I am currently building a step down transformer with multiple secondaries and a single primary. The core material is standard silicon steel EI laminations.

The primary is spec'd out to use 127 turns of magnet wire with the secondaries being proportional to that in the normal way for voltages. I have 3d printed a custom bobbin made out of abs plastic.

The problem is after I wind the entire transformer and plug it in for some reason under no load 8 amps of current are drawn on the primary which leads to A LOT of heating requiring me to shut down the transformer to prevent damage. What are some possible reasons for this high current draw when the transformer under max load was designed to draw no more than a single amp on the primary?

EDIT: Detailed Design

The core is made of silicon steel rated to 2T with a center leg width of 22.23mm and a stack height of 90mm.

Primary - 127 Turns of 24AWG

Secondary 1 - 14 Turns of 18AWG

Secondary 2 - 9 Turns of 18AWG

Secondary 3 - 6 Turns of 24AWG

Secondary 4 - 17 Turns - Center tap - 9 Turns - Tap - 17 Turns


simulate this circuit – Schematic created using CircuitLab

Above is the basic test schematic, just insert the additional secondaries which were also open.

  • \$\begingroup\$ Please specify more details; which (and how many) laminations, secondary turns, etc. as well a simple drawing of how you've wired the test circuit. \$\endgroup\$ Commented Jul 19, 2013 at 12:24
  • \$\begingroup\$ Is the wire enamal coated? - I know it seems a silly question... \$\endgroup\$
    – Spoon
    Commented Jul 19, 2013 at 12:29
  • \$\begingroup\$ Yes they are enamaled, will post more details \$\endgroup\$ Commented Jul 19, 2013 at 12:43
  • \$\begingroup\$ Did you measure the primary with RLC meter before adding secondaries? Did you test the primary without the secondaries? \$\endgroup\$
    – Vasiliy
    Commented Jul 19, 2013 at 13:56
  • \$\begingroup\$ The primary without the secondaries draws 8A. I don't have an RLC meter but it has a resistance of around 4ohms \$\endgroup\$ Commented Jul 19, 2013 at 14:04

1 Answer 1


Due to an error in my calculations, the transformer's impedance was way too low, allowing a large amount of current to flow.


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.