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 Post subject: Leave 2.5V winding open
PostPosted: Jun Sat 15, 2019 11:13 pm 
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Do you think or know if it would be OK to leave a 2.5V filament winding disconnected and just use the 5V, 6.3V and HV windings of a power transformer?

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Sat 15, 2019 11:25 pm 
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Absolutely yes. Never a problem to leave a winding unused. The only thing that may be affected is that the voltage on the other windings might be ever so slightly higher in the absence of the usual load on the 2.5 V winding, but you will likely not notice.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Sun 16, 2019 7:08 am 
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Test the transformer to find out if there is any voltage between one
wire of the 2.5 winding, and the metal shell. Do this carefully.

If you find any voltage, find out what resistance makes it drop 50 %

Then ponder.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Sun 16, 2019 7:30 pm 
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radiotechnician wrote:
Test...

Then ponder.

I think you're trying to be nice and diplomatic in hinting that something important may have been missed here.

Feel free to fill in the missing information. Don't worry, if I've missed something important, my feathers will not be ruffled.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Sun 16, 2019 7:42 pm 
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Of course, also use the 120V input winding.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Sun 16, 2019 9:46 pm 
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Tom Albrecht wrote:
radiotechnician wrote:
Test...

Then ponder.

I think you're trying to be nice and diplomatic in hinting that something important may have been missed here.

Feel free to fill in the missing information. Don't worry, if I've missed something important, my feathers will not be ruffled.


The lay of the transformer is of interest. What winding is wound above and below the 2.5 volt.

With no information about type, date of manufacture, previous use, condition of
insulation can be probed by how much electrostatic coupling there is between primary,
high voltage secondary, and frame.

If you did find a voltage, with a resistance box, you can further find out just how
much current could flow from the unused winding.

If none, what to do ?

Ground one end, or connect a resistor of several hundred thousand ohms to case.

Just thoughts of an electrician. I could wire your house. But would probably screw it up.

However after 20 years working in industry with new, and very old, insulation, voltage stress
is always on my mind.
Attachment:
Avoid an extended argument about the  2 point 5 volt winding use Build this to spiff up your project Use a 3AP1.jpg
Avoid an extended argument about the 2 point 5 volt winding use Build this to spiff up your project Use a 3AP1.jpg [ 437.2 KiB | Viewed 578 times ]


Attachments:
Avoid an extended argument about the  2 point 5 volt winding use. Build this to spiff up your project Use a 3AP1.jpg
Avoid an extended argument about the 2 point 5 volt winding use. Build this to spiff up your project Use a 3AP1.jpg [ 437.2 KiB | Viewed 578 times ]

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 12:26 am 
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If the 2.5V winding is no longer connected or used in any way, why are we concerned about leakage currents to or from this winding? I think it would be a case of tape off the wires and forget it.

Seems to me that more concern might reasonably be directed to windings that are being used.

An interesting and useful subject, but transformer leakage current was not what the original question asked about.

Ted


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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 12:45 am 
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The question was whether to leave it disconnected, so discharging it to ground would
also make sense. Radio case histories have instances of clicking caused by discharges
that are solved by such a resistor.

The example circuit showed at least one transformer described in the OP was made 82
years ago.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 4:14 am 
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The power transformer is on a $10 chassis I bought yesterday, a Grunow 9A, used in the 1933/4 Model 109 and 209 radios:

https://www.radiomuseum.org/r/general_ho_902.html

http://www.nostalgiaair.org/pagesbymode ... 008431.pdf

I do not expect to ever find a cabinet for it, and really bought it for the tube shields, but now realize that chassis may provide an excellent power transformer and choke to use to build another amplifier, or to rebuild my recent push-pull 6L6GC amplifier with higher B+ voltage.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 7:17 am 
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Nice. Tape up each of the two 2.5 volt wires and leave the center tap grounded.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 3:59 pm 
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If the transformer is for 110-155vac line, the unused winding can be pressed into service as a buck winding for the primary. Won't save much but if the 6.3 vac is too high under load that 2.5 may be helpful...

Chas

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 4:44 pm 
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Thanks, the bucking function is a good idea, as long as the center tap is easily disconnected from ground.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 5:11 pm 
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A winding insulated for 1.25 volts above ground, in a transformer made in 1933,
connected in series with a 120 volt primary winding ? :shock:

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 5:53 pm 
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Also a good point, although I bet they used the same varnish on all of the windings.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 10:50 pm 
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So as usual I am lost here. What is the schematic of the oscilloscope supposed to prove? Both of the six volt windings are also connected to ground on one end and to aphabet letters on their other ends, same as the 2.5 volt one. It means that they are continued elsewhere on the schematic but the lines were not drawn for clarity. It does not mean they are disconnected.

Regarding the original question, yes it is perfectly okay to leave unusued transformer windings disconnected. Just insulate the ends of the unused leads well so they cannot short out to anything.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Mon 17, 2019 11:17 pm 
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I just remembered that many of us have used 12V windings to make bucking transformers.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Tue 18, 2019 5:21 am 
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Chris108 wrote:
So as usual I am lost here. What is the schematic of the oscilloscope supposed to prove? Both of the six volt windings are also connected to ground on one end and to aphabet letters on their other ends, same as the 2.5 volt one. It means that they are continued elsewhere on the schematic but the lines were not drawn for clarity. It does not mean they are disconnected.

Regarding the original question, yes it is perfectly okay to leave unusued transformer windings disconnected. Just insulate the ends of the unused leads well so they cannot short out to anything.


Attachment:
Mistakeless oscilloscope.jpg
Mistakeless oscilloscope.jpg [ 270.98 KiB | Viewed 429 times ]

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Tue 18, 2019 3:35 pm 
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I also have no idea why the oscilloscooe schematic has been posted. All of the filament windings in it are used.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Tue 18, 2019 4:44 pm 
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oops

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Last edited by battradio@ on Jun Tue 18, 2019 4:47 pm, edited 1 time in total.

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 Post subject: Re: Leave 2.5V winding open
PostPosted: Jun Tue 18, 2019 4:47 pm 
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He is just being OSC , about an unused winding , have seen unused windings from the factory , they either got a great deal on them or use them in other models that need the extra winding .

Much todo about very little

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