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Power line disturbance magically damaging MSP430FE423A. How to protect it?

Purushottam Kamat
Prodigy 180 points
Other Parts Discussed in Thread: MSP430FE423A

I am in such a mess!! This is how my setup looks like.

My metering part consists of a MSP430FE423A placed in MSP430FETU64 kit and program it using JTAG debugger of TI. The voltage to be measured is fed through resister divider circuit as shown.

It is being used to measure line voltages and currents. I use a auto transformer to vary the voltages and currents under measurement.

 To protect the metering part, I have used for now a commercial line filter from EMIS. Hence, I was confident that my circuit had enough protection for my circuit from electrical disturbances from power line and had postponed my filter design for later stages of development.

 My MSP430 draws its power from Debugger, which is connected to Laptop, who runs on battery and not connected to power line.

 Now somehow, when I turn ON or OFF (mostly OFF) the switch of the auto-transfomer the MSP430 gets disturbed or damaged.

Disturbed = IAR kickstart which I use to debug the design stops communicating with MSP430 and I have to restart the session. And then it works

Damaged (type 1) = the DVCC,AVCC,DVSS,AVSS all get shorted. Debugger stops giving supply (both green and red led off)

Damaged (type 2) = MSP430 is still working, as confirmed by blinking LED. But can reprogram it.

 This happens even if auto-transfermer is set to give 0Vrms output.

 I just don’t get it! How can a disturbance good enough to do damage, get through the line filter? (Trust me, its high quality one)

 How does it damage the MSP? How to protect it? Is it a disturbance from power line or something else? What am I missing? What should I do?

  • 0
    Expert 2740 points

    I would connect the floating laptop USB shield to Load Neutral.

    You should  connect your  pin 7 input to a local reference voltage 

    Peter

  • 0 in reply to Peter Dvorak
    Prodigy 180 points

    Thanks Peter for your kind help. Can u please help me understand what is happening? I mean, looking at the symptoms of failure (disturb/ damage1,2) can you tell what might have attacked it? Would it be possible to protect the MSP430 by adding something to design so that the design itself becomes robust? So that it can work in adverse environment.

  • 0 in reply to Purushottam Kamat
    Guru 227245 points

    The original schematics was designed to be potential-free. In theory, this will decouple the MSP and the Laptop from mains and therefore should be safe. However, everything forms a small capacitor between itself and earth. So the circuit is never 100% potential-free. As small as this capacitance may be, there's still a chance that some electrons are pushed in and cause damage. This can be avoided by making an intentional GND connection that limits/shortcuts any virtual voltage on this capacitor.

    In the original circuit, you have a differential voltage between V1+ and V1-. But both, V1+ and V1- also have an absolute voltage to MSP VSS. Only that it is not defined and can be anything. By this small capacitance to earth, it may be a huge absolute input voltage. if you tie GND to V1-, you limit the absolute voltage to the differential voltage.

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