I found a transformer from an old kit I had, I think it was the 10 in one electronic lab from GeoSafari. I'm really confused, as it only has three leads coming out of it (instead of the minimum of four I usually see), and can't find the model or part info. Guessing that the center terminal is the common ground and the other two are the input and output leads. Has anyone else ever encountered a transformer with a similar configuration (only 3 leads, all on one side)?

(Sorry if I look like I'm an idiot. I've never really worked with transformers.)

Abused the leads pretty badly lol

enter image description here

Leads are abused pretty badly. The transformer is about 3/4" on each side.

I just did some testing with LEDs. Current will flow between the second and third pins (in both directions), but not between the first and second pins or the first and third pins. I'm starting to think that the first pin is broken or disconnected, or just a dummy pin.

  • \$\begingroup\$ Do you have a picture? \$\endgroup\$
    – Dave Tweed
    Commented Dec 27, 2013 at 16:33
  • 1
    \$\begingroup\$ Whatever you do don't plug it into the mains because it doesn't seem to be a power transformer intended to transform the mains voltage to a lower one. \$\endgroup\$
    – alexan_e
    Commented Dec 27, 2013 at 16:35
  • \$\begingroup\$ It could be an auto transformer... where one lead is the supply, one is the output and the third is neutral .... not a transformer that should be connected to low voltage beginners citcuits \$\endgroup\$
    – Spoon
    Commented Dec 27, 2013 at 17:04
  • \$\begingroup\$ Realized that I forgot the pics until now... But this looks like a tiny transformer, about 3/4" on each side \$\endgroup\$
    – swe100
    Commented Dec 27, 2013 at 17:06
  • \$\begingroup\$ can you get a close shot of the bottom side? As a kid, I removed quite a few like that from circuit boards only to snap off a lead or two. It's possible there may be a winding on the other side that has snapped-off leads. \$\endgroup\$
    – JustJeff
    Commented Dec 27, 2013 at 17:45

3 Answers 3


It looks like an audio transformer, with probably two leads snapped off.

These transformers were used to transform power between a high-impedance transistor amplification stage and a low-impedance loudspeaker. Simple one-transistor amplification stages with low-power transistors needed these.


These may be either of an autotransformer, or a tapped inductor. In principle, rule out the possibility of an autotransformer, due to the small size of the nucleus, could not deliver more than 5 W. Anyway, it probably is not a transformer, so you should avoid connecting as such.

  • \$\begingroup\$ This part came off of a cardboard mount labeled 'transformer'. I'm pretty sure that this some form of inductor or transformer, but I assumed it was a transformer because it was labeled as such. \$\endgroup\$
    – swe100
    Commented Dec 27, 2013 at 17:30
  • \$\begingroup\$ OK. Then surely worked as an autotransformer, for instructional purposes. It did not miss a cable? \$\endgroup\$ Commented Dec 27, 2013 at 17:35
  • \$\begingroup\$ Not sure if this is an autotransformer or a very strangely mounted inductor. The first lead appears to be an open circuit, while the second and third leads appear to be connected by an inductor. Don't have the equipment to test inductance, though. \$\endgroup\$
    – swe100
    Commented Dec 27, 2013 at 17:39
  • \$\begingroup\$ After detailed reading of the wiki article, I think that my transformer is partially broken, with the windings between the primary and secondary leads intact but the windings between the common node and the other two leads broken. \$\endgroup\$
    – swe100
    Commented Dec 27, 2013 at 17:44
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    \$\begingroup\$ Is it possible that the transformer has lost a cable? It happens that the soldering is broken, or the wire is cut on the inside of the coil and is not seen from the outside until it succeeds in opening the winding ... \$\endgroup\$ Commented Dec 27, 2013 at 18:25

It looks like a trigger coil. Or the primary leads have been yanked out from the other end (a high possibility). Look around and see if the inside coil has been disturbed.


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