The Electrician
- Joined Oct 9, 2007
- 2,971
I found a similar transformer in my junk box. It is an old transistor radio output transformer. It's dimensions are almost identical to yours, but the DC resistance measurements are a little different. The primary is a 1k ohm winding with a DC resistance of 65.3 ohms. The secondary is an 8 ohm winding with a DC resistance of .61 ohms.Hi all,
I received an audio transformer for a project from my prof and went ahead and started to characterize it before using it.
CT-side DC resistance: 46R
Non-CT-side DC resistance: 2R
Voltage gain ratio: 1:13
The part is an audio transformer (1.2K primary, 8R secondary) 60-282-0
While doing LC tank measurements to figure out the inductance, I determined the resonant frequencies with different parallel capacitor values (1nF, 1.5nF, and 10nF) and both the primary and secondary windings had roughly the same inductance 10-12 uH.
Maybe I'm just double-guessing myself but shouldn't the ratio of inductance be roughly the turns ratio and not one?
For each measurement, I left the other winding open and unconnected.
Thank you,
JP
Your method of determining the inductance of the windings is not going to work because you end up measuring at a much too high frequency.
I made some measurements on my transformer with an impedance analyzer. This analyzer can measure over a wide frequency range and can display several parameters such as impedance magnitude, impedance angle, inductance, capacitance, Q, etc.
Here are two plots of impedance magnitude and angle over a frequency range of 10 Hz to 1 MHz. The plots show impedance in green and angle in yellow. The scale is shown on the plot; there are two makers labeled A and B. The A marker is at the left end of the plot--at 10 Hz; the B marker is at 1 kHz.
The vertical scale of the impedance plot is logarithmic with the grid shown and the angle plot is linear with 100° at the top and -100° at the bottom with a linear grid not shown.
The first plot shows the impedance and its angle for the high impedance winding (the 1k ohm winding) with the impedance scale ranging from 100k ohms at the top, to 100 ohms at the bottom; the other winding (the 8 ohm winding) is left open for this measurement.
The impedance is not purely inductive. You can see this because the angle (yellow) is not a horizontal line at +90° (that would be nearly at the top of the display). One could calculate the inductance at any particular frequency by using the imaginary part of the impedance (the reactance). Divide the reactance by (2 Pi f) and you have the inductance. The impedance also has a real part which represents loss in the inductance.
The second plot shows the impedance of the 8 ohm winding with the other winding left open:
Note that in both plots, the impedance (green) reaches a maximum just past 10kHz. The high impedance winding consists of many turns of very fine wire, and that winding has a self resonance which is responsible for the impedance maximum.
The 8 ohm winding also has its own self resonance which is at a much higher frequency and doesn't show up in these plots.
The impedance peak in the second plot above, which is of the 8 ohm winding, has an impedance peak because the self resonance of the 1k winding is reflected into the 8 ohm winding by the coupling between the two windings; it is not due to the self resonance of the 8 ohm winding itself.
We also see a sharp dip in the impedance of the 8 ohm winding at about 108 kHz. This is due to the resonance between the leakage inductance of the 8 ohm winding and the reflected impedance of the 1k ohm winding, which is a capacitance at this frequency (since this frequency is above the self resonance of the 1k ohm winding).
This transformer is designed to be used at audio frequencies and you can't characterize it by connecting capacitors which cause resonances at frequencies as high as 500 kHz
Look at the impedance curve for the 1k ohm winding. It varies from about 164 ohms at 10 Hz to about 20k ohms at 10 kHz. But, it's supposed to be a constant 1k ohms at all frequencies, isn't it? Its impedance is steadily increasing from 10 Hz to 10 kHz; what's going on? I'll explain in the next post.
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