1/4 inch bolt transformer?

[flat5]

In the 1970s I needed an inductor to isolate the power supply feeding a crystal oscillator. Not having one, I wound some wire on a bolt. It worked.

I don't have any magnet wire to try this but to make an audio transformer (not a good one) might it be possible to wind 50 turns (guessing) on a 1/4 inch steel bolt. Bring the ends out. Then wind maybe 200 to 400 turns over the first winding ending up with a transformer?

 

[Nigel Goodwin]

It would be a transformer, but a really poor one, and very low impedance.

 

[flat5]

Thanks...and with a steel bolt I'd guess it's high frequency response would be terrible.

The number of turns was just an uneducated guess.
What makes a real transformer more efficient?
A "round" core that contains the field?

 

[kblack]

lamanated steel, and the steel designed for the correct permability. I've salvaged the cores from old transformers and made crude coils. It does work, but if given the option I'd buy one with known specs.

 

[crutschow]

For an efficient transformer you need a complete, low magnetic impedance, magnetic loop. Just like electricity, the magnetic field needs a complete return path. That's why most transformers have a "C" core or toroidal core where the complete loop consists of a good magnetic material. A bolt's magnetic return path would be from on end of the bolt, through the air, to the other end of the bolt. Since air has a very high magnetic impedance (permeability) as compared to iron, the transformer would not be very efficient.

You could improve the bolts characteristics by using a "U" bolt with a magnetic plate bolted across the open end to complete the magnetic loop. But it would still be a poor transformer as compared to one using a proper transformer core with good magnetic characteristics and laminations to minimize eddy current losses.

 

[MrAl]

Hi there,

To add to the other posts...

Another problem with plain steel (bolt or nail) is that the max saturation level
isnt that high so you need a lot more turns to make a transformer.
Also, there are eddy currents which eat up power and heat up the core.

Nothing says it better though than a picture. Below is a picture of a
BH curve of a material that is designed just for the sake of making
transformers and inductors, and if you look real close and zoom in
on the very center of that curve you'll see another curve in blue,
and notice how much smaller that blue curve is than the larger
big 'S' curve is. Well, the big curve is the magnetic material,
and the very small blue curve is the plain steel bolt or nail.
Quite a difference.

I've also included a 5x zoomed view of the small blue curve...

**broken link removed**

This is just an illustration, as in real life the blue curve could be much smaller too,
as small as 200 times less than the big curve, which says that a bolt or nail
saturates 200 times easier than a good grade of magnetic steel. This is
partly because the steel is specially made to be used with magnetic fields.

Total saturation basically means the steel core disappears, magnetically,
and the construction, even though the core is still inside the wire coil,
turns back into an AIR CORE COIL until the current level drops back down
to some much smaller level where the steel can again become magnetically
active.

 

[flat5]

Thank you, crutschow, and MrAl.
I'll re-read that a few times.
Some of it should sink in

A bolt can be used to make an acceptable electromagnet but not a transformer.

 

[MrAl]

Hello again,


Well, with dc current there is no eddy current losses so it's a little bit better,
however any electromagnet would benefit from a core made from a
good magnetic material.
The force is 'amplified' by the magnetic properties of the core, and
a good core amplifies more than a just so-so core. A plain steel
core might amplify by 100, but using a good magnetic material
the amplification can be 1000 to 20000, which is much much more
meaning the electromagnet would pick up a heavier weight.
For comparison, an air core doesnt amplify at all.

 

[Hero999]

Transformers can't convert DC.

If there's a DC current there would still be eddy losses and the core would saturate much more easilly.

 

[flat5]

Do relays use fancy cores?

 

[Nigel Goodwin]

Do relays use fancy cores?

It's just an electromagnet, why would it?.

 

[flat5]

Require less power to operate.
Perhaps be lighter in weight.
Quicker.

Maybe other reasons?