Re: [sdiy] Mysterious Transformer Wire

[Roman]

That must be shield between primary and secondary windings.
The AC voltage you can measure comes from capacitance between
the shield and windings. Connect 1k resistor in paralel with DVM
or use grandpa's analog meter and you'll see there's no voltage.

Roman

---- Wiadomość Oryginalna ----
Od: Glen <mclilith at charter.net>
Do: synth-diy at dropmix.xs4all.nl
Data: Thu, 18 Dec 2003 02:05:59 -0500
Temat: [sdiy] Mysterious Transformer Wire

>Are power transformers ever made with a wire that doesn't connect to any of
>the windings, neither primary nor secondary?
>
>I am looking at an old transformer that I salvaged from an old Kimball
>organ, and there is a fairly small gauge blue wire that was originally
>connected to ground, before I removed the transformer from its chassis. I
>first thought that this wire might be a center tap for one of the
>secondaries, but it's much smaller in gauge than any of the secondaries. It
>also has no DC electrical connection to any of the secondaries.
>
>The primary has a tap to allow for operation from different mains voltages.
>The "220 VAC" connection was not connected to anything. (This was a 117 VAC
>organ.)
>
>Judging from some old service manuals, I feel confident that I've correctly
>identified the 3 different primary wires. I have identified one secondary
>winding that was used to power a string of 6V lamps. I have also identified
>another secondary that has 7 connections, including a center tap.
>
>The only thing remaining is this small-gauge blue wire, which was
>originally grounded. I can't measure any DC continuity between it and any
>of the other transformer wires. I also can't measure any DC continuity
>between it and the housing of the transformer. I also can't find any
>mention of it in the service manuals.
>
>Any suggestions?
>
>The transformer did pass the smoke test, at least it did when I hooked it
>up without a load. All the identified secondaries had appropriate AC
>voltages present. The odd thing, is that, with my DMM, I could measure AC
>voltage between the blue wire and the two secondaries, even though there is
>no measurable DC connection between the blue wire and anything else on this
>crazy transformer. If the blue wire is for a shield of some sort, should it
>do that? Oh yeah, this transformer does have an outer metal cover
>(shielding), in case you were wondering, but it doesn't seem to be
>connected to the mysterious blue wire.  
>
>
>thanks,
>Glen