AVR465 power meter: using shunt resistor instead of CT

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Hi!
I would like to build the AVR465 Power meter, but I was considering using a shunt resistor instead of the CT (current transformer).
Being aware of the pros and cons of both options, I would like to know what should I change in the analog front end to use a shunt resistor.

Can I just change the CT and resistor for a shunt resistor in the Neutral line?
Can I remove the Capacitor (C30 and C40)?

Thanks!

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Quote:
Can I just change the CT and resistor for a shunt resistor in the Neutral line?
No problem if you are suicidal. Do yo understand what all those lightning bolts in the PDF mean?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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js wrote:
Quote:
Can I just change the CT and resistor for a shunt resistor in the Neutral line?
No problem if you are suicidal. Do yo understand what all those lightning bolts in the PDF mean?

Yes, I understand.
I also understand that the signal ground is connected to the Neutral line and that Vcc is floating on the mains voltage. That means it's dangerous to touch any part of the circuit.
So, connecting a shunt resistor in series with the Neutral line won't increase the danger.
In fact, one terminal of T2 secondary is connected to signal ground, hence, connected to the Neutral line.

I forgot to mention that I just want to measure current in the Neutral line.

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You would obviously need to modify the software to remove the GFCI function if you are only measuring neutral current. Otherwise, it should work. If you are in 110V land, you will need to change the dropping capacitor in the supply and the divider resistors for voltage measurement.

Make sure you have proper isolation on anything connected to this circuit (serial port, ISP, scope, logic analyzer, meter) or all sorts of bad things will happen. Remember that the ground of most instrumentation and computers is tied to power-line ground.
/mike

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Quote:
connected to the Neutral line.
It may be OK for 1 case but can you guarantie that it will not be the ACTIVE line due to incorrect wiring?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Or worse still.. floating star point

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Another not-so-minor concern is the integrity of the shunt element during a fault condition on the load side. The shunt must tolerate the full fault current until the circuit breaker feeding the circuit can trip. If the shunt blows before the breaker, you could have a dangerous situation with unexpectedly energized equipment.

Tom Pappano
Tulsa, Oklahoma

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Thank you all for the warnings. I'll take them into consideration.

But what about the analog front end modifications to use a shunt?
Does anyone have ideas or experience with that?

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I have significant experience in developing power meters. The only shunt I would consider is on the secondary a metering CT. There are many variables, CT ratio, shunt resistance, gain stage and A/D parameters. If you desire the meter to be reliable and overload, and transient and protection must be considered. It is also necessary to preserve accurate phase relationship between V and I.

It all starts with a mental vision.

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How are you powering the circuit? Can you use a seperate (isolated) PSU? If so, then things are bit easier. Otherwise, it is tricky to implement if not impossible. I have done similar design using ADE7753 + shunt + M88 + online powered .

Please check Analog Devices website for ideas on input connections (hint capacitor divider psu).