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I try to make a 110VAC energy meter connected to my computer out of the CS5490

This IC measures voltage and current over a voltage divider and shunt resistor respectively and then calculates the power. Besides connecting the measure inputs, the IC also needs power. In a stand alone metering application this power needs to come from the same line and that is the example they give in the datasheet. But in my case I have an external power supply that I would like to use. Also I think (correct me if I am wrong) one should not connect the Neutral from the line to the ground from a PC.

Fig 21 on page 54 of the datasheet is then plain wrong?

What is the correct way of doing this? Can I do it without the need of adding another power supply? And what does a pointing down triangle mean compared to the more usual ground symbol?

It's an awesome group of EE's here, any help is highly appreciated!

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Also I think (correct me if I am wrong) one should not connect the Neutral from the line to the ground from a PC.

Right. Never connect neither the phase nor the neutral of the mains to any node of a PC. If you did that, touching any node (including the chassis) of the PC could kill you.

Fig 21 on page 54 of the datasheet is then plain wrong?

It is not wrong. It just assumes that all nodes around the CS5490 and the "Application Processor" (including the "triangular" ground) are electrically isolated from any end user. This means that, if you want to connect a PC to the CS5490, you MUST insert isolation between them. Easiest: insert isolation at digital links like RX and TX (isolating analog links is much more complex), using either optocouplers or magnetic isolators like these ones (I've used many of them, and they work very well. They allow you to work at higher frequencies than with usual optocouplers. Some of them even allow you to transfer power to the other side (the isolated one). Look for "isoPower" in that same page).

Can I do it without the need of adding another power supply?

You need a power supply for the CS5490, which cannot be the same as your PC's power supply.

And what does a pointing down triangle mean compared to the more usual ground symbol?

Conceptually, the same. All of them mean grounds (0 V reference points, for their "local" circuitries), but different ground symbols mean different grounds.

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  • \$\begingroup\$ Thanks for your explanation, it starts making sense now. I still find it hard to believe that the whole IC waves up and down with the 60Hz 110V without impacting its performance at all. Of course I know, the IC has no way of knowing about the real ground if it is isolated from there, but still,.. What if it was 60MHz, would it still work fine? (For any electronic circuit in general) \$\endgroup\$ – Bastiaan May 8 '12 at 16:09
  • \$\begingroup\$ @Bastiaan Think of the neutral staying fixed, and the line wobbling positive and negative with respect to neutral : ) \$\endgroup\$ – Adam Lawrence May 8 '12 at 17:23
  • \$\begingroup\$ Yes, but you can plug-in your Kill-a-watt up side down right? At least when you're in Eurozone where the two connectors of a power plug are identical. Still it will work fine, while the "ground" of your IC is connected at 220V (!) \$\endgroup\$ – Bastiaan May 8 '12 at 17:44
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This sensor is mains-connected. The power supply inside (or outside, if it's a laptop) your PC will have some form of isolation transformer which electrically isolates the mains from the PC power rails. This means your PC is isolated from the mains.

The datasheet is 100% correct. There's no electrical reason why an 'application processor' cannot be referenced to the same return as the sensor. Inexpensive 'white good' appliances like washing machines and microwave ovens often have all their control circuitry non-isolated from the mains, and provide isolated user interfaces (buttons and knobs).

That being said, PC != 'application processor' in the context of that datasheet. You cannot mix isolated and non-isolated circuitry without dire conesequences.

As Teleclavo says, you need some form of isolation to bring those signals to and from the PC. You need to be extremely careful when you're working with mains-referenced circuitry - not only the inherent electrocution risk, but other dangers like PCB creepages and clearances. If you're just doing hobby work, I would strongly recommend using the reference design PCB as your platform, since the isolation is built in already.

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In some countries, the Neutral is Earth (i.e. here in the UK) as it enters the home. Indeed there is no Neutral, just three live phases and earth. (It's called PME) However a miswired plug could easity reverse that fact so it cannot be relied on for safety past the consumer unit.

It is very dangerous to connect any mains kit to a PC. To be sensible, you must use galvalic isolation.

Idealy power your sensing circuit using an isolated DCDC converter. Then, even if there was a fault and any power or signal on your sensing circuit became live, there would be no risk to life (or the health of your PC)

Indeed it is possible to create sensing circuits where the circuit 'floats' at 250Vac mains.

You can use optical, magnetic or GMR based isolators to transmit the signals across the isolation barrier.

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