Why Does My Custom Toroidal Transformer Blow the Breaker When Connected to 115V Line?

Hi, I am trying to make a Linear voltage regulator power supply. I am using a custom wound toroidal transformer, from 115V U.S. Line Voltage to 12VAC (10:1 windings). The toroid is the blue ferrite core, 20mm O.D. I am only needing about 1 VA of power.

My question is probably very common sense. I am winding the transformer according to the standard transformer design you see everywhere. However when I hook it up to line voltage it blows the circuit breaker. It would seem that this would be the case because the primary windings are basically shorting out the Neutral to the Line wires of the line voltage. Am I missing something obvious? I have a couple of other transformer power supplies and the Neutral and Line test as open-circuit, while mine tests as closed circuit with a small resistance - obviously it would short out and blow the breaker. What am I doing wrong?

William,

the transformer primary must have enough "magnetizing inductance" to limit the unloaded current to a value that does not saturate the core.

A rough rule-of-thumb is to make the magnetizing inductance such that the magnetizing current is 1/5 the operating current. At 115 VAC and 1 W the operating current is approximately 10 mA and so the magnetizing current should be 2 mA. For this to be so the primary inductance is 150H. Using the Al value of the core you can calculate the number of primary turns.

As mentioned, this method is rough. The proper method is to calculate the core flux density and design the transform to not exceed this. At 60 Hz core losses are negligible and the flux density can be run close to the limit.

But, due to the relatively low allowable flux density and permeability of ferrite, 50/60 Hz transformers _always_ use laminated silicon steel cores.

Wow, thanks David, this is exactly the answer I couldn't find anywhere. I knew it had something to do with the magnetization, and likely the inductance of the primary. I may have to switch core material to achieve the required inductance.