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[SOLVED] Power metering AC live as DC ground?

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cmotif

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Power metering AC live as DC ground.

Hi All,

I came across this power metering circuit that does 230VAC to 5VDC conversion.
However I do not understand why it uses the AC switched line as DC ground.

it powers a microcontroller and a debugger cannot be connected while the circuit is connected to live. the debugger is connected to a pc which is grounded to earth.

Is there some reason to this, or is it just bad design? is there some way to design the circuit so that I can use a debugger on the live circuit?

The circuit is as attached to the post.

Thanks!
 

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It uses switched AC as DC ground because it is powered using a capacitor-resistor ultra-low cost power supply. The only reason for using this power supply is low cost and simplicity.

The first option is having an isolation transformer so you can work on a safe isolated AC line.

The second option: If you make sure your PC power supply is fully isolated and take very serious precautions you can use your debugger on the live circuit. You can also work with a laptop. Always make sure that a full galvanic isolation exists at your PC power supply and take care not touching the circuit. In addition I would recommend connecting everithing before powering the circuit.

The third option: using an isolator for your debugger. USB isolators exist.

Good luck and follow all safety recommendations!
 
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    cmotif

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Thanks for the reply! I did not know USB isolator existed.

I have another question, I'm quite new to AC circuit design hence bear with me. =)

Is there a difference if I used AC neutral for DC ground instead?
 

No difference from your circuit point of view since the circuit is floating and there is no earth connection.
However, note that the fuse will be connected on the Neutral instead of the line. In case of any faulty condition line will not be interrupted and fault currents to earth could be dangerous. Use a high sensitivity Residual Current protection device (RCD) before the circuit to protect yourself. Have you checked that your Neutral line is at low voltage from protective Earth? Some circuits have high residual voltages at the Neutral.
 
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    cmotif

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Is there a difference if I used AC neutral for DC ground instead?
Click to expand...
There's no difference from a safety point of view. Both terminals have to be considered as potentially carrying dangerous voltages which need safe insulation against accessible circuit parts. There's guarantee for AC "neutral" to keep a safe voltage limit against protective earth.

You should consider, that the schematic is apparently showing a commercial product, not a DIY construction plan. You can expect, that the designer had state-of-the-art equipment available when developing the circuit.

Besides transformerless supply, the direct mains connection is inforced by using a current sense shunt, as many simple energy meters do. I don't see a particular reason, why it's connected to "L" rathen than "N". Both variants are basically possible.
 
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    cmotif

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Thanks all for your replies, I now understand the circuit better. :smile:
I just checked my AC neutral and yes it is on same potential as protective earth.

I was thinking of using an offline switcher like the LNK306PN instead to implement the transformer-less power supply in attempt to cut down on the idle power consumption of the circuit. The series resistor is dissipating around 1W of power. I hope it works!
 

I am using isolation transformer between the AC Mains and Input to the Energy Meter. Then I have measure the energy meter ground and debugger ground. It show 2.5Vdc and 95VAC If connect will it work.
.
emontllo said:
It uses switched AC as DC ground because it is powered using a capacitor-resistor ultra-low cost power supply. The only reason for using this power supply is low cost and simplicity.

The first option is having an isolation transformer so you can work on a safe isolated AC line.

The second option: If you make sure your PC power supply is fully isolated and take very serious precautions you can use your debugger on the live circuit. You can also work with a laptop. Always make sure that a full galvanic isolation exists at your PC power supply and take care not touching the circuit. In addition I would recommend connecting everithing before powering the circuit.

The third option: using an isolator for your debugger. USB isolators exist.

Good luck and follow all safety recommendations!
Click to expand...
 

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