Electronic – these 50Hz noises from

mainsnoisepower supply

Circuit structure:

  1. An AC to DC power converter MeanWell 220V to 5V. The output is 5V. The power grid runs at 50Hz.

  2. We build DC-DC converters(Power Module) with multiple orders of Low pass filters to provide different DC voltages needed for our main circuit: +5V, +15V, -15V

  3. Main circuit uses the voltages generated above. It provides control signals to its target and reads feedback signals. It connects with target through 2.54 mm dupont wires, which are around 0.3 meters long. It will adjust control signals according to the feedback signals. The target device has purely differential receiver and shall not be influenced by common-mode noises.

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Problem:

  1. The whole circuit runs normally at our companies lab. The circuit is not connected with earth.

  2. When used in some Lab with many devices(scopes, signal generators and etc.), it does not function well any more. The feedback signal looks like following, which is 50Hz noise.
    Noises

  3. When the circuit ground is connected with earth, the 50Hz noise will be reduced greatly. It starts to work.

  4. There are still 50Hz noises which do not exist when in my Lab and it harms the device performance.

Questions

  1. Where are the 50Hz noises from?

  2. Why the noises are reduced when connecting the circuit with earth?

  3. How to remove the 50Hz noise totally?

Best Answer

  1. The 50Hz noise is mains hum. Any time you pick up noise at the same frequency as your mains hum, assume it's from the mains (once, maybe twice in a lifetime it'll be a wrong assumption, but every other time it's right)

  2. Pretty much the only way to get rid of picked-up noise (mains hum, emi/rfi, whatever) is to ground it out.

  3. From your updated question post, it looks like the noise is appearing on a (possibly long) wire carrying a non-differential signal & coming as data feedback from a (mains powered?) "slave" device.

Either the hum is originating from within the "slave" device due to some power supply noise to it (likely due to poor wiring & other noisy devices on same circuit), or it is being picked up by the wire as EMI/RFI.

If it is being picked up as EMI/RFI (some part of it almost surely is), then switching to differential signalling could "fix it."

If it is coming from the "slave" device somehow, then there may be a need to add some 50Hz filtering &/or grounding in that device, to eliminate the noise before it gets onto the data line.

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