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I am attempting to build an I/O board to integrate with software I have running on a Raspberry Pi 4. I need the board to monitor voltage and current (power would be a bonus) on 8 circuits running 240VAC 40A, and a way to get the information to the Pi every half second or second.

I have been looking at the datasheet for Microchip's MCP39F511N and it looks promising. Each chip measures 2 circuits so I would need four of them. Here are my questions about the sample circuit shown on page 3:

  1. How much current can the 2M shunt measure before it overloads itself or the 3.3V limit of the IC?
  2. Could I use a Hall effect sensor (such as one in the SCT013 series) instead, for isolation and to avoid generating extra heat?
  3. The sample circuit's voltage divider has a 200:1 ratio, so that would be fine for 240VAC, correct?
  4. It lists L and N, but in the case of 240V I have L and L. Would that affect anything? (I only see one V+ pin so is it assuming the other wire is neutral?)

Edit: The shunt is actually 2m and is listed as part of the example circuit in the IC's datasheet; I'm not familiar with shunts so I don't have any more information to go on than what's there. Additionally, this is not a 3 phase supply. I'm working with single phase 240V, so X and Y instead of L and N.

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  • \$\begingroup\$ You can't monitor power; you have to calculate it. Please add data sheet links for the sensor and shunt that you mention. If you are monitoring L and L in a 3 phase supply then 200:1 is probably not enough. Requests for product recommendations are off-topic. \$\endgroup\$
    – Andy aka
    Apr 13 at 12:50
  • \$\begingroup\$ Your questions are OK. The request for product recommendation at the very end is what had this closed. If you remove that part it might be re-opened. \$\endgroup\$
    – Maple
    Apr 13 at 15:58
  • \$\begingroup\$ Thanks @Andyaka, I added what clarifications and data sheets I could. In the case of the IC I asked about in the question, it handles power calculation. I perhaps worded it poorly. \$\endgroup\$ Apr 13 at 19:02
  • \$\begingroup\$ Thank you for the explanation @Maple, this is one of my first posts here. I've edited the question accordingly. \$\endgroup\$ Apr 13 at 19:02
  • \$\begingroup\$ What’s a 2M shunt? \$\endgroup\$
    – winny
    Apr 13 at 21:17

1 Answer 1

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  1. You need to read datasheet more carefully. The nominal input of current ADC is +/-1V and the maximum is +/-2V. The shunt is 2 mOhm, not 2M. This means nominal peak current of 500A and absolute maximum 1000A.

  2. Yes, you can use current transducers and other types of sensors to monitor current. In fact, this is how it is often done in commercial meters: Single phase 3-wire metering Image from EKM Metering site.

But you still need direct connection to measure voltage. For this reason the isolation is provided on the communication lines, you don't need more than that.

The "extra heat" you talking about is entirely in your hands. You can set PGA gain as high as 32x, for differential input range 0.01875V. Then 62uOhm shunt will dissipate only 0.1W at 40A current.

  1. The datasheet specifies nominal V+ input 1V and maximum 2V. For 240V you'd need to use higher divider ratio than 200:1.

  2. This chip is designed for single phase 2-wire power monitoring.

For single phase 3-wire supply a different chip, like Cirrus Logic CS5463, may be a better fit. The diagram in Figure 21 of the datasheet shows how to connect it to 240V single phase 3-wire supply without neutral wire. Just like MCP chip it provides a lot of information for you, calculates RMS etc.

Of course if you are getting 240V from some other supply configuration, like 3 Phase Open Delta, then you need entirely different hardware setup.

Update

Just an after-thought. While using 2-channel monitoring chips looks appealing as a way to reduce component number, for 8 independent circuits I'd suggest making a small single-circuit board with isolated outputs. Eight of these boards can be placed right next to the corresponding loads, minimizing high voltage wiring. Then they can be connected to central Pi node using low voltage off the shelf cables.

This way you will have flexibility of the location and number of monitored circuits, plus simpler setup and debugging.

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