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Transformer question?

I took a part an ooooold boombox and got some cool parts from it, one of them being a heavy, good-sized transformer. The boombox was using the transformer to step down the voltage before rectifying it.(Right before rectification it goes through a 500mA fuse if that helps...) From what I can tell, the mains power is connected directly to the transformer. I tested it with low voltage AC source and found that it is roughly a 10:1 ratio transformer. I wanted to reverse this and use it as a step-up transformer, but I don't feel comfortable hooking it up directly to mains in case it draws too much current and flips a breaker or something (I read a biography of this one dude and when he was a kid he tried to make an electromagnet using mains power.... knocked out the power on the whole block and the police came...). Will it draw too much current putting it on the low voltage side? I saw one guy say that using a 50 ft extension cord will prevent it from drawing too much. Will that work? If not any other ways? (or just not worry about it?)

edit: It is roughly the same size as this one  https://www.instructables.com/id/LM3886-ChipAmp/

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framistan6 years ago
It will and it wont. If you try connecting the transformer BACKWARDS to the mains wall-outlet, it will fry the transformer because it is not made to withstand the HIGH voltage that will come out of it. It would also be dangerous voltage for a minute or 2 untill transformer starts to SMOKE. so... shock-hazard... fire-hazard.... I recommend DONT DO IT.

However... if you just want to connect a couple 1.5 volt flashlight batteries to the SECONDARY and see if you can get 50 or 75 volt pulses out of the PRIMARY... then yes that will work. Each time you touch the terminal to the battery you will get a pulse.
And a pulse on each break.
If you applied the mains voltage to the low voltage side, the output on the primary side would be 10 times that, which ends up being 1200volts (or 2200 volts if you are in the UK). The insulation on the wiring of the primary is most likely not rated for that high a voltage. The transformer was designed to have the 120 or 220 volts applied to the primary, and it the manufacturer would unlikely spend another cent to use wire with higher rated insulation.

Adding an extersion cord would add resistance, and reduce the maximum current that would be drawn. The degree to which it would limit the current would depend on the size of the wires used in the extension cord. At any rate, that idea is not a very good one, as you are just wasting power in the resistance.

The more safe approach would be to wind an electromagnet yourself, and drive it using a lower voltage. Depending on how powerful of a magnet you need, you could just use the transformer to step down the mains voltage, rectify it, and use that. If the fuse on the mains side is 500mA, you could probably assume it would be safe to draw 250mA from the mains. This would provide a 2.5 amp output on the secondary, which could drive a small electromagnet.

7654321 (author)  LargeMouthBass6 years ago
Haha, I dont need a magnet that was an example from a book. Is it feasible to wind my own transformer? If so, any links?
Try looking at some of the instructables for home made arc welders. I think there is info on rewinding old transformers in there.