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 Nov 3 '16 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 Nov 3 '16 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 Nov 3 '16 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 Nov 3 '16 at 19:12

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 Nov 3 '16 at 19:13
  • \$\begingroup\$ It's got to be worth posting a picture for safety reasons. Electrocution hurts! \$\endgroup\$ – Andy aka Nov 3 '16 at 19:15
  • \$\begingroup\$ Well, i don't have it wired up at the moment. But how should i ground it properly? \$\endgroup\$ – joel Nov 3 '16 at 19:16
  • \$\begingroup\$ Posting a picture is the best way to get decent answers. \$\endgroup\$ – Andy aka Nov 3 '16 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 Nov 3 '16 at 19:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.