Noise due to ground coupling resets uC

[expert_vision]

Noise due to ground coupling

I am trying to measure the current in the mains trough a shunt resistor by amplifying the voltage across it with an op amp and feed it into a microcontroller's ADC, like in the schematics below.

If I connect a high impedance load in the mains everything is OK, but if I connect a low impedance one, the uC resets due to huge spikes in the ground, no matter how much I try to stabilize the Vcc at the uC.

How to filter this huge spikes coming through the ground from the mains when a load is connected?

 

[scanman]

Re: Noise due to ground coupling

Hi expert_vision,

I would try using a toroid inductor for AC current measurement. This will allow you to isolate uC ground from AC mains ground.
Otherwise you will need an isolated instrumentation amplifier.

**broken link removed**

Regards,
scanman

 

[mtwieg]

Don't connect the circuit ground to neutral, for starters. Use a differential amplifier with a common mode input range with a resistive shunt, or use a current transformer.

 

[expert_vision]

I was kind of avoiding toroidal inductors because from what I've read they are inaccessible, large in size, they saturate fast (small range) and imprecise.

- - - Updated - - -

Hmm .. I guess I'll have to experiment with a ferrite tor, but I don't think it will sense small currents

 

[schmitt trigger]

Purchase either a hall cell current sensor or a ready made current transformer.