Power sensor signal conditioing

[ericgibbs]

Woops typo.! Its this one.

Adc Value = [Vsig/Vref]*1023

As you are working down close to zero, the -Vref isnt included, just the +Vref

 

[charbel89]

Quick update.

The new proposed diagram worked. The signal is arriving at the PIC as expected. The AD736 is now correct.

As I suspected the AD736 output was not correct.

When clamp not connected the AD736 is zero, however th CA3140 is still showing 5mv on my DMM and 3 on my software..

I tried to connect Pin 1 and Pin 5 to 0V using a 10K pot however this only cut the 5mv in half on my DMMand down to 1 on my program, I know I can allow for it in my program but it will be nice if I can make sure I get zero as output of the CA3140.

Many thanks for all your efforts and time.

Woops typo.! Its this one.



As you are working down close to zero, the -Vref isnt included, just the +Vref

 

[ericgibbs]

hi,
Thats good news.

Do you a have a low voltage supply on the project, say about -5Vish.?

What tolerance are the two 100K resistors that give the Vs/2.?

 

[charbel89]

They both have a Gold band, which means 0.1 I think.

With regards to -v I do not think the demo board provides -v.

Any other way I can achieve this?

hi,
Thats good news.

Do you a have a low voltage supply on the project, say about -5Vish.?

What tolerance are the two 100K resistors that give the Vs/2.?

 

[ericgibbs]

They both have a Gold band, which means 0.1 I think.

With regards to -v I do not think the demo board provides -v.

Any other way I can achieve this?

Ideally the CA3140 should be powered from +/- 5V supplies in order to get absolute zero output. Most OPA when working on a single supply will not go to absolute zero. The CA3140 is a good choice for single supplies as will go close to zero output.

I would use the program to remove this small zero offset, the offset value will remain constant over the full output range.
The only other alternative is a another small battery, which is a pain.!

The reason for asking about the 100K's if they were low tolerance, its possible that the junction of the two would not be exactly V/2, which would give a slight dc offset.

Personally I think the results you are getting are OK.
Once the project is laid out on a pcb the noise should be reduced.

I would be interested when you have finished building, to see your test results.

 

[charbel89]

Thanks Eric,

I am already allowing for this in my software. At the moment I am deducting 2 from the values I am reading.

Plugging my laptop charger: is showing a reading of:

AD736 output......CA3140 output......Software output...reference system
2mv....................11.3/11.4mv..........(4 raw) .............. 0.045KW

My watts would be : ((2/10)*240)/1000 = 0.048 KW

Adding my 2000 watts hair dryer to the load:

77mv....................230mv...............( 76 raw)............ 1.790 to 1830KW

My watts would be : ((74/10)*240)/1000 = 1.776 KW

I think those are pretty good figures especially that they match very closely a reference system I am using.

Is it possible for me to send the schematics to someone who can build the PCB for me without remortgaging my home to pay for it .

My other alternative is to do it on a veroboard, and I was thinking of adding an LED pushbutton to switch on and off, 2 banana sockets for the clamp and a plug for +5v, Gnd, and output, and place all this in nice case with battery compartment.

Ideally the CA3140 should be powered from +/- 5V supplies in order to get absolute zero output. Most OPA when working on a single supply will not go to absolute zero. The CA3140 is a good choice for single supplies as will go close to zero output.

I would use the program to remove this small zero offset, the offset value will remain constant over the full output range.
The only other alternative is a another small battery, which is a pain.!

The reason for asking about the 100K's if they were low tolerance, its possible that the junction of the two would not be exactly V/2, which would give a slight dc offset.

Personally I think the results you are getting are OK.
Once the project is laid out on a pcb the noise should be reduced.

I would be interested when you have finished building, to see your test results.

 

[ericgibbs]

hi,
It would be a good idea to check the actual mains voltage when you doing these calibration tests.

As you know Power = V * I or V^2/R

So if the mains isnt what you think it is, it will throw your results off.

If you want to measure the Wattage of a 'resistive' load you should have a adc channel for the mains voltage.

I would lay the board out on veropcb, its a simple pcb.
I understand there are some freebie strip board layout programs on the web.?

 

[ericgibbs]

hi Charbel,

I found these two layout freebies, I hav'nt tried them myself.

**broken link removed**

Roger Lascelles VCad Page

 

[charbel89]

Cool!!

I will check them out. I found few but they are not free.

hi Charbel,

I found these two layout freebies, I hav'nt tried them myself.

**broken link removed**

Roger Lascelles VCad Page

 

[ericgibbs]

Cool!!

I will check them out. I found few but they are not free.

Hi,
This one looks about the best sofar..

Download DIY Layout Creator 1.23 - A useful tool that will help you easily create board layouts for various components - Softpedia

 

[charbel89]

This is a good one especially that it can handle different types pf boards.


I found this one:
FreePCB: freeware PCB layout software

However I do not think it is good for veroboards

Hi,
This one looks about the best sofar..

Download DIY Layout Creator 1.23 - A useful tool that will help you easily create board layouts for various components - Softpedia

 

[ericgibbs]

This is a good one especially that it can handle different types pf boards.


I found this one:
FreePCB: freeware PCB layout software

However I do not think it is good for veroboards

EDIT:

Download DIY Layout Creator 1.23 - A useful tool that will help you easily create board layouts for various components - Softpedia

just a quick try, looks a nice easy program to use.
this sample layout of your project has NOT been checked for accuracy

 

[charbel89]

Yes I am using it now.

Seems to be missing an important feature ability to drag components around which is a bit strange. I will check the help and tutorials.

Edit:

Just found it. It is on the bottom right corner 4 arrows

EDIT:

Download DIY Layout Creator 1.23 - A useful tool that will help you easily create board layouts for various components - Softpedia

just a quick try, looks a nice easy program to use.
this sample layout of your project has NOT been checked for accuracy

 

[charbel89]

Here is the diagram.

I hope I did not miss anything,

 

[ericgibbs]

Here is the diagram.

I hope I did not miss anything,

hi,
I'll look it over.. that was quick!!!!

Always leave room at the board corners for mounting pillars.

 

[ericgibbs]

hi,
There is missing connection from one end of the 100K's

also whats a 9V led.?

Correct and repost.

 

[charbel89]

Thanks.

Here is the update one I missed the removal of the track break.

The 9V LEd I was thinking of something more like:

EAO | Relays, Switches and Indicators | Switches | Switch Accessories | 8-10 mm diameter cut out

hi,
There is missing connection from one end of the 100K's

also whats a 9V led.?

Correct and repost.

 

[ericgibbs]

hi,
With exception of the missing 0V line for the supply it looks OK.
Cut the track at the spot marked 'X'

EDIT: just noticed you have linked the 0V near the centre of the pcb

 

[charbel89]

Please find one that I have connected with lines.

I think 0v is already there.

And I am not so sure about the link in the centre.

Please review.

hi,
With exception of the missing 0V line for the supply it looks OK.
Cut the track at the spot marked 'X'

EDIT: just noticed you have linked the 0V near the centre of the pcb

 

[ericgibbs]

Please find one that I have connected with lines.

I think 0v is already there.

And I am not so sure about the link in the centre.

Please review.

Your 'thick' 0V link in the centre is OK, I did notice it after I had posted, so I posted the edit.

You have made decent layout in quick time, that layout program is nice and simple to use.

Consider the LED drain on the 9Vbty.