Fun with a variac

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John Fields 13 years ago

From an earlier thread about the frequency response of variacs, I realized that a variac could be used as a variable inductor.

As such, it could be used to vary the frequency of an oscillator.

I kluged this together with stuff I've got around the shop just to see what tuning range I could get. (View with a fixed-pitch font)

. +--------------------------------+ . | POWER AMP | . | +---------+ | . +-|IN OUT|---+-10k-+ | . | GND | | | | . +----+----+ 100R 100n | . | | | | . +--------+ +-----+ | . | | | | . +->VARIAC [10k]

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Jan Panteltje 13 years ago

On a sunny day (Sat, 29 Jun 2013 15:44:15 -0500) it happened John Fields wrote in :

Does the inductance - frequency change a lot of you magnetize it with for exampel a permanent magnet or some extra turns and a power supply? Can you play theremin with a magnet? ;-)

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Jasen Betts 13 years ago

so you've got the shorted turns there to eliminate the mutual inductance of the top-end windings?

?? 100% natural --- news://freenews.netfront.net/ - complaints: news@netfront.net ---

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Tim Wescott 13 years ago

Fun. Now put a servo on it and you'll have a building block of a really slow PLL.

Tim Wescott Wescott Design Services http://www.wescottdesign.com

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Fred Abse 13 years ago

You have half the winding shorted.

With the slider at 50%, and a short from slider to bottom, the variac I measured showed 15 millihenry from top to slider. With an end-to-end inductance measured at 10.3 henries, that would seem to be a believable leakage inductance.

A plot of frequency versus rotation would be interesting.

I see no resonating capacitance. I suspect that your amplifier parameters are playing a significant part in what you see.

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

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John Fields 13 years ago

The arrowhead is the slider, which leaves the turns above it unshorted and, thus, the inductance of the variac at that point.

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John Fields 13 years ago

With the thing idling at around 5kHz, you can get about a -20% change in frequency by waving one of these around the outside of the can: http://www.kjmagnetics.com/proddetail.asp?prod=SX0 If you hold it still after waving it around, or let it latch on to the steel can, the frequency eventually drifts back up to its idling value.

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John Fields 13 years ago

Oops... Schematic error! Good catch, Thanks. :-) It's really wired like this: . +--------------------------------+ . | POWER AMP | . | +---------+ R2 | . +-|IN OUT|---+-10k-+ | . | GND | R1| |C1 | . +----+----+ 100R 100n | . A1 | | | | . +--------+ +-----+ | . | |L1 |R3 | . | VARIAC [10k]

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John Fields 13 years ago

No, it was a schematic error. Here's how it's really wired: . +--------------------------------+ . | POWER AMP | . | +---------+ R2 | . +-|IN OUT|---+-10k-+ | . | GND | R1| |C1 | . +----+----+ 100R 100n | . A1 | | | | . +--------+ +-----+ | . | |L1 |R3 | . | VARIAC [10k]

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Fred Abse 13 years ago

Thanks.

^^^^^^^^^^^^^^^ I didn't write that.

I didn't spot the 100nF at all, I have difficulty with ascii circuits, it's a perception thing, hate the goddamn things.

In view of the 100nF that I hadn't spotted, it's irrelevant.

Sorry.

I'm still intrigued by the shorted winding, though, if I'm reading the ascii right.

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

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Fred Abse 13 years ago

Where's the variac slider, now?

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

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John Fields 13 years ago

Right. I wrote it as part of my reply and forgot to edit it out. Sorry.

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Jan Panteltje 13 years ago

On a sunny day (Sun, 30 Jun 2013 14:51:18 -0500) it happened John Fields wrote in :

Cool :-)

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John Fields 13 years ago

. +-----------------------------+ . | POWER AMP | . | +---------+ R2 | . +-|IN OUT|---+-10k----+ | . | GND | R1| |C1 | . +----+----+ 100R V 100n | . A1 | | A | | . +--------+ R

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Fun with a variac

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