# Sensing signal on a power line from high side

I have a circuit design as above. I'm trying to sense the voltage difference accross the $0.1 \Omega$ resistor. I physically connected the sensory circuit to the low side of the AC line, and saw that it is working alright.

I'm going to make a boxed circuit of this, and I'm going to connect it to the wall outlet with a standard plug, seen as below:

The plug is symmetric. I can accidentally connect it reversed (on the other hand, i don't want to check the polarity every time I'm connecting it to wall outlet). In that case, the sensory circuit is connected to the high side as seen below:

My question is, What happens if the sensory circuit is connected to the high side? Would a terrible thing happen? Do I need to do any modification on the overall circuit for making it connectable to the high side?

Supose that,
RMS of the AC is 230V.
VCC is 5V, and it is electrically isolated from the AC by a transformer.
All circuit elements are rated to work between -5V to +10V. The electrical device drains small current (below 10A RMS).

• If you do not know the answer to the question already, you should not be connecting anything directly to the mains. You will kill somebody. – markrages May 7 '12 at 18:35
• At least tell us that VCC and GND and all electrical connections are completely isolated from the end user, with no exposed ground screws or connectors. Because VCC and GND have a 50% chance of having 230VAC superimposed on them. This is still a dangerous (possibly illegal) construction method. We don't have AC/DC radio sets anymore for a reason. – markrages May 7 '12 at 18:38
• @markrages This is not going to be a commercial product. I'm just making a project in my home-lab. If I were to make a product for other people, of course, I would take required safety cautions. – hkBattousai May 7 '12 at 22:25
• @hkBattousai I hope you will not sneeze and accidently kill yourself by touching somewhere you shouldn't. – abdullah kahraman May 8 '12 at 8:18
• @hkBattousai by the way, why don't you introduce some circuitry to lessen the death risk? It will be trivial and you will learn, if you do not know. If you know, then I think your are too lazy to work with high-voltage :) And yes, I know nothing about high voltage.. – abdullah kahraman May 8 '12 at 8:24

If VCC and GND are electrically isolated from Phase, Neutral and Earth, it doesn't matter the direction with which you connect the plug.

Your sensory circuit doesn't even know there's 230 VRMS somewhere else. It will just see the voltage across the current-sense resistor.

• Given that the way he has drawan it and the way he has described it do not match, there is no way to be sure tha your answer is correct, or safe. – Russell McMahon May 7 '12 at 15:05
• @RussellMcMahon I don't understand. The OP wrote "VCC and GND are electrically isolated from Phase, Neutral and Earth" in his figures. I assume he knows what "electrically isolated" means. Otherwise, answers are too long, and don't focus on the point. – Telaclavo May 7 '12 at 15:33
• His diagrams show a connection from one pin to the "ground of his circuit - whatever that means. I assume that given he is asking a fundamentally very simple question here he may NOT "know" what electrically isolated means in practice. eg if the 0.1 ohm sense resistor goes O/C - as well it may - so that 230 VAC is then connected across the sense input terminals - as it would be - is his circuit still "isolated"? Would this destroy the sense circuitry. Will the sense circuitry conflagrate and break down the isolation? Will ...? – Russell McMahon May 7 '12 at 15:43
• @RussellMcMahon You are right, they are not electrically isolated, but a one-wire connection between the two "worlds" won't do any harm to the circuit. It may do to him, if he touches any node in the sensory circuit. Of course, if the current-sense resistor breaks, he'll have problems (I've pointed that out in other answers). – Telaclavo May 7 '12 at 15:54
• I'm hoping that his diagram is wrong and that in fact he has genuine isolation despite the diagram. If not then "all bets are off". Allowing mains in to an "isolated" circuit by a single wire gives Murphy license to do about anything at all. – Russell McMahon May 7 '12 at 17:32

The answer depends on your "sensory circuit". As you have provided zero information about how the sensor works and have not included the utterly obligatory actual circuit diagram then it is not possible to answer your question with certainty.

You say to ignore safety but the fact that you are asking this question at all strongly suggests that for a competent person to answer this question and to ignore safety would be dereliction of duty.

Indications are that the circuit will work as well with the sense element in either lead. Your statement that "it is working alright" should be treated with great care. The connection between sense circuit ground and one input pin does not make sense if the unit is electrically isolated as stated.

If the 0.1 ohm sense element goes open circuit then some of all of the whole input side of your interface goes to phase in either configuration shown. This may be a safety issue but in the absence of a circuit we cannot be sure.

Your transformer must, of course, be rated to handle worst case voltage input (say 250 VAC+) and inductive spikes and general surges or dips.