ADE7753 ADC - AC Connection and CT Question

[wgarrido]

I just wanted to double check the AC connection on the reference design in the datasheet on page 15.

http://www.analog.com/static/imported-files/data_sheets/ADE7753.pdf


Figure 30 shows V2P connected to a voltage divider and what I understand to be the Hot\Live connection from the AC input. But what is confusing me is it shows the GND symbol the same as all the other connections. On the PCB I am designing I have it connected to a screw terminal which I was going to connect the live AC input. Is that correct? Then the GND would be connect to neutral and earth?

Thanks, I am working on a breakout board so I can test this before making it part of my PCB.

-William

[Magician]

Then the GND would be connect to neutral and earth?

AFAIK, you need to connect a common ground to Neutral only. Earth connects to conductive enclosure, for safety. Make sure, a person couldn't touch any part of the circuits, all user interface (buttons, pots/knobs, LCD) has to be insulated.

[wgarrido]

Thanks, that is what I thought. I had seen examples of them connected together. In my case the earth would just pass through since my case is plastic.


So then neutral would goto common ground and hot would goto the voltage divider to the ADC. Also the hot would go though the CT and pass to the out outlet for load monitoring.

One more thing

Can I use any Current Transformer with corresponding burden resistor? Then I just need to set the closest gain? Digi-Key only has 2 CT's that use 2 PC Pin footprint and one is out of stock, which was the one from another example. So would the other one work?


Out of stock from another examples
http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=121038154&uq=634862356714545170

20Ohm?


In stock
http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=121038205&uq=634862356928335270

18Ohm I think


Thanks

[retrolefty]

I would highly recommend that you use a 1:1 isolation transformer for the AC voltage input as shown in figure 30. Directly wiring neutral to your low level logic circuitry common is just not a good practice and is probably against all code laws or restrictions. Why take the risks?

Lefty

[wgarrido]

I must be missing something because I don't see that on the schematic.

[retrolefty]

I must be missing something because I don't see that on the schematic.

Don't see what? The fig 30 drawing doesn't show using a 1:1 transformer, that is just my recommendation.

[wgarrido]

Oh lol.

So the transformer would isolate both hot and neutral and then connect to the voltage divider(hot) and ground(neutral)?


I had seen both examples. At this point for the first time around I just want to see if I can get the IC working.

[jumpjack]

Does it exist any breakout PCB for this IC? I am not able to solder SMD components!

[wgarrido]

I was making one to sell but the 2nd revision blew up on me. Its on my list to get back to. Btw SMD isn't that hard to solder. I tried and haven't looked back. I need to build some AC/DC isolation and then I think my PCB will be really to sell.

Currently I am working on a OLED backpack for this: http://www.mobilewill.us/2012/12/our-first-product-usb-tester.html

[Docedison]

There are other reasons for an isolation transformer and that is both common mode noise (ground/neutral) and the ability to better 'condition' the signal prior to measurement. Line transients are much easier to deal with when passed through a small transformer as the transformer becomes a series reactance that can be utilized to enable smaller transient devices (Mov's/tranzorbs, etc) to work by limiting the available transient current by the limiting factor of the smaller isolation transformer. Since the 'spikes' are are of short duration  they pose no problem for the transformer.
Attempting to prototype any type of mains operated measurement is extremely dangerous... Primarily to the device used for measurement, protect your hard work and your health... Use an isolation transformer... Mainly because you don't really know about all the other equipment connected and whether there are any 'issues' with anything else connected to your project.

Bob

[wgarrido]

Thanks! I need to get back to this but I stopped for a bit after blowing up the prototype, Arduino Uno Rev 1 and almost my USB port. Good thing a reboot resumed the USB port.


Do you know of a small transformer that would work for this? I am trying to keep the size down. I am really close to get this to work. The first version worked. The only difference was I removed the jumpers for earth and neutral. I didn't even use the earth one. Not sure why the 2nd version didn't work.


Hmm maybe I will take a look at this weekend, would be nice to get at least one PCB prototype working.

[Docedison]

First, how small is small? Getting one that fits on the PCB is inadvisable because of the necessity of running mains wiring on the PCB. A small (< 1VA) chassis mount type type transformer shouldn't be too hard to find.
"Baack... in the Day..." I used small "filament" transformers back to back, mainly because they were cheap and readily available.
The last isolation transformer I made was 2 24V 240 VA transformers that I bought at a flea market... Lost it in a move... the movers thought it was junk.
However the device is required for all the fore mentioned reasons but mostly for your personal safety and the life of the device being developed.
As to the transformer, try Mouser. That company always has something that can be bent into working shape.

Bob

[tack]

You'd definitely want some kind of isolation. If you connect a battery source to the same neutral/earth of a power system then there are things that can kill your electronics. Under fault conditions on the power system you will get a voltage rise on a neutral earth, called a transfer potential. This is transient in nature but could potentially last long enough and rise high enough to take the ground connection above a small battery positive voltage. Typical safe values for touch and step potentials her in the UK on power Athens is 400v. Similar voltages can be transferred to a neutral earth system for fractions of a second.

Transformers in consumer equipment also serve the purpose of isolation, as well as stepping dowon voltage before rectification.