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 Post subject: Auto transformer Question
PostPosted: May Sat 12, 2018 11:11 pm 
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Auto transformer question
So I have a transformer with four primary taps.
0 v
105v
115v
125v
If I use it in conjunction with one of the primary taps besides the Vin what does that do to the secondary VA?
In other words, if I have 0 and 125v primary and 6.3v secondary but decided to use 0 and 105 to feed another circuit, autotransformer, what effect does that have on the secondary side?

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 Post subject: Re: Auto transformer Question
PostPosted: May Sat 12, 2018 11:24 pm 
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Not possible to give an absolute answer without doing a few tests. In general, a transformer will have an overall VA rating and / or current ratings for each output winding.

With a tapped primary, and one or more secondaries, there is normally not going to be a rating for using the primary as an autotransformer.

We can bound the problem: Suppose you have the one 6-volt winding, rated at 5 amps. That's 30VA for the whole transformer. You can probably add 5-10VA to the primary side, but probably NOT another 30.

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 Post subject: Re: Auto transformer Question
PostPosted: May Sat 12, 2018 11:47 pm 
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My whole post went to the bit bucket
Will try back later

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 Post subject: Re: Auto transformer Question
PostPosted: May Sun 13, 2018 3:59 am 
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Secondary?

I thought it was an auto transformer?

I suppose you could call the output side a "secondary."

Rich


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 Post subject: Re: Auto transformer Question
PostPosted: May Sun 13, 2018 5:51 pm 
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Transformer has a kvA rating. You can't exceed it.

And, there is no ISOLATION between the AC line and the load. Those transformers were not intended for 6.3 volt filament operation, if that is part of what you were asking. Filament current will quickly eat up kvA ratings.

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 Post subject: Re: Auto transformer Question
PostPosted: May Mon 14, 2018 2:25 am 
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In most cases transformer windings are made of the same size wire from end to end. There were some exceptions to this, but if the same size wire is used throughout a winding, then the current anywhere in the winding is fixed by the maximum allowable temperature. That in turn is determined by the cross sectional area of the wire, and the insulation on it.

Practically speaking you have no control over the wire or the insulation once the transformer is manufactured. What you can control is the load on the transformer according to its VA rating. In order to be complete, the voltage or the current at which the VA number obtains must also be known. For example, if I have a "universal" 240-V/120-V travel autotransformer that is rated for 60 VA output at 120-V, I know that in order to obtain 60 VA at 120-V, the winding must be able to handle 0.5-amps.

Now if the transformer has a 100-volt tap, it does not mean that I can keep on drawing 60 VA by raising the current to 0.6-amp. That's 20% more current than the winding was made for, and it will likely overheat. Instead, what it means is that the VA's taken by the load must be decreased a like amount so the transformer stays within its ratings. You could still draw 0.5 amps but at 100-volts the maximum load would be 50-VA.

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 Post subject: Re: Auto transformer Question
PostPosted: May Mon 14, 2018 2:36 pm 
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My interpretation was a 6- volt transformer with a tapped primary. Not intended as an autotransformer but could certainly be used as such.

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 Post subject: Re: Auto transformer Question
PostPosted: May Mon 14, 2018 3:40 pm 
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So if the 105 tap is connected to a 125 source, the secondary voltage will rise a proportionate amount. That will skew the VA's a bit but unless the load side is excessive no big deal.

What are you trying to accomplish in re-cycling this transformer? In the grand scheme, just knowing one fact and considering it is O.K. for the balance of the hookup could be risky...

YMMV

Chas


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 Post subject: Re: Auto transformer Question
PostPosted: May Mon 14, 2018 3:45 pm 
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Well, how about posting connections and resistances. "Auto" transformers have a wide range of interpretations, some to tame higher "current" line voltages for older appliances, some to power low voltage stuff like tube filaments, or whatever. Whatever, be careful!


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 Post subject: Re: Auto transformer Question
PostPosted: May Mon 14, 2018 9:32 pm 
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I am sorry that I didn't express myself very well in the original post.
Pixellany seems to have read my mind fairly well.
Here are some pictures to help me explain what I intend to do.
I will only need 6 amps of filament current so there seems to be enough beef in the transformer to do what I need to.
The crt transformer is too much over on the filament voltages, as my house is close to 124 volts.
My proposed "fix" is like this.
Attachment:
connection.jpg
connection.jpg [ 100.61 KiB | Viewed 607 times ]
Attachment:
filament_xfmr.jpg
filament_xfmr.jpg [ 48.27 KiB | Viewed 607 times ]
Attachment:
crt_xfmr.jpg
crt_xfmr.jpg [ 37.09 KiB | Viewed 607 times ]

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 Post subject: Re: Auto transformer Question
PostPosted: May Tue 15, 2018 12:43 am 
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I don't see 6 amps anywhere....

Regardless, the way I read this, you're using the 1st transformer strictly as an auto-transformer (no load on the secondary) to get from 125 to 105. But why? the second transformer is speced at 115, so why not feed it that?

You could also use a 12-volt filament transformer in a bucking configuration.....giving you 125 - 12 = 113 volts.

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 Post subject: Re: Auto transformer Question
PostPosted: May Tue 15, 2018 1:10 am 
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Six amps is from the filaments of the scope amplifier plate rectifiers and amplifier tubes.
The crt transformer provides filament power for the crt and hv rectifier.
The plate transformer is not shown here and the plate rectifiers and amps are not shown.

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 Post subject: Re: Auto transformer Question
PostPosted: May Tue 15, 2018 2:13 am 
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The second transformer is speced at 115 but real time measurements ( no load) are overvolt. That makes me nervous because I have ruined some of my long lost prized possesions from a mere 1/2 volt too much on the filament.

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 Post subject: Re: Auto transformer Question
PostPosted: May Tue 15, 2018 3:25 am 
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If we knew what type of equipment you are working on, the voltage available, the voltage required, and the problem you are attempting to resolve, the solution may be as simple as using a buck transformer on the equipment. More information is required.

Dave


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 Post subject: Re: Auto transformer Question
PostPosted: May Tue 15, 2018 1:44 pm 
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Greetings to the Forum:

It seems to me that Mark has defined the problem pretty well. He has an oscilloscope that has at least two transformers in it.... a 6.3 volt filament transformer to operate all the tubes in the vertical amplifier and sweep sections and a "scope" transformer to develop the high voltage for the CRT and provide a high voltage isolated filament supply for the CRT so that its cathode can be run at a high negative voltage with respect to ground. Presumably, there is a third transformer to provide plate power for the vertical amplifier and sweep circuits.

I further assume that Mark does not wish to squeeze an additional bucking transformer into a chassis that may be already crowded with three transformers. Therefore, he wishes to reduce the primary voltage supplied to the "scope" transformer by running it from a tap on the primary of the filament transformer. Since the filament transformer is capable of 10 amps or 60 VA, and he is using 6 amps, or 36 VA, then the question becomes: Does he have sufficient capacity in the filament transformer to allow it to perform both functions?

Mark, please jump in here and let me know if I have interpreted this correctly.

I do have a couple of thoughts: I did the ratio and proportion calculation and the "scope" transformer 6.3 volt winding should deliver 6.85 volts with 125 volts on the primary. This could be dealt with by simply inserting a resistor in series with the CRT filament winding. Worst case, if the scope tube draws the full .6 amp the transformer is capable of, then the resistor would be about .92 ohms and dissipate about 1/3 watt.

I would be a little concerned for the high voltage windings, however... the 3 KV winding would have about 3,261 volts across it. That's about an 8.7% over-voltage. That is not too bad in terms of percentage, but it all depends on how good the insulation is in that particular transformer.

My second thought is that measuring the unloaded filament voltage will probably give you an erroneous high reading. You should load the filament winding with a suitable resistor to simulate the CRT filament and then measure the voltage. You might find it to be below my calculated figure, in which case, you should be fine.

Regards,

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 Post subject: Re: Auto transformer Question
PostPosted: May Wed 16, 2018 10:52 pm 
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Thanks everyone, and thank you Jim.
Jim's assessment is accurate.
Actually, the scope is a homebrew project. Here is the link for anyone who may be interested.
viewtopic.php?f=12&t=339878
One of my favorite things to do is build things with tubes, especially old t.v. tubes that nobody wants.

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