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I'm planning a PIC-based power measuring circuit that will transmit measured data through an RF module to an other PIC. I'd like the circuit to be placed inside one of those casings in which you can plug in an AC device (just like all those digital timers and power measurement devices you can find everywhere). So the whole thing is plugged into the wall, and the AC device is plugged into the casing.

My circuit will include this stepdown encapsulated transformer, a bridge rectifier and a 7805 regulator to power the PIC (I don't want to use an external power supply).

It will also include current sensor and op-amp as described in this project.

My questions are:

  • Can I place the transformer close to the RF module (as required by the limited space in the casing) without any risk of distorted signals due to EMI from the transformer?
  • How do I, if necessary, protect the op-amp from EMI and noise from the transformer?

I don't have a circuit diagram yet as I'd primarily like to learn about the pitfalls and useful tips etc. with this type of circuit.

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Radiated noise depends more on household devices that use triacs or SMPS circuits without PFC. If any Op Amp line connected circuit had high impedance with a large loop circuit between ground and neutral, it could pick up noise like an antenna.

I do not see an issue with your AC transformer without more details. Ground could be used as a shield and any external data ought to be opto-isolated or properly designed serial port. If just using an RF Tx, take care on supply and return noise with suitable filters.

You do not want to connect neutral to ground and generate ground currents, other than a noise suppression caps to ground with 0.5mA leakage max total , if used. The RF transmitter should have good immunity from radiated 60Hz if low impedance inputs, but conducted noise must be eliminated. Your ground path must be carefully designed, whether it is neutral or gnd. Common mode chokes do well to suppress external conducted noise coming in.

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  • \$\begingroup\$ Thank you for your answer. I realized that a better solution would probably be to integrate a small USB adapter instead (such as the iPhone charger) for the DC voltage and a SparkFun ACS712 breakout board for the measurement. I think both will fit inside the case. \$\endgroup\$
    – Alex
    Oct 18, 2012 at 16:50

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