DC magnetization in low power toroidal transformer

[nenea dani]

Hello everybody ,
It is considered a 220V / 12V / 20W toroidal transformer. I would like to use it as an output transformer in a low power single ended electronic tube amplifier. How can I test this transformer to find out which is the largest direct current that keeps the core in the linear area of the magnetization?

[Weston]

A common way of finding the saturation current of an inductor is to connect it to a stiff voltage source and measure the current with the oscilloscope. The slope of the current is proportional to the inductance. Eventually you will see a "knee" in the slope where the slope rises. This is when the core starts to saturate and the inductance decreases.

There are a bunch of test circuits for this on the internet, here is a random example: http://www.vk2zay.net/article/200

In your case though the inductance is going to be large and the saturation current rather small, so you could probably get away with just connecting the terminals to a lab power supply by touching the wires / a switch. No need for a large capacitor bank or anything.

[Gyro]

If it's a dual primary or secondary transformer, you could maybe try injecting balancing current in a spare winding to counteract the DC current in the primary. You probably wouldn't need a constant current source, just a series resistor.

[nenea dani]

     It seems that things are more complicated than they look. In question is the Indel 220V / 16V, 20VA transformer; 4.48H / 222Ohm, 102.5mH / 2.7Ohm. I did numerous tests with various frequencies between 25Hz and several thousand Hz with the Sch1 from attachment scheme. At frequencies below a few hundred Hz it begins to become nonlinear below 30mA DC. At frequencies above 1000Hz it is linear right over 70mA DC. I noticed that at a few tens of Hz it starts to become nonlinear below 10mA DC. With my 30V/5A lab. source I can't go beyond 70-80mA in this set up. All currents were read from the source ammeter so they are somehow average values. The duty cycle matters a little, but the frequency strong determines the maximum current for linearity. I would have thought that at low frequencies it saturates at high currents, but it's exactly the opposite.
 I have attached the oscillograms for SCH 1 taken on the 4.7Ohm resistor for 242, 470 and 3250Hz so that no limitation or nonlinearity occurs. Voltage values are on the top left screen.
     For a better check I thought of a test that is closer to my goal, namely the behavior in the simultaneous presence of a sound signal and a direct current . I used the Sch 2 scheme  (see attachment) together with an online sine generator or even music. In principle, with music, when a current of more than 30-40mA  DC is introduced, only the low frequency (bass)  level decreases. However, the sound does not become distorted at least for the ear. It's not the same when I turn on the oscilloscope. I did tests with a moderate signal, around 1.4V true RMS on a 16Ohm speaker at various frequencies, all sinusoidal between 20 and 20000Hz. Each time I watched to see what happens when I introduce a voltage between 0 and 30V in the two primers transformer connected in series, which corresponds to a current up to 80mA. At low frequencies there is a strong nonlinearity and after 1000-2000Hz the signal remains pure sinus, even at 20000Hz. At a few tens or hundreds of Hz with the increase of the DC current, artefacts appear on both even and odd harmonics. For rectangular input signals, the odd components remain unchanged as the current increases, but  even components seem to increase as the DC component increases. For a sinusoidal signal at 500Hz I  can introduce a maximum of 30mA DC and from 1000Hz and up I can feed with 70-80mA DC and the wave stays clean without any harmonics. It is possible that there is a connection between these currents and the fact that the transformer data shows a maximum current of 90mA but for 50Hz.
In conclusion, why saturate the toroidal core rather at low frequencies?

[Kleinstein]

Saturation is faster to happen at the low frequencies, as the limiting number is the voltage times period length. This also applies to the case without a DC bias.

A torroidal transformer may not be a good choice with a DC bias, as there is very little air gap and thus a rather high inductance.