCE. Never again. Printers and scanners ONLY.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I had to laugh when Carly bought Compaq. One brand that I won't buy, bought out by the other brand I won't buy.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Long story short, I own an IT consulting company... Adobe long ago sold out to the bloated software gods. The package is pure garbage, conflicts with a large portion of popular software (including MS Office

2003 and XPSP1 and SP2). The software is burdened with numereous bugs, memory leaks and inconsistant behaviors. Adobe is more worried about branding and co-branding than they are about turning out quality software. It is all about the number of desktops, not the quality of the product. Ever notice how they shove the yahoo toolbar (another company that has gone down the shady path of adware and other dubious practices) down your throat? Have you noticed the ads that now are part of Acrobat?

Ever notice that even though you turn off ALL automatic updates in Acrobat, that it still forces you to go through the autoamtic update dialogs a few times a month? Imagine trying to manage that crap on a

200 desktop network!

Try to install Acrobat and Reader on the same machine if you want a real struggle. Who wants to wait for the Full Version (or light) of Acrobat to load every time they want to look at a PDF. Remember they started bundling the full version with many DELL, GATEWAY, HP and other desktops that had MS Office small business edition installed.

We will not even get started on the Macrovision and registration nightmares with the Adobe flagship products. I have had to re-authorize Photoshop and Studio over a dozen times for some of our clients who are legal license holding customers.

Sorry get so OT in my own thread...

Secondly, to confirm what some other have said. Yeah the HP and COMPAQ desktops are garbage. They are crippled with bundleware and poor architecture. The Compaq servers are not bad, but then they are not designed and made by the same folks. I would not have ANYTHING but a business class HP printer. HP printers are work horses and parts are available anywhere, dirt cheap. Getting 2 million pages out of an HP is a piece of cake. The same can not be said about the competition.

Back to logging current via a CT hooked to an A/D

I will be ordering the specified CT for this project ASAP. In the meantime I am going to hack a Xformer off of a modem or something in an attempt to breadboard the project to garner a better understanding of the circuit in a working state.

Question: Is it feasable to use a small pot in place of the resistor that grounds the Rectifier (or a pot and Resistor in parallel for safety). In my mind that would allow fine tuning of the CTs output scale to match the A/Ds full scale voltage.

Tried DejaVu?

Use a good quality fixed resistor. Use a pot across that if you must.

[snip good heads-up about Adobe and HP]

Your latest schematic cmon2.jpg has some errors, and is also way more complex than needed. Also, I don't think you need to worry too much about these little CTs bursting into flame or exploding if the secondary is open circuited, but it is always wise to use precautions. A true current source would generate an output voltage high enough to arc across the terminals or break down internal insulation, but these small CTs will just saturate and put out at most maybe a hundred volts. If there are high frequency components involved, it could be much higher, however.

I'll try to draw a suggested simple schematic that should work:

+-----+ AC-----)|(----|~ +|--+---R2---+----+----AD1 )|( | | | | | )|( | BR1 | R1 C1 Z1 )|( | | | | | AC-----)|(----|~ -|--+--------+----+----GND T1 +-----+

I would suggest R1 about 200 ohms to get about 3 VDC at 10 AAC input. R2 can be about 10K, C1 about 5 uF, and Z1 a 4.7 V zener. You could put a pot across R1 to make it adjustable, but you can also do that in software.

For best accuracy, however, I still suggest doing the rectification with an absolute value routine in software, and eliminate the bridge rectifier. Here's that circuit: +----R3----> Vcc | AC-----)|(-----+---R2---+----+----AD1 )|( | | | )|( R1 C1 Z1 )|( | | | AC-----)|(-----+--------+----+----GND T1

R1 should be about 150 ohms to get about 4 volts P-P at 10 amps input. R2 and R3 should be about 2K, and C1 should be about 0.1 uF. The input to AD1 will be just about 2.5 VDC with an AC signal varying from about 1 V to 4 V, which is easily sampled by the A/D at about 300/sec. This should be done in an interrupt service routine (ISR) using a microcontroller's programmable resettable timer (PRT). You should read, accumulate and store the A/D at the start of the routine, and then start the next conversion. When you have accumulated a certain number of readings, send it out the serial port or whatever you will use to display it.

Breadboard the above circuits and measure values to fine tune the resistors and capacitors for best results, and to become familiar with how this works. Use a scope for better visualization. You can worry about the details of the A/D later.

Paul

[snip]

At 60Hz, this should be fine.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Hello Jim,

The most concise outline for someone who is new to these filters (and doesn't speak German) seems to be this nice introduction:

WDF's have been presented by Prof.Fettweis and not all stuff was written in English. But I do have quite some material. So if you need more let me know and I'll email. Best may be to email me because my NG computer made some signs this morning that it may be about to croak.

Was out yesterday. Perspectives 2006 in Sacramento, moderated by Tom Sullivan. But the best were the speakers. Bob Dole, Tom Ridge, Tom Daschle, Soledad O'Brien, Dave Barry, Roy Firestone. This means I'll have to work today but it was absolutely worth it.

--
Regards, Joerg

http://www.analogconsultants.com

I've been to several Bakersfield Business Conferences... great speakers, but lousy accommodations in Bakersfield :-(

I'm off this morning for breakfast with Steve Forbes and Senator Kyl, courtesy of the oldest daughter, Republican Party Chair of Yuma ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I do something similar to design digital filters, although pretty much devoid of theory: I design an opamp-based analog filter, then "model" it in uP code or in an FPGA. It seems to be not only easy to design, it's usually is easier to code, in that the internal scalers seem to be less outrageous than the factors you get when you use more conventional IIR DSP filter design techniques.

.SBTTL . ANALOG LOWPASS FILTERS

; INPUT SAMPLES ARE TAKEN AT THE 139-HZ RATE AND SOFTWARE LOWPASS ; FILTERED. THE FILTER IS A 4-POLE, STATE-VARIABLE (INTEGRATOR-BASED) ; SORTA GAUSSIAN GADGET WITH A 'TRANSITIONAL' (EG, HOMEBREW) TRANSFER ; FUNCTION.

NODE 1 NODE 2 NODE 3 NODE 4 | | | | IN --(+)-[K1:INT]--(+)--[K2:INT]->-(+)-[K3:INT]--(+)-[K4:INT]-->--OUT | | | | | | | | | | | | | ^---(-1)--- | | ^---(-1)--- | | | | | | | | | '------

Caution: most pots will tolerate only small wiper currents.

And yes, Adobe products are flakey garbage.

John

They just saturate and put out some spikes. No big deal. But they can get magnetized if run unloaded, and that can subsequently degrade the low-current linearity.

John

1) The latest schematic was cmon3.jpg. I had hoped to fix the errors in cmon2.jpg. Here is the URL

Can you please comment on this (cmon3.jpg)

2) With regards to being more complex than needed. I do understand that there are many ways to skin a cat. Can you please explain your reasoning and the pros and cons of your proposed schematic vs the one Jim and John have helped me construct? I have (24) of these to build.

I don't mind more complex if there is a compeling reason or enhanced functionality.

3) I would rather use as little sofware as possible. I am not AT ALL versed in uC programming so just bit banging the A/D sample to the serial port will be hard enough. I had planned on simply using a service loop to sample the A/D ports in sequence and average every so many. ISR and other complex uC work is again something I am not ready to tackle. I am VERY new to uC coding. I would not know how to begin using software to write an ABS routine to calculate the voltage in VB, let alone doing it in uC speak.

The goal at this point is to use (3) PIC uC to do 8 A/D inputs each. The PICs will either all be tied to another PIC that is tied to the PC, or simply tied to (3) serial ports on the PC. I have a spare DIGI serial board around here somewhere.

Lastly, regarding hte POT and wiper current. What size wattage resistors and pot need to be considered for the burden and R1 and R2? I understand the rest of them can be standard 1/4 watt types.