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Old 15th June 2007, 12:57 AM   #1
jarthel is offline jarthel  Australia
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Default is it safe to connect these secondaries in series?

transformer specs: 825V 400mA, 250V 250mA, 90V 250mA, 150V 80mA and various others secondaries for tube heaters

Is it safe/possible to connect the 250V secondary and 90V secondary in series?

thank you.
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Old 15th June 2007, 02:35 AM   #2
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as long as you phase them corectly, but why do you want to do that anyway?
planet10 needs your help: Let's help Ruth and Dave...[B
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Old 15th June 2007, 03:14 AM   #3
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Yes you can connect them safely in series and you only need to worry about the phasing (which way round you connect the 90 volt to the 250V) from the point of view of the voltage you want out.

Connecting the 90V in one way (the one I assume you are after) will give you 340V , with the 90V connection reversed you will get 160V. Both connections are safe. When the current capabilities of the 2 windings are different (not so in your case) maximum current will be determined by the winding with the lesser capability.

This technique is used a lot to adjust output voltages when the transformer will not give you exactly what you want. You can even (for example) use a 5V secondary intended for a tube rectifier heater (it will have good voltage withstand capability) in series with the primary to either tweak all of the other secondaries up or down a little.

A multimeter set on AC Volts will quickly tell you if you have it the right way round.

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Old 15th June 2007, 12:28 PM   #4
ilimzn is offline ilimzn  Croatia
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Originally posted by gingertube
You can even (for example) use a 5V secondary intended for a tube rectifier heater (it will have good voltage withstand capability) in series with the primary to either tweak all of the other secondaries up or down a little.
With due respect to Ian (Gingertube) I felt it important to emphasise a part of his post.
There is also another criterion for series connecting windings, and this is the isolation quality with respect to other windings.

Ian's example is a great one - a 5 or 6.3V tube rectifier winding is normally connected to the resultant DC rectifier output, which means that it is at B+ potential (at least) WRT ground, while the AC input is +- 1.41 * (AC voltage) WRT ground. Worst case instantaneous voltage between primary and rectifier winding for a typical case will be a good 600-800V, possibly more, easily the same as between the rectifier winding and the B+ or other ground referenced heater windings. Because of this the rectifier heater winding is well isolated from other windings, hence the high voltage withstand ian mentiones - and this makes it possible to connect this winding, if otherwise unused, in series with any other winding of that same transformer, without fear that it might arc over to other windings.

Now for an example when series connecting would not be possible - say you have a transformer with a mains AC winding, and two equal, bifilary secondaries, say 20V each. Something like this is quite typical for toroids, fo instance. Suppose your primary is 115V and you wanted to make the secondary less than 20V by connecting the primary in series with one of the secondaries, to form a 135V winding. Conecting this to 115V mains would then give you 17V on the other secondary. Unfortunately, this exercise would end up in smoke, as the isolation between the two secondaries is only the wire enamel. This can withstand 600V if so specced, but in a bifilary set of 20V secondaries, it is likely the manufacturer did not use multilayer enamel, so it will be signifficantly less. Such a connection would easily result in compond primary to remaining secondary arc-over, because the separate secondaries are not sufficiently insulated from each other and do not have a high enough voltage withstand capability.
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