YES, YES, YES

First, read this Wiki, which discusses the power distribution grid. There are differences in different countries, but this description likely covers your specific situation in your country...

I have prepared the following diagram and simulation. The box on the left represents the Electrical Distribution Grid. I show it as a 230V AC voltage source, one side of which (doesn't matter which) is referenced to earth ground. This is intrinsic in your country's grid. All voltages shown are with respect to earth ground.

Look at the voltage at the node "hv" V(hv), green trace. Note that is always positive. It is a half-wave rectified wave, with a peak voltage of 325V (1.414*230V). Now look at V(arduinognd), red trace. Note that it is the mirror image of V(hv), but delayed by 180 degrees. Finally, look at the current that flows through R1, I(R1) blue trace. It is as you expect, the normal full-wave rectified waveform.

The reason that this circuit is a death trap is because the instantaneous voltage on the ArduinoGnd side of the circuit goes to a peak of -325V. Imagine that you have the USB cable plugged between the Arduino and your computer. Now either of two very bad things can happen:

All desktop computers I have ever worked with have a direct connection from their metal box to earth ground. This is shown in schematic as the dashed line. The USB plug 0V pin is connected to the computer's metal box. The instant you plug this in, there will be a huge explosion, with a huge current flowing through the Arduino, along the USB cable, through the computer to earth ground. You will blow something up...

If your computer is a laptop, which might have a transformer-coupled isolated power supply, you may avoid the explosion described above, but you have now connected the exposed metal parts (d-sub shell, video connector shell, modem cable, usb cable, network cable, phone line, touch-pad) of the lap-top computer to -325V.

If there are any grounded peripherals plugged into the laptop, chances are they get back to earth ground some other way, and you are back to the explosion. If nothing touches earth ground, then the metal parts of the laptop, and all thing plugged into it are hot... Imagine your kid coming to use your laptop to play a game...

Sorry to be blunt, but if I have to explain this to you, you have no business playing with transformer-less, off-line circuits...

Mike: Mi field is not electronics and thanks for your time and help but I still a little confuse. You provided mi with the link to this circuitry. This is a treadmill controll that is comercially aproved and widely use in that industry. Are you telling me that it is not safetely designed because they did not use a transformer to step down the voltage or you are advicing me not toprototype with the arduino without isolating the high voltage from it?

 

It is the act of replacing the built-in micro-controller (which is totally hidden from the user inside an insulated box) with an Arduino (which exports its ground though its power supply, and/or through its USB connection) that turns your project into a death trap!

 

Beside the safety issue, I need your advice me about reprogramming the microcontroller. I will like to modify the way that it works. This circuitry works fine with my motor with the exception that the acceleration and disaceleration of motor takes to long for my aplication. I tried adjsting the pot that it comes with but it does not help much. If I need to modify the way the microcontroller works. I do not have knowlage of what pin 3, 17and 4, are inputing. Where can I look for advice and help? Thanks again.

 

... Where can I look for advice and help

Per the "terms of service", not on this forum...

 

I often don't think some threads should be closed, but this one should be because he doesn't think and probably never will think there is a danger.

 

Are you telling me that it is not safetely designed because they did not use a transformer to step down the voltage or you are advicing me not toprototype with the arduino without isolating the high voltage from it?

Commercial products will often contain circuitry that is not for discussion here, but that doesn't mean it is a poor design, only that it is an inappropriate project for discussion here. A sealed, finished product is very different from a hobby project. Consider the radiator fan under the hood of your car. It would be crazy to help a kid with a hobby project involving that, but it's not a safety problem for drivers that know of the danger.

 

I often don't think some threads should be closed, but this one should be because he doesn't think and probably never will think there is a danger.

That's a good point.

The only thing more dangerous than danger... Is when someone refuses to accept that it is dangerous.

 

Hello,

There have been several requests to use isolation for the circuit.
As the circuit can be lethal at use, I will close the thread.

Bertus