I was given an unknown transformer that I need help with. It has three taps on one side and 5 taps on the other.

I was told that it was a 120 volt input transformer but when 120 volts is applied it trips the breaker in my building. I need help figuring out the input and output voltages.

I am new to all of this so please be patient.

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  • 3
    \$\begingroup\$ Surely you had a low-current rating fuse in the 120 V line that you used to test the transformer?! \$\endgroup\$
    – Transistor
    Dec 30, 2020 at 17:15
  • \$\begingroup\$ Where did you apply the 120 volts? \$\endgroup\$
    – Hearth
    Dec 30, 2020 at 17:16
  • \$\begingroup\$ I applied 120 volts to the two outer taps on the transformer. The center on is turned up. On the three tap side two taps are flat and one is turned up it that matters \$\endgroup\$
    – chrisky1
    Dec 30, 2020 at 17:22
  • \$\begingroup\$ @chrisky1 Can you give us any photos of the transformer? Especially any markings on it. It's possible that the three terminal side is the secondary, not the primary. And please do be a little more careful with any further tests you run at 120 volts! \$\endgroup\$
    – Hearth
    Dec 30, 2020 at 17:29
  • \$\begingroup\$ @chrisky1 I would also recommend, rather than hooking it up to mains again, measuring the resistances between each pair of terminals with a multimeter; that will help us figure out how it's connected internally. Incidentally, please use the @ feature on this site to notify people when you reply to them; I only saw this one because I'm in the habit of checking because many new people don't realize that we don't get notified if there's no @. \$\endgroup\$
    – Hearth
    Dec 30, 2020 at 17:30

1 Answer 1


Your transformer most likley is from a automotive battery charger. I've seen lots like it. Check the windings with an ohmmeter, the windings with the highest resistance should be the primary.


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