Electronic – arduino – Split-Core Current Transformer – Disconnecting secondary load questions

arduinocurrent measurement

I have heard that disconnecting the secondary load from a current transformer while the primary line is passing current causes a huge voltage spike (from a question about CTs that I can't find again). I am planning to build a similar power monitoring system as one shown on openenergymonitor.org, but I don't have a dedicated Arduino, and I would like to disconnect it at times to work on other projects.

1) What causes the voltage spike/Is this info correct?

1) Is there any way to disconnect the secondary load without any issues?

2) Is the voltage spike in the primary or secondary lines?

3) Will any harm come to the CT without a secondary load?

4) Is the burden resistor (the resistor used for voltage measurement) enough of a load?

5) If not 4, what is considered to be the load? The thing measuring the resultant voltage
or the 5V I must supply to the circuit?

Best Answer

  1. The voltage spike is caused by the secondary coil not having a discharge path: Inductors oppose change in current flowing through them. Disconnecting the coil forces the current flow to stop instantly. Thus the inductor must find a path to discharge the energy stored in it, and unless it finds such a path, the behavior is of voltage across the coil rising to an arbitrary level. This mechanism is very effectively used in a variety of designs requiring very high voltages, such as some automotive ignition coils and other such devices.
  2. Keep the load resistor in place. Disconnect the sensing connection alone.
  3. In the secondary.
  4. Yes, the burden or load resistor is designed to provide a current path, with the voltage read across it. The sensing device's impedance appears in parallel to the burden resistor, splitting the coil current. If the sensing equipment is high impedance, e.g. a solid state multimeter as opposed to a low-impedance vintage coil voltmeter, this parallel load is insignificant.
  5. No.
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