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I have designed a 12V electric fence energizer. It works as follows: A flyback converter charges a 50uF capacitor to 400V. After the capacitor has been successfully charged, a thyristor discharges the capacitor on the primary winding of a transformer, so a 10-12kV pulse appears on the secondary side of the transformer. (One-half of the secondary winding must be connected to the fence, and the other half must be grounded) The electric fence energizer is controlled by a microcontroller and it also has a small LCD display.

My problem is that when I put the terminals of the secondary coil of the already mentioned transformer close to each other, the display starts to behave interestingly due to the resulting spark. Sometimes its content disappears, and other times the text on it rotates 180 degrees.

During an oscilloscope measurement, I noticed that at the moment of the spark there is a 80-90V ringing of a few MHz between the GND and any other point on the board. (It appears for example, on the 12V input, but also on the 3.3V LDO output, which provides power for the microcontroller and the display.) I believe that this ringing or noise is what causes the display's interesting behavior. (The mentioned 80-90V ringing or noise can also be measured between the GND pin of the display and the other pins of the display).

The type of display I am using is HX1230 and here is the schematic: https://drive.google.com/file/d/1r-dnNyj1L3xkxInHm3HGnrcYFK1zSzCi/view?usp=sharing

My question is how can I filter this noise so that it doesn't reach the display?

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  • \$\begingroup\$ If all of this is on the same PWB, this will be difficult to fix. A picture will help. \$\endgroup\$
    – Mattman944
    Oct 4, 2023 at 20:27
  • \$\begingroup\$ Most probably layout related. Post schematic and PCB layout. \$\endgroup\$
    – winny
    Oct 5, 2023 at 19:54

1 Answer 1

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Without a schematic and parts layout, one can only answer in generalities.

There are various mechanisms to filter EMI (electromagnetic interference).

  • Inductive: put an inductor in series with power leads.
  • Capacitive: put a capacitor from supply to ground.
  • Faraday cage: put a conductive shell around circuits.
  • Magnetic shielding: put a mu-metal or ferrite shield around parts.
  • Distance: separate power and sensitive parts and lines; have their wiring at right-angles rather than parallel.

Here are some things to try:

  • Put the sensitive parts in a copper enclosure, with large-value feed-through capacitors for power line, and small-value feedthroughs for signal lines (enough to attenuate EMI, not so large as to attenuate control signals appreciably).
  • Put ferrite inductors in series with both HV generator and CPU/display power lines.
  • Space lines and parts carrying high-voltage pulse farther from CPU/display, and orient them at right angles, rather than parallel.
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