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> Light Dimmer Using Toobs
Jimthecopierwrench
Posted: December 23, 2011 12:09 am
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Muse slash thought experiment.

A transformer's primary impedence depends on it's secondary load, correct?

So if one were to use a transformer primary in series with a given load, then loading the transformer secondary would proportionally reduce the primary impedence, allowing the trafo to act as a reasonably linear AC current regulator - correct?

So going further, a pair of inverse parallel triodes directly across the secondary could be used via a ganged pot for grid bias as a dimmer control for a lamp in nseries with the primary?

Oh, I'm not talking efficiency, cost, or even sheer mass. Can already imagine 8 pounds of hundred dollar iron and copper, and a pair of 6L6's for ~40W of less than full range light dimming - But the circuit could actually work, yes?

If not, why? or what would it's electronic limitations be?



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Geek
Posted: December 23, 2011 12:47 am
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Eeeeeeeeeasy! Saturable reactor smile.gif

Cheers!


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Jimthecopierwrench
Posted: December 23, 2011 01:39 am
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Ah, http://en.wikipedia.org/wiki/Saturable_reactor

Not a very indepth article - though many that are end up quickly over my head.

So it would work then. Although I'm not sure where the point is (gross load wise in W) where one might as well simply employ a massive rheostat.


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Geek
Posted: December 23, 2011 01:55 am
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Someone had a YouTube link (Tim, was it you?) on a little potentiometer + battery using a regular Rat-Shack tranny controlling a bigass headlight.

Cheers!


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CWB
Posted: December 23, 2011 03:26 am
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a simple device ...
try a coil in series with the load that you slide an iron core in and out of .

something else that might be of interest is a suitable "magnetic amplifier" circuit .


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AwesomeMatt
Posted: December 23, 2011 04:20 am
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QUOTE
Not a very indepth article - though many that are end up quickly over my head.


Tim once explained saturable reactors when he was helping me figure out power control on my MOT-based welder I was/am building.

Lemme dig...

http://www.dutchforce.com/~eforum/index.ph...15&#entry301815 <-- Here, and the couple following posts.

http://www.dutchforce.com/~eforum/index.ph...79&#entry303679 <-- Again here for a post or two.
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Sch3mat1c
Posted: December 23, 2011 06:10 am
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Some day I'm going to wind a saturable reactor for these induction heater thingies. No one else in the industry does it.. gotta dig around in the machine and unbolt bus bars...or worse!

Anyways, three ways you can control AC power with toobs...
1. Thyratron. C6Js are powerful enough to have been used in machine tools for decades. Use 'em just like voltage mode SCRs.
2. If you want to use a vacuum tube (you usually don't, you need huge cathodes to get the voltage drop low enough to be useful), you can do it AC with a tube per polarity, or you can "short" across the DC output of a FWB -- even vacuum diodes have lower drops than anything with a grid, so this isn't horrible, or it's better if you use gas diodes.
3. Biasing a saturable reactor, or variants thereof (magnetic amplifiers).

My video is here, good memory:
http://www.youtube.com/watch?v=oPg1Cbr-VC0

Tim


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Geek
Posted: December 23, 2011 06:48 am
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That was yours I found from the one I was thinking of thumbsup.gif

Here it is: http://youtu.be/DBX1-POuJMw

Go 35 secs in...

QUOTE
Some day I'm going to wind a saturable reactor for these induction heater thingies. No one else in the industry does it.


Not even the Russians?

Cheers!


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johansen
Posted: December 23, 2011 07:59 am
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saturable reactor for a double digit kw induction heater?

it might make sense actually if weight isn't a concern, and neither is hitting 95% efficiency.

short version: if you want a reactor to control 0-100% of a resistive load, it has to have enough turns to take the full line voltage without too much magnetising current. and at 100% load it has to pass the full line current without that much voltage drop.
In effect it must be as big as a transformer for the same kva rating.
of course if you only need 50-100% load then it can be about half the size.
i've got a book on my hard drive somewhere with a whole chapter in it just for saturable reactors and magnetic amps, written in the 70's.. *still looking for it.


hmmm... all kinds of sick twisted fun you can have.

have a delta magnetic inverter.
http://books.google.com/books?id=tUG1OkBXa...nverter&f=false
with just an oscillator and 6 SCRs


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Geek
Posted: December 23, 2011 08:11 am
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QUOTE (johansen @ December 22, 2011 11:59 pm)
*still looking for it.

egrep wink.gif


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johansen
Posted: December 23, 2011 08:29 am
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it wasn't ocr'd. i'm worried it got deleted somehow, it was an old book, had all the rules of thumb for calculating leakage flux inside dc motors too.


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Geek
Posted: December 23, 2011 09:08 am
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Oh, that would be such a loss sad.gif


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johansen
Posted: December 23, 2011 09:19 am
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Colt45
Posted: December 23, 2011 10:58 am
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Yeah, thyratron is the name of the game here (assuming line voltages, not so hot for automotive).
Been meaning to piddle with some thyratron circuits myself, one of these days when i get some lab time.


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Sch3mat1c
Posted: December 24, 2011 05:19 am
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QUOTE (johansen @ December 23, 2011 01:59 am)
saturable reactor for a double digit kw induction heater?

it might make sense actually if weight isn't a concern, and neither is hitting 95% efficiency.

short version: if you want a reactor to control 0-100% of a resistive load, it has to have enough turns to take the full line voltage without too much magnetising current. and at 100% load it has to pass the full line current without that much voltage drop.
In effect it must be as big as a transformer for the same kva rating.
of course if you only need 50-100% load then it can be about half the size.

Not sure about efficiency, if overbuilt it shouldn't be too bad. Cores near saturation always incur extra losses so you might have that.

Weight isn't a problem, we sell tons of units that have three huge iron / ferrite cored inductors/transformers in them. It takes two men to lift a 10kW unit, let alone the 30kW! Even so, core size drops at high frequency, so it shouldn't be too bad. What worries me the most is figuring how much voltage turns up at the output -- that's the limiting factor in core area * turns, and with Q multiplication, it could be quite high.

As long as the core path is tight with no gaps, magnetizing current can be very small indeed, so it can be used effectively as power control. This goes for anything, of course. You might be surprised how much on-to-off ratio you can get with a simple tape wound core. Those little plastic-boxed toroid cores you see in computer PSUs are actually mag amps, there's a tape wound amorphous core inside. You can put a resistor in series with the core and hook it up to a square wave generator and observe the saturation, it's very sharp and clean. I've seen sub-microsecond risetimes from simple saturation.

Tim


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johansen
Posted: December 24, 2011 05:59 am
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ohhh... you were thinking of saturable reactors on the highfrequency side.

i figured you meant on the 60hz three phase side.


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