I have setup a step down transformer to transform my 115v AC down to 5v AC. I ordered a transformer and hooked it up in parallel (there are two coils) and my oscope is showing 10v on the output side. There isn't any load on the circuit. Is this similar to show an unload DC circuit will show higher voltages when not loaded.

Thanks in advance!



  • \$\begingroup\$ 10V what? 0-to-peak? peak-to-peak? RMS? 10V zero to peak could be possible, bit high but unloaded not excessively extreme. Peak to peak is low. RMS is too high. Also "show your work": I.e. pictures and schematics. We're, unfortunately, still not psychic about what you bought and how you connected it exactly when you describe it like that. \$\endgroup\$
    – Asmyldof
    Jun 16, 2015 at 19:08
  • \$\begingroup\$ What is the make and part number of the transformer? \$\endgroup\$ Jun 16, 2015 at 19:12
  • \$\begingroup\$ @Asmylodf, 10v zero to peak. I will work on putting up a schematic, still drawing it in Eagle. \$\endgroup\$ Jun 16, 2015 at 19:30
  • \$\begingroup\$ @PeterBennett digikey.com/product-detail/en/0/237-1557-ND is the link of the item I ordered. \$\endgroup\$ Jun 16, 2015 at 19:31
  • \$\begingroup\$ That transformer has outputs that can be connected in different ways. Do you have the secondaries in series or in parallel? If you have them wired in series then you will get around 10V. If you have them wired in parallel you should get 5V. Otherwise, it is very common for mains transformers to be off. That is the reason why you should connect a regulator after your rectification and smoothing circuitry (assuming you want 5VDC out, not 5VAC) \$\endgroup\$
    – DerStrom8
    Jun 16, 2015 at 19:40

1 Answer 1


As you have 0 to peak 10 volts, this means a RMS of around 7 volts. If you now add a small load it should drop to 5V.

This is very common. Especially the internal resistance of the transformer leads to a fairly high idle voltage.

When you rectify it and add a suitable capacitor you should have something like 5.5V or the like.

  • \$\begingroup\$ thank you, now that I have done as you said, I ended up with almost exactly 5.5 volts. Once I completed my circuit it acted as expected. Thanks to the other that helped as well. \$\endgroup\$ Jun 17, 2015 at 0:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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