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Old 7th April 2007, 07:25 PM   #1
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Default Autoformer as choke?

Well, I thought I had found a killer deal at the surplus store on some 150VA 240VAC-120VAC step down transformers to use for tube PS's. On bringing them home I found I had purchased some autoformers

So, do they have any value as PS chokes? Is there an easy way to roughly estimate inductance without purchasing a meter?
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Old 7th April 2007, 07:58 PM   #2
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Default Re: Autoformer as choke?

Quote:
Originally posted by leadbelly
Well, I thought I had found a killer deal at the surplus store on some 150VA 240VAC-120VAC step down transformers to use for tube PS's. On bringing them home I found I had purchased some autoformers
Bummer.

Quote:
So, do they have any value as PS chokes? Is there an easy way to roughly estimate inductance without purchasing a meter?
I would say that the value for PS choke use would be extremely limited as they stand right now. Power xfmrs don't include core gaps, and so can't tolerate very much DC without excessive DC magnetization and possible core saturation.

You might possibly be able to take the cores apart and restack with a gap. That way, they might be useful as ripple chokes.
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Old 7th April 2007, 08:01 PM   #3
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Hi Leadbelly,

First thing you'll need to do then is restack the laminations into a gapped core to handle the DC. Gap size depends on their VA size. 150VA would probably need .6-1mm gap.

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Old 7th April 2007, 08:11 PM   #4
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This requires at least a DVM, but one crude way is to check its impedance of a 60hz (or other) signal, best to use real, fused stepdown/isolation xfmr as source (Im assuming no real test gear.)

Set DVM to ACV, hook it across a small (ohmic value) resistor in series with the inductor and use Ohm's law to see how much current flows when the inductor is shorted across the source. The resistors and source resistance add some impedance, but really shouldnt be significant with an iron choke. If the choke saturates or just isnt very inductive this wont be valid. The choke will likely act as a swinging choke as it wont have much airgap stock, these are tricky to measure properly with a meter...

Toroidal core autotransformers such as seen with most variacs have no airgap at all and will saturate readily. Sometimes people take shot variacs and cut the core to press them into ballast service.

This is quite a crude method and only gives a guess, in actual service there will be DC to contend with. Im sure there are better ways.

Inductor Impedance Calculator.
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