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You need a closed magnetic loop for an efficient transformer. You need laminated soft magnetic steel for transformers up through the audio range and generally some type of ferrite material for frequencies above that. Any form of solid iron or steel for a core will give high eddy current loses in the transformer, which will generate heat.
DO you have a particular need for one that you wna to build or is this a just for fun learning project?
If your just looking to experiment pick up some old battery chargers or microwave ovens. They are the simplest and easiest transformers to rewind and many have the primary and secondary windings side by side.
For HF rewinds the small toroid inductors from the line filters can work rather well with very little work involved in rewinding one of them. Or the ones you can find in most electronic devices can be reworked too!
But for application specific there are many variables that have to be acounted for before one can be hand built.
lol, ya , i luv these flybacks, but its too big(volume), all i really want is the 2ndry coil and the core, but the blodys all glued together, so i can get the secondary out ok, but i keep shatterin the core, ,, i wanna actually set a few up in series on the same core, and form my own primary loop.
In another expirment i had a 57khz flyback(running off a 2n3055 + sine generator) , when i took the leads from the primary coil , and put them on the feedback loop , the resonant frequency went up to 420khz , and my voltage potential trippled, this also cut my current draw from 0.50 A (which is typical for flybackdriver circuit) to 0.08A, however i need to resort to plan b since im haveing problems with a 5-9v clock ringing that fast (iv used 555 timers and hex inverter clks)
The core of an ignition coil is a solenoid (straight) That's why you can pick them up on an AM radio, the magnetic loop isn't closed. Proper manufacturing of a toroid or pot core ignition coil would be prohibitively expensive. You should have had a loop of wire around that coil. Maybe attached to the case itself.