Large adjustable inductor?

[BiOzZ]

I'm doing a introduction to electronics thing at a local highschool and were reaching inductors and i thought it would be fun to show using a large adjustable inductor to show the cleaning up of DC signals (along with some other fun stuff) but i cant find one that can get much more over 9mH and im looking for 1-5H or so that can go down to like 10mH

anyone know of anything out there i can grab or build up? if i cant find anything i will make my own using a bobbin and some sort of screw jig

[dannyf]

You can make one out of an opamp + capacitor + resistors.

Otherwise, lots of iron / copper.

[c4757p]

Variac?

[BiOzZ]

Variac?

Both of my vaiacs have large input filters on them so i think i will go the way of using large core-in-bobbin on a screw ... i found a few on ebay but there replacement parts for rare stuff so there real expensive
were snowed in today so i have 2 weeks to finish it now so that seams simplest

i dont want to simulate an inductor as also suggested as i would have to explain that and i cant point and explain how an adjustable inductor works hands on

[Jay_Diddy_B]

Hi,

Have a look at Hammond:

http://www.hammondmfg.com/153.htm

These are made with E and I construction. You can remove the frame and make the air gap adjustable.

The alternative would be to get two or three different value ones in series and make a stepped inductor.



Jay_Diddy_B

[dannyf]

Get a bunch small audio transformers and put them in serial -> you have an adjustable inductor in the 10H range, assuming that you link enough of them .

[Conrad Hoffman]

Look at the construction of the old GR type 107 variable inductor. It didn't go that high, but might give you ideas. You can also drop a ferrite into a coil to raise the value, and maybe drop in some brass tube to reduce it- look up "tuning wand" for coils.

[T3sl4co1l]

Unmodified transformers (or variacs) aren't so great because they will saturate with much DC through them.  Gapped cores are the way to go, as has already been mentioned.

Depending on how much current capacity you need, it might be interesting to look at microwave oven transformers, too.  It takes some work cutting the welds so you can vary the gap, but if this is an option, you should have piles of inductance at your fingertips.

Tim

[calexanian]

The construction style of something with that much inductance will make it hard to have a variable inductor. Especially anything that can handle a DC bias or even low frequency. I can recommend two things. First being a variable core transformer from a simple AC arc welder. THey are only going to be up to maybe 100mh but they do vary. Realistically though just for demonstration purposes an inductor with many taps would be best. 

[tszaboo]

If I remember correctly, at highschool we had some big 5x5x10 cm toroid inductors with some 1000 turns. The inside we could put in a ferrite, the more it was in the higher the value was. Very good for demonstration, and to play with.
I don't remember the inductances though.