How does this transformer work?

Question

I've watched this latest video from Great Scott (relevant timestamp) and he came across and used this transformer successfully.

I tried to simulate it in a circuit simulator but I must misunderstand something.

Given a 50 Ω sense resistor, no matter what limit resistor or inductance is set, the simulator shows that there's always a lot more current flowing then the 2 mA rated current the datasheet claims.

The only way I found to be able to achieve 2 mA and the stated input/output voltage transformation through resistors in the simulator is by increasing the primary inductance to at least 20 H and raising the resistor values by multiple magnitudes. I've run all my simulations with 50 Hz, 230 V RMS voltage on the input as that seemed to be the use case for this device.

• How does this transformer work in the configuration from "Figure II"?
• Is it a different, uncommon, type of transformer?
• What range of primary inductance would be common for such a device?

Answer 1

I used LTspice for a simulation, with coupled 1 H inductors with 50 ohms resistance, and the results are as follows:

Answer 2

Thanks for the comments and thanks for trying it in LTspice! After re-learning some basics I think I got it now. I indeed misunderstood some things.

• Is it a different, uncommon, type of transformer?

It is a current transformer. Most current transformers have high ratios in the number of windings of the secondary to the primary coil. Allowing them to transform higher currents, like 10, 40 or even 100 A into smaller, more manageable ones like 20 mA. This particular one has a 1:1 ratio which is uncommon for current transformers.

• How does this transformer work in the configuration from "Figure II"?

It works the same as any other transformer, however current transformers are used much differently inside circuits. The main thing to consider here is that these components are meant to transform and transmit current, not voltage.

The configuration from "Figure II" reduces the voltage over the primary coil in order to limit the current trough the coil. This is primarily done using a resistor. As this is a 1:1 transformer the secondary can not reach higher voltage than is seen by the primary. Other transformers primarily use magnetic reluctance to limit the current trough the primary coil. The difference is, that with the resistor most of the energy flow is limited and some converted into heat. Most doesn't reach the transformer. With magnetic reluctance, some of the energy is stored in the magnetic field in the transformer and is released by the primary and secondary winding, thus a transformer relies on magnetic fields to be able to transfer energy to the secondary side. More powerful transformers generate more powerful magnetic fields and vice-versa.

In order to use the output signal suitable electronics have to be utilised that can work with the current directly or with low voltages. This situation also happens with photodiodes and is called low impedance. To shift the signal to other desired levels trans impedance (integrated) circuits and amplifiers are used for this purpose.

So. Even though this is a transformer that is meant for mains AC voltage there is no way to use it to obtain any arbitrary voltage between mains and zero. The vast majority of power from the mains does not reach the transformer and its secondary side. This transformer probably has a power rating below 1 mVA.

• What range of primary inductance would be common for such a device?

I wasn't able to find an answer to this.

The voltages over the primary and open voltage over the secondary are dependent on the inductance and coil resistance.