I have a HVAC which is having a 24V 40VAC transformer. I might have shorted the output of the transformer while working on the wires.

I noticed that the fuses on the input side of transformer blew off and I replaced them with time delay 2A fuses. I unloaded the output of transformer (open circuit) to test if transformer is providing the right volts. Here the fuses blew off again.

Is it because the transformer is bad or I am using wrong fuses here ?

Transformer: Input: 240V Output: 24V 40VAC

  • \$\begingroup\$ I might have sorted the output Do you mean shorted? I unloaded the o/p of transformer what does o/p mean? If you want help, at least make your question clear and don't use abbreviations. Did you replace the fuse with a new fuse having the same ratings? \$\endgroup\$ May 26, 2020 at 11:53
  • \$\begingroup\$ It had mda 2A fuse which i replaced with mdl 2a fuse. The both are slow blow fuse but not sure if that's what is causing it. \$\endgroup\$
    – Mohit
    May 26, 2020 at 12:03
  • \$\begingroup\$ Does the transformer name plate read 'Primary 240V~ 50Hz, Secondary 24V - 40VA'? Does it have only 2 windings and have you inadvertently applied 240V on the secondary side? \$\endgroup\$
    – vu2nan
    May 26, 2020 at 13:22
  • \$\begingroup\$ The transformer take either 208V or 240V and the inputs are labeled. The o/p is 24V 40VA. I provided input to the 240V \$\endgroup\$
    – Mohit
    May 26, 2020 at 14:34
  • 1
    \$\begingroup\$ If disrupting the current in a transformer you will generate voltage spikes. U=L*dI/dt. Så large/fast current change dI in short time dt will give a large voltage spike. L is the inductance of the windings. This could have damaged the isolation in your transformer and led to a short circuit. This you can measure with an ohm meter provided you know what value is should be! \$\endgroup\$ Feb 23, 2021 at 11:38

1 Answer 1


If the input side of the X-former is blowing fuses with no-load on the output, you killed your transformer when you shorted it before. In a no-load situations, using a previously known good transformer, the fuses (even if not the right size) should not be blowing.

  • \$\begingroup\$ is there a possibility of inrush current with no load on secondary causes fuses to blew quickly? trying to understand how i can properly test the transformer to make sure its not bad. \$\endgroup\$
    – Mohit
    May 27, 2020 at 2:17
  • \$\begingroup\$ No. An open secondary will result in nearly zero inrush current. Re: testing - You're already testing it with fuses ;) No point in me typing out "how to test a transformer", when you can google it... i.e. hunker.com/13414369/how-to-test-a-24-volt-transformer \$\endgroup\$
    – Kyle B
    May 27, 2020 at 5:45
  • \$\begingroup\$ Actually that was a lame link. Sorry. youtube.com/watch?v=D0OQvuux6CE is a little better. Unfortunately, you can't always prove a transformer is good with just an ohmmeter, but you can sometimes prove it's bad. \$\endgroup\$
    – Kyle B
    May 27, 2020 at 6:21

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.