Probably this is a noob question but i can't find an answer on the web.

I salvaged a transformer from an old radio. I wanted to test it, so i began to wire live to the primary coil and neutral to the other end of the primary. I didn't do anything with the ground wire, should i? I connected my voltmeter to the secondary coil. I plugged it in and... the fuse switched off. The fuse in the meter cupboard, i didn't have any connected to my transformer.


  • \$\begingroup\$ several things to check here: a) what is 'the other end of the primary'? b) are you sure it is a mains transformer and not something like a speaker LF transformer (SMT)? c) what do you get when you measure the resistance of the primary winding? \$\endgroup\$
    – dlatikay
    Commented Nov 3, 2016 at 19:01
  • \$\begingroup\$ A) The primary coil has 2 wires, i meant the other wire of the primary coil (Sorry, english isn't my first language) \$\endgroup\$
    – joel
    Commented Nov 3, 2016 at 19:05
  • \$\begingroup\$ B) i'm quite sure C)maybe i messed up with the primary and secondary. As i have wired it as i explained in the question, the primary has 1 ohm and the secondary 133 ohm. \$\endgroup\$
    – joel
    Commented Nov 3, 2016 at 19:10
  • \$\begingroup\$ Yes. the primary winding's DC resistance in a mains transformer would be higher than that of the secondary. As for the ground wire, it should be, well, grounded :-) so your circuit breaker would save your life in case it is shorted to its core. \$\endgroup\$
    – dlatikay
    Commented Nov 3, 2016 at 19:12

1 Answer 1


the primary has 1 ohm and the secondary 133 ohm

Sounds like you got primary and secondary the wrong way round - always, for an AC step-down transformer the primary DC resistance will be significantly greater than the secondary DC resistance.

However, without a picture there's no guarantee that the primary is suitable for AC mains voltages.

  • \$\begingroup\$ Thanks alot for your help! I knew it was a noob question :) \$\endgroup\$
    – joel
    Commented Nov 3, 2016 at 19:13
  • \$\begingroup\$ It's got to be worth posting a picture for safety reasons. Electrocution hurts! \$\endgroup\$
    – Andy aka
    Commented Nov 3, 2016 at 19:15
  • \$\begingroup\$ Well, i don't have it wired up at the moment. But how should i ground it properly? \$\endgroup\$
    – joel
    Commented Nov 3, 2016 at 19:16
  • \$\begingroup\$ Posting a picture is the best way to get decent answers. \$\endgroup\$
    – Andy aka
    Commented Nov 3, 2016 at 19:18
  • \$\begingroup\$ if it is meant to be grounded (as you said), then usually you would place it in a case and attach a triple-pole mains cable, properly strain-relieved with the ground wire longest so that it will detach last. and use a standard mains plug (in EU: "Schuko") \$\endgroup\$
    – dlatikay
    Commented Nov 3, 2016 at 19:20

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