I'm new to Arduino, and have a quick question. We're trying to interface the Arduino with the Kill-a-watt to measure the current and voltage, and are following the tweet-a-watt project. However, instead of using the XBee, we'd like to interface the Arduino directly. I'd like to be a little wary before I make the connections, seeing as the Kill-a-watt uses a transformlerless power supply scheme.
Do you think if we connect the voltage and current reading directly to the analog inputs of the Arduino, we'd have to worry about any safety issues? The schematic of the Kill-a-watt is below:
What I'd like to do is to connect the pins X3-3 and X3-4 on this schematic to the Arduino analog inputs. Since the Arduino itself is currently powered through the USB port of the laptop, I don't want to cause any issues with the grounding.
Under "Transmitter schematic," you'll see what I mean. (I imagine your smelling sense must be very active right now from all the Darwin awards)
My worry is that I am powering the Arduino from the USB of my computer, while the input is coming from the transformerless setup. The two have separate grounds. Do you think this will be an issue?
You are comparing "apples and pears"
When in use the tweetawatt has no physical user interface so there is minimal risk to the user.
However connecting an arduino implies a user interface (switches, usb to computer, pots, etc) so there is a real risk to the user. Therefore an opto-isolator is essential to minimise end user risk.
Thanks for the reply. In this instance, the implication you speak of is not as clear. We'd like the Arduino to make these measurements, and control two other shields that transmit this data wirelessly. Ultimately, the Arduino will be entirely powered by this transformerless power supply, similar to the Tweet-a-Watt.
For the prototyping though, we obviously would prefer having a permanent USB connection that we can upload the new code, and read some values from the computer. As I understand, this is when the safety issues arise, right?
So in essence, we should use the opto-coupler during the prototyping phase, but not once we get things running? Is this a fair assessment?
Also, out of curiosity, is there a way to have the analog sensor's ground be set independently of the power supply ground?
If you use a coupler for development and commissioning, why remove it for normal operation. As to isolating grounds, the practical way to do that is via the optic coupler. Apart from that, when you develop a circuit that contains a particular component, eventual removal of that component may have a material effect upon calibration. You've already bought it and used it so why not keep it in operation, unless of course you like living dangerously.
That way you gain some protection while working with the circuit. However, as soon as you hook up any device that is grounded (PC, scope...) that protection is GONE.
And having the arduino connected to your PC while another part of it is connected to mains without any form of isolation barrier is not a good idea at all.
--> code code code ... upload new firmware ... disconnect from PC ... connect to kill-a-user ... test ... disconnect ... repeat
Google for "AVR465". Maybe it will give you some inspiration.
After nearly 50 years in the electronics industry, I can tell you that in the engineering profession one DOES NOT 'Mess with Mains AC".... Without an isolation barrier and a very good one at that.
Should you just destroy the equipment under test would be your lucky day, the more conservative the design the better. Assuming that this device is for your own use... It is Still Most Dangerous
and Not for the general public. One more thought... In the US we have various safety organizations that would prohibit the sales of that device as unsafe and Europe is a great deal more strict
