I'm conducting an experiment for my Physics class. I'm testing different transformer cores and comparing the transmission efficiencies.

For my circuit, I have a variable autotransformer connected to a receptacle. From there, I'm measuring AC current with a multimeter in series with the hot wire which then connects to one of the lead wires of my primary, and my other lead wire is connected to neutral. I also have another multimeter in parallel across both connections for an AC voltage reading.

This is a step-down transformer with a winding ratio of 2:1. On the secondary side, it's the same setup as with the primary. I have a multimeter in series with one of the secondary lead wires and am measuring the current. That then connects to the load which is a small, low-power bulb that you find in car headlights, so 12 V and less than 2 A. After that it goes to the other lead wire on the secondary, again with a multimeter in parallel reading the AC voltage.

When I started testing, I was getting no power through. The secondary side was 0 A and 0 V, while on the primary it was 0 A and 20 V.

The transformer wasn't vibrating so I knew no power was going through.

When I disconnected the multimeter reading current on the primary side and did straight connections, power went through and I was getting readings on the secondary, but it was about 0.2 A and 1 V on the secondary with the primary running about 5 V to 10 V maybe. The light bulb (load) wasn't lighting up, and when I started stepping up the voltage on the primary through the autotransformer, it started smoking so the insulation on the copper wires must have been burning.

I don't have a current reading on the primary because it didn't work when I first tried it out, but I know it must have been a high current because it was burning the insulation. I don't know why there's a high current in the primary to begin with when my load doesn't require high voltage or current.

Please give me any ideas or solutions.

  • 3
    \$\begingroup\$ It would help for you to draw a schematic of your connections, and maybe a picture of your setup. \$\endgroup\$
    – PStechPaul
    Commented Nov 19, 2022 at 1:47
  • 1
    \$\begingroup\$ Need a sketch of your setups. Show where magic smoke was released. Did you use an isolation transformer before your variac? Info on diy transformer? How did you design your transformer? Equations. \$\endgroup\$ Commented Nov 19, 2022 at 3:47
  • \$\begingroup\$ What’s your core cross sectional area and how many primary turns? \$\endgroup\$
    – winny
    Commented Nov 19, 2022 at 9:52
  • \$\begingroup\$ This is a good point to start ... eepower.com/technical-articles/… \$\endgroup\$
    – Antonio51
    Commented Nov 20, 2022 at 19:02

1 Answer 1


Sound like you’ve blown the fuse on the ammeter that was on the primary.

You should have a fuse on the variac secondary. It’s possible to have destructive secondary current with relatively low primary current if the output voltage is set low.

If you have too few turns (for the voltage, frequency and core) on the core, excessive current will flow, even with no load on the secondary of the transformer.


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