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distortion in ac output from transformer

Thread starter #1
Hi guys,
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?

ac3.png
 

audioguru

Well-Known Member
Most Helpful Member
#5
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.
 
Thread starter #6
I pulled the transformer from a circuit board which is used to charge a 6V sealed rechargeable battery. The above output is from the secondary coil when a 220V is applied to the primary.

Try to Reduce the input Voltage.
The transformer works on 220v AC, as its primary coils were directly connected main supply wires before it's pulled out from the circuit board. That means it designed to work on 220v mains.

Is it normal to have this kind of output in transformers?
 

crutschow

Well-Known Member
Most Helpful Member
#7
Is it normal to have this kind of output in transformers?
No, the output should look like the input sinewave.
Are you sure the transformer was connected directly to 220Vac?
 
#10
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.
 

audioguru

Well-Known Member
Most Helpful Member
#12
I have a Chinese +9V, 100mA wall wart that produces +17V with no load and +9V with 100mA load. It gets hot with and without a load.
I think its transformer core is made of rice or poop.
 

JimB

Super Moderator
Most Helpful Member
#13
How to calculate the required resistor?
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.

JimB


PS, Anybody know the magnetic permeability of rice?
 

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