Crude Variable Inductor / Transformer

[Raven Luni]

Greetings,

This idea was sparked by the joule thief experiments but I decuded to make a new thread because something like this has a wide range of potential uses.

If you were to put a whole bunch of individual windings on a rod of (core meterial of your choice), would I be right in assuming that connecting a number of these in series would simply be seen as a single winding with the total number of turns? What about the joins? Would they have any effect on the field since they are basically interruptions in the coil shape? Am I also right in assuming that unconnected windings would have no effect since nothing is induced in them?

The idea is basically to have a bunch of windings - say 10 turns each that can be used to make a transformer with a variable ratio (as this seems to be one of the important variables in JT efficiency). It also allows for the possibility of multiple secondaries (which I've seen in a few of the popular variants).

 

[(*steve*)]

Subject to them all being wound the same way, and the frequency being low enough that signal propagation through the coil is not an issue, then yes, you can do that.

Beware though that additional turns on the end of a coil can act as an autotransformer winding and you can get surprisingly high voltages in some cases.

Another way to vary the inductance is to have a fixed number of turns, but slip the core part of the way out of the coil.

 

[Raven Luni]

Interesting - was just reading about autotransformers which lead to the question "can I build a variac with the stuff I've got?" - and I ended up wondering - why are there no designs that use a ball bearing or roller bearing instead of a brush. It seems like such an obvious way to do it with much less friction and wear and it would be great for commutators in motors. I searched and couldnt find anything similar. Have I just invented something?

 

[duke37]

As I remember, a Variac has a twin brush, glued together so that there is a resistance between one turn and the next. If a ball bearing were used, there would be a shorted turn when placed on two turns, resulting in a considerable amount of smoke.

 

[poor mystic]

I was once zapped by a series LC arrangement that was running on a low supply voltage. The 'zapping' occurred because even though the impedance of the reactances was quite low when measured in series, the reactances of the L and C when measured individually was quite high, and the voltage I felt was quite high, too.

Just mentioning that for interest's sake, (and safety's sake too) since you're out to play.

 

[Raven Luni]

Ah - found one with a roller - so they do exist

 

[Raven Luni]

As I remember, a Variac has a twin brush, glued together so that there is a resistance between one turn and the next. If a ball bearing were used, there would be a shorted turn when placed on two turns, resulting in a considerable amount of smoke.

It took me a while to get that but I see what youre saying now - the shorted turn would act as a separate winding with high current right?

 

[duke37]

Yes, that is the idea. I saw somewhere that modern variacs used carbon fiber brushes to do the same job but don't know if this is correct. My little variac came out of a skip and I fitted it with a 'brush' made from a 6B pencil.

The roller transformer you show is interesting, are there two magnetic circuits there? There seems to be one slab on top of another.

Electrical distribution transformers have taps to adjust the voltage on load, I am not clear how they do this.