Re: Current control

> I do have access to old transformers out of 36v 25A battery chargers 
> but I don't think the core pieces would be long enough.  Do you know 
> anyone who has built such a device?

I asked Richard Hull from Richmond the exact same question about 
using old transformer cores, and he suggested using 6" PVC pipe for 
the form and packing it with welding rods.  You should use the coated 
iron rods with powdered iron mixed in the coating; there will be a lot of 
iron to concentrate the flux and the coating will insulate the rods from 
one another to reduce eddy currents and the associated losses (I 
bought type "7014" rods if I remember correctly).  You can then wind as 
many layers of wire around the form as you want to get any inductance 
you need, bringing out taps every layer or whatever.

As an alternative to taps and the discrete control they would afford, I am 
working on a controller with the rods in one pipe that will slide in and 
out of a slightly larger pipe that will hold the windings (identical to what 
you are referring to).  I don't think a lot of precision will be necessary, 
though, so you shouldn't need a complicated control mechanism.  I was 
initially going to use an insulated handle to move the core in and out by 
hand, possibly going on to build a simple arrangement using a knob to 
move a rope attached to pulleys to adjust the core (sort of like a clothes 
line with pulleys, or the frequency indicator on an "old" analog radio 
that slides back and forth).  Regardless, once you get your variable 
inductor built, figuring out a way to set it should be trivial.

Steven Roys (sroys-at-radiology.ab.umd.edu)