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Unknow current in transformer


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With an unknown transformer you must take precautions to ensure your safety:
1) Make certain that it is a mains-power transformer, not an audio one.
2) Make certain that you know which wires are the primary that connect to the mains.
3) Make certain that the primary voltage matches the supply of your country (115V or 230V).
4) Connect the primary to the mains and measure its secondary voltage. Subtract about 5% to allow for its voltage to drop when loaded. This will approximate its rated secondary voltage.
Then you should visit your electronic parts supplier and find their transformer that is the same voltage and size. If its current is marked then you have a match. Sometimes transformers are rated in voltage and power (VA). You can calculate their current by dividing the VA rating by the secondary voltage rating.

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Mikail, measure the resistance of each winding. This will give you the ratio. Use the basic formulas for transformers to go from there. DO NOT connect this transformer to mains until you know that it is the correct type of transformer.
Be safe!


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I have never heard of measuring the resistance of a tranformer's windings to determine its voltage ratio.
The resistance of each winding is not only dependent on the number of turns (length). It is also dependent on wire size.
For a voltage step-down transformer, thin wire is sometimes used for the primary winding and heavy wire for the secondary. They can be overloaded without much heating.
Also, in a transformer that has one winding wound on top of the other, the top winding will have a greater length, therefore its resistance will be higher than expected.
Just measure the voltage.

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The question was how to find the current of the secondary. Unless some things have really changed since I was in school, you look for impedance on a transformer to calculate current. Impedance is the ratio of voltage to current. Z = E / I. This is the beginning. From there you go with the other formulas to find other parameters. Remember, the transformer was designed using formulas. Therefore, formulas will tell you what you need to know from a few characteristic measurements on the device.
BTW- Never connect an unknown transformer to mains to find out what it will do.


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  • 4 weeks later...

common MP,
What the other guy has been telling is correct.Transformer winding ratio has nothing to do with the resistance values of either winding.Their is no way to findout the ratio other than actually applying a frequency sweep to the primary and plotting the amplitude at secondary.Well..., may be i dont know another method.


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The other guy that you are agreeing with mis-read the original post. The original post was not about voltage ratio. It was about current. There are formulas for this. Winding resistance has everything to do with current. If you have an unknown transformer, you are going to run a voltage through it to find out what it will do? How much voltage? You have to make a guess. I have a poor opinion about bench tests that start with guessing and work their way from there.
This site is filled with beginners looking for answers. They do not have high tech equipment. So, when a guy says just connect it to the mains and see what it does, that worries me a bit.


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The amperage part is going to be difficult to know. Just guess by the size of the transformer. It's interesting how high the current can be. The rated amperage is the average current and not the peak current. You will notice that a fuse is very small for the rated current, this is because it deals with peak current.

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