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I want to know, why I am getting a distorted output from a step-down transformer? Why is it not a proper sine wave? Is it normal?
What may be the problems? Does adding a 100uF capacitor solve this problem?
If this is the output from a loaded mains transformer then its core is saturating because this transformer is overloaded.
At first I thought it is a vacuum tubes audio output transformer that is overloaded. But if the vacuum tubes are clipping then the upper and lower horizontal clipped parts would be slanted due to the phase shift of the transformer.
Many small transformers do this, the size found in small wall warts, as chemelec said its core saturation, a design feature!! Thats why such transformers have a surprisingly high offload power (often a watt or so). Somewhat annoying if you want a reference sine for example, that can be solved by incorporating a primary series resistance to lower the voltage enough to stop the saturation or you can by an EXPENSIVE transformer for such purposes.
I am not sure that there is a quick and easy calculation that can be done without knowing a lot more about the transformer.
If you really want to use that transformer to to give a sinewave, the easiest way would be to experiment with a few different resistors and see which works best.
To start with, try something between 10 and 100 Ohms and see what happens.
If the resistor gets hot and smokes, you need one with a higher power rating.
Of course, putting a resistor in series with the primary of the transformer will reduce the voltage regulation. So if you try to draw any power from the secondary, the secondary voltage will fall. Just like AGs rice cored transformer.
PS, Anybody know the magnetic permeability of rice?