[R-390] Question about VARIAC

[Sheldon Daitch]

Barry,

OK, thanks for the clarification.

As others have pointed out, yes, there is absolutely nothing wrong with
putting the line voltage across the entire winding of the variable 
transformer.

As you note, by doing so, the output of the variable transformer will 
never be
any higher than the line voltage.

Thanks for the interesting discussion.

Sheldon



Barry wrote:
> Hi Sheldon,
>
> No, that really isn't where I was going with it.  I just wanted to confirm 
> that connecting the entire set of windings to the input and forcing the 
> variac to stay within 0V to input voltage wouldn't be the wrong thing to do.
>
> WIth my current house voltage running about 121V, this causes the high end 
> of the variac (as wired per the side panel) to run to something over 145V 
> and I don't want that (too much chance of setting the output voltage too 
> high).  Of course, the variac (wired as per the panel) can provide up to 
> 135V which is far too high for these older radios to run comfortably; I'm 
> just trying hoping to avoid this situation too easily.
>
> I think I'm going to simply go with a bucking transformer (6.3V will bring 
> things down quite nicely) and still be able to use the variac with its 
> standard dial plate of 0V too 135V.
>
> I'm just weighing my options.
>
> Thanks!
>
> Barry - N4BUQ
>
> ----- Original Message ----- 
> From: <sdaitch at kuw.ibb.gov>
> To: "R-390 HF Receiver List" <r-390 at mailman.qth.net>
> Sent: Sunday, April 19, 2009 2:48 PM
> Subject: Re: [R-390] Question about VARIAC
>
>
> Bob and Barry,
>
> Looking at Barry's concern again,
>
> "If I were to tap the input at one turn
> (or two or three), then I assume this
> would appear as a very low impedence
> and wouldn't work very well (most likely
> incurring a very heavy current draw and
> hopefully trip the circuit breaker before
> burning out the transformer."
>
> I am almost thinking Barry is looking at
> running the INPUT to the variable transformer
> in the first few turns, making it a quite
> large step-up transformer.  Short (and pun
> intended) of either modify the unit to do
> so, or feeding the input into the variable
> contact (not normally recommended, I beleive)
> there isn't enough winding in the transformer
> to make it operate as a transformer, but more like
> a very low inductance coil.
>
> Certainly that isn't the way the variable transformer
> is designed to operate, but when Barry discusses
> a few turns, "very low impedance" and high current
> draw, it is almost like the discussion is on the
> theoretical, "if you used only a few turns" on
> the variable transformer as the primary.
>
> Barry, correct me if that is not where you were
> thinking.
>
> 73
> Sheldon
>
>
>
> ----- Original Message -----
> From: rbethman <rbethman at comcast.net>
> Date: Sunday, April 19, 2009 9:37 pm
> Subject: Re: [R-390] Question about VARIAC
>
>   
>> Barry,
>>
>> The impedance of a transformer "usually" does NOT come into play
>> UNTIL
>> there is a short-circuit condition.
>>
>> Variacs/autotransformers, have a current rating based on conductor
>> size,
>> "primarily", and can be run throughout the variable ranger that
>> they are
>> built for.  The number of turns used, determines the voltage out.
>> You
>> are not matching an impedance as we are accustomed to with RF.
>> Power
>> transformers are really a different animal.
>>
>> Bob - N0DGN
>>
>> Barry wrote:
>>     
>>> I guess what I was getting at was impedences.  If I were to tap
>>>       
>> the input at
>>     
>>> one turn (or two or three), then I assume this would appear as a
>>>       
>> very low
>>     
>>> impedence and wouldn't work very well (most likely incurring a
>>>       
>> very heavy
>>     
>>> current draw and hopefully trip the circuit breaker before
>>>       
>> burning out the
>>     
>>> transformer.  If there were double, triple, etc., the amount of
>>>       
>> turns, this,
>>     
>>> too, would affect the input impedence, would it not?
>>>
>>> I guess what I was trying to say is that I figure there's a
>>>       
>> point where the
>>     
>>> number of turns matters but just wasn't sure where that point
>>>       
>> is.  Is that
>>     
>>> incorrect?
>>>
>>> Thanks,
>>> Barry - N4BUQ
>>>
>>>       
>> -- 
>> Bob - NØDGN
>>
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>
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