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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

schematic

simulate this circuit – Schematic created using CircuitLab

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

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  • \$\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\$ – Adam Lawrence Jul 19 '13 at 12:24
  • \$\begingroup\$ Is the wire enamal coated? - I know it seems a silly question... \$\endgroup\$ – Spoon Jul 19 '13 at 12:29
  • \$\begingroup\$ Yes they are enamaled, will post more details \$\endgroup\$ – secretformula Jul 19 '13 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 Jul 19 '13 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\$ – secretformula Jul 19 '13 at 14:04
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Due to an error in my calculations, the transformer's impedance was way too low, allowing a large amount of current to flow.

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