I found a fairly big, old transformer. I unwinded all the coils and winded them again. My primary coil has 20 turns, and the secondary coil has 200 turns. I didn't use any tools while doing this, so it isn't precisely woud.

Now, when connecting this transformer directly into the power outlet, the power fuse goes out. I have 3 theories for why this is happening:

  1. Maybe I shouldn't connect the transformer directly into the outlet - do I need to put something "in between" the transformer and the power outlet?
  2. Maybe I am connecting the transformer in a wrong way? Currently I connect it by putting first primary coil wire into first hole in my power outlet, and second primary coil wire into second outlet hole.
  3. Is it possible that there is something wrong with the wires? I put isolation tape all over the primary coil so that those two won't touch.

What is wrong with my transformer?

  • \$\begingroup\$ From your description in statement 2, you're connecting one lead of the primary to one outlet hole and one lead of the secondary to the opposite outlet hole? This means you have 2 unconnected wires, one from each coil of the transformer. Is this correct? \$\endgroup\$ – Shabab Oct 31 '13 at 17:25
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    \$\begingroup\$ Also, regarding statement #3, did the insulation (either plastic or enamel coating) wear off the wires? Did you confirm there is no short-circuit between the primary and secondary coils with the use of a ohmmeter/multimeter? \$\endgroup\$ – Shabab Oct 31 '13 at 17:27
  • \$\begingroup\$ @Shabab: If insulation on the wires (not between the coils) is broken, can it cause short-circuit? Coils are well insulated, but wires may have their insulation broken. \$\endgroup\$ – ojek Oct 31 '13 at 17:39
  • \$\begingroup\$ Your 24v is AC correct? What is the resistance of the coil? Are you drawing more current then your fuse can handle? Just use a multimeter to debug your coil, make sure nothing is shorted. \$\endgroup\$ – Matt Clark Oct 31 '13 at 18:24
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    \$\begingroup\$ You wrote that you unwound the windings, approximately how many turns had the transformer before you started? \$\endgroup\$ – jippie Nov 1 '13 at 18:14

If it is a step down transformer then the coil with the most turns connects to the AC supply. I can't imagine that a 20 turn primary is going to function correctly and is definitely going to stand a bigger chance of blowing fuses.


  • \$\begingroup\$ In my power outlet I have 24V, I need it to be 240V. \$\endgroup\$ – ojek Oct 31 '13 at 17:34

First, you likely have the primary and secondary flipped. A 10:1 voltage ratio makes a lot more sense than a 1:10 ratio. In other words, you should be plugging the 200-turn winding into the wall, not the 20-turn winding.

Second, maybe you broke something while messing with it. Was the insulation intact on all wires? Did it look to be in good shape? Did you keep sharp tools away from the wire?

Another thing to be aware of is inrush current. On some transformers the core is made of material that sortof has memory. If it was last turned off with little load and the magnetic field at one of the peaks, then turned on again at the zero crossing before that peak, the core can become saturated. This greatly reduces the apparent inductance of the primary, allowing it to draw too much current over the first 1/2 line cycle.

This large inrush characteristic is particularly likely to happen in toroid transformers, because the magnetic field is mostly in the core material. There is no air gap or similar to help against saturation.

  • \$\begingroup\$ It is a step-up transformer, I need to convert 24V into 240V. It is likely that insulation on the wires may be broken, I just wasn't sure if that was a factor here or did I connect something in a wrong way. \$\endgroup\$ – ojek Oct 31 '13 at 17:38
  • \$\begingroup\$ @ojek: If it's a step-up transformer meant to take 24 V in, then blowing a fuse when you connect it to 240 V in is no surprise. If it is really a 24 to 240 V stepup transformer, then you can test it by connecting 240 V to the secondary and seeing if you get around 24 V on the primary. Overloading the primary by a factor of 10 is causing it to draw excessive current, which is why your fuse is blowing. \$\endgroup\$ – Olin Lathrop Oct 31 '13 at 17:43
  • \$\begingroup\$ Then how can I get a 240V from that 24V power outlet? I thought that transformer was the way to do it... \$\endgroup\$ – ojek Oct 31 '13 at 17:44
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    \$\begingroup\$ @ojek: You have a power outlet that provides only 24 V? Is it really AC? If you have unusual circumstances, you should say so in the question. When you say "power outlet", we can assume 120 V or 240 V depending on what part of the world you are in, which you also haven't specified. Remember, filling in your profile is a courtesy to us. \$\endgroup\$ – Olin Lathrop Oct 31 '13 at 17:58
  • \$\begingroup\$ @ojek: is this 24 volt outlet 24 volts AC or DC? Transformers don't work on DC. Connecting a trnasformer to a 24 volt DC source will likely draw excessive current, and blow the fuse in the source. \$\endgroup\$ – Peter Bennett Oct 31 '13 at 17:59

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