How to reset a current transformer core

Question

I use a current transformer for pulsed current measurement (with DC offset). It seems to me the core stores some energy after pulse disappear. The transformer works at about 30khz a it has 2 turns on primary and about 300 on secondary. The secondary is loaded with 100 ohm resistor with 300mV voltage across.

Is there some way how to reset the core so the output signal will not be shifted? I am thinking about disconnecting the secondary from 100 ohm right after pulse and quick discharge to higher impedance.

^{simulate this circuit – Schematic created using CircuitLab}

Top is primary current, bottom is 100ohm resistor voltage:

Answer 1

The first thing you have to realize is this: -

So, if you want to recover the waveform (so that it looks like the input waveform) you have to process it with an op-amp clamp that keeps the most negative voltage at 0 volts. Or use an MCU to process the signal.

Answer 2

The easiest way to counter a DC offset on a current transformer is to add an additional "reset" winding alongside the primary and apply a DC current through that, calculated (and/or trimmed) to oppose the DC offset.

^{simulate this circuit – Schematic created using CircuitLab}

This will cancel the DC component of the flux in the core, and allow you to sense larger currents without saturating the core. The current in the reset winding needs to be \$I_{rst}=I_{pri,DC}·\frac{N_{pri}}{N_{rst}}\$; use a multi-turn reset winding if your primary current's DC component is large.

Answer 3

When the source is switched and unidirectional, the burden must also be switched and unidirectional.

An example application from my archives:

^{Source: my website, https://www.seventransistorlabs.com/Images/Mag_Amp_PSU.png}

The bottom half of this can be ignored for present purposes.

Notice the CT primary (1T) is in series with the switching MOSFET, so the source is switched and unidirectional. The secondary (150T) has a large burden resistor across it (1k), which determines the peak reset voltage; then a diode into the nominal (small value) burden resistor.

Mind that this doesn't show phasing for some reason, which should be set in the obvious direction (transistor on = FR102 on).

As long as duty cycle is less than some maximum (the time required for the CT to reset into the 1k, several L/R time constants), the reading will remain zero-based.

Notice that another CT and diode could be wired-OR into the burden resistor. If you needed continuous current monitoring (say for an average current mode control), a second CT could be placed in series with the SBL2040, the secondary wired in exactly the same way (with its own reset resistor and diode). Pulse width would then have to be limited at the top and bottom end (i.e. it only reads correctly for, say, 5 to 95% PWM). I would generally recommend a Hall effect current sensor over two CTs, as it's less limited, but it is an option sometimes.

Answer 4

To reset a magnetic item (in this case, the current transformer core) takes an AC induction that slowly decreases. One might accomplish this with a switched incandescent light bulb and thermistor current limiter. This connects to a winding on the current transformer, which is out of circuit until the reset button is pressed.

^{simulate this circuit – Schematic created using CircuitLab}

The PTC thermistor starts cold, at low resistance, but warms up in a few seconds to drop the AC applied to the reset winding; the meter winding is also on the same core, so it is a transformer (i.e. you don't want the reset winding to exceed normal range of the metering circuit; select lamp or other dummy load, and thermistor, accordingly).

The pushbutton switches the lamp circuit ON, drawing initial current according to the lamp and thermistor's cold resistance, then that current diminishes (as the thermistor resistance rises with thermistor temperature). After a few seconds, release the pushbutton, and allow a few minutes of cool-down time before resetting again.

Answer 5

1. Turn off the power: Before working on the CT, make sure to turn off the power supply and ensure your safety.

2. Disconnect the secondary circuit: If the CT has a secondary winding or conductor loop, disconnect it or short-circuit the secondary terminals.

3. Remove primary current: If the CT is connected to a live circuit, disconnect the primary current flowing through it.

4. Demagnetize the core: Pass an alternating current through the secondary winding in the opposite direction of the primary winding. This creates an opposing magnetic field that demagnetizes the core.

5. Gradually reduce the current: Start with a high magnitude of alternating current and gradually decrease it over time. This allows the magnetic flux in the core to decrease to zero.

6. Remove the current: Once the current is completely removed, the opposing magnetic field dissipates, and the core is demagnetized.

7. Reconnect and reassemble: Reconnect any disconnected components and reassemble the CT as necessary.