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Fun with a variac

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]

Reply to
John Fields
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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? ;-)

Reply to
Jan Panteltje

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 ---
Reply to
Jasen Betts

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
Reply to
Tim Wescott

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)
Reply to
Fred Abse 

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

--
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.
Reply to
John Fields 

--
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]
Reply to
John Fields 

--
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]
Reply to
John Fields

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)
Reply to
Fred Abse

Where's the variac slider, now?

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

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

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

Cool :-)

Reply to
Jan Panteltje

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

Reply to
John Fields

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