Unknown problem with transformer

[Tetradigm]

hello there, i have a question involving a possible bad wiring i did.

i have a transformer with a 120v input, and a 120v 1.9A output. it has a total of 7 output wires, 5 of which were isolated to avoid any contact with anything.

on the thickest 2 of the wires i applied a load of probably about 30-50ohms of graphite. in between the wall and transformer i have a simple rocker switch attached to one of the input leads, rated at 125v16a/250v10a.

upon plugging the transformer in, something inside of the transformer box i built sparked quite loudly and made some smoke. i removed the plug from the wall, and everything inside was still room temp, and nothing seemed or smelled burnt or melted. the graphite was unaffected.

now i can plug it in, and flipping the switch does nothing at all. i have opened up the switch to check if its still good and it is, so that only leaves me with the transformer. i know that i connected it properly from the wall, as i removed it from an old stereo and payed close attention to how it was connected, as well as the two green wires on a different side from all the others are clearly hooked to the primary coil.

i have not removed the other wires from isolation yet to check them, but honestly im not all that interested in them, but could someone possibly explain to me if i blew one of the coils somehow, and why?

 

[MikeMl]

Do you have and know how to use an Ohmmeter? If so, set it to low-Ohms scale, and find which leads have DC continuity to which others. Note all of the wire colors. Post all of the wire-pairs, and DC resistances. Some may have continuity to the iron core.

During this test, all wires must be disconnected. Since it has 9 wires, this means that there are a lot of pairs, but usually the wire colors will give you a clue as to which wires go together to make a pair to feed a particular winding. Some of the windings may have a center-tap, so those three wires will have continuity to each other, but be isolated from wires going to other windings.

Its likely you hooked 120Vac to a low-voltage winding, and burned a wire inside the transformer.

 

[Tetradigm]

i have a cheap harbor freight centech multimeter. it has several settings for ohms, from 200-2000k.

wires are:

2 green wires on one side, connected to a coil of larger gauge wire
on the opposite side:
brown, orange, black, yellow, and red.
on the same side, but the top not bottom, a red and white.

the green wires do not register any change with any wires.
the orange, brown, black, red, and yellow wires on one end all display 0 instead of the default 1, i have no idea what this means. i just bought and changed the battery.
the white and red wire on the other end also display 0
this remains true across all ohm settings.

 

[Tetradigm]

oh also, the 2 greens were my input leads, while the red and white that display 0 were the output.

 

[MikeMl]

A winding that shows 0 to a number like 2. or 3. on the 200Ω scale is showing that that winding is intact (has continuity). If you get a 1__., that means that there is no winding or that the winding that should otherwise have been there is now burned out.

What does it read when you connect the probes between the two green wires? If you see a low resistance then the primary winding is intact. If you see the 1__. then the winding is bad.

 

[Tetradigm]

well then it appears i did in fact destroy that coil then. i dont possibly understand how though, when that was the original 120v input. oh well, i guess its time to rewind one or just buy a variac then. my thanks for the help anyways.

 

[chemelec]

well then it appears i did in fact destroy that coil then. i dont possibly understand how though, when that was the original 120v input. oh well, i guess its time to rewind one or just buy a variac then. my thanks for the help anyways.

More Likely, that Transformer has a Built in Fuse on the primary and you burned it out.
Sometimes you can carefully remove the outer paper, find thefuse and replace or bypass that fuse.

 

[Tetradigm]

no, not unless the fuse is held underneath the coild. i can see where the leads connect to the coils and outside of the coils, so i know theres no fuse out there. upon closer inspection however, i now notice what appear to be burn marks on the plastic parts that separate the coils from the leads. im tempted to just say screw it and hook it to the red and white leads and see if that works, but im not in the mood for blowing my fuse box at the moment.

 

[MikeMl]

I think I know what happened: (From Wikipedia

Power transformers used in North American vacuum-tube equipment often were color-coded to identify the leads. Black was the primary connection, red secondary for the B+ (plate voltage), red with a yellow tracer was the center tap for the B+ full-wave rectifier winding, green or brown was the heater voltage for all tubes, yellow was the filament voltage for the rectifier tube (often a different voltage than other tube heaters). Two wires of each color were provided for each circuit, and phasing was not identified by the color code.

Audio transformers for vacuum tube equipment were coded blue for the finishing lead of the primary, red for the B+ lead of the primary, brown for a primary center tap, green for the finishing lead of the secondary, black for grid lead of the secondary, and yellow for a tapped secondary. Each lead had a different color since relative polarity or phase was more important for these transformers. Intermediate-frequency tuned transformers were coded blue and red for the primary and green and black for the secondary.[6]

 

[Tetradigm]

well that might make a little bit of sense, even though it was a solid state amp it used a vacuum fluorescent display, which sadly did not survive the disconnection process.

im pretty sure that if the primary winding is still intact and i simply mistook which it was, that it is the white and red wire, as the other 5 are all connected to each other.

ill link a picture of what im talking about.

**broken link removed**

 

[Tetradigm]

the wires on the right and left end are only connected to 1 coil each, the 3 inside coils are each connected to 2 coils each.

the red and white wires connect to only 1 coil each, i imagine they are the same coil. gonna look for a fuse holder and then hook up the red and white to the wall and see if it works as i want or not, if not then the whole things being rewound.

 

[Tetradigm]

well it seems that it still works....although my brilliant idea of "screw it, short it" managed to blow the 4a fuse i put there. i know the total output is 1.9a, though no idea which wires output what, nor which ones i actually killed. any suggestions on the fuse size to use, and also is there a better way for me to wire the fuse? currently its just a small glass fuse holder that came with the stereo, wired in between one of the input leads and the wall.

 

[Tetradigm]

one more thing, i think the fuse was a fast blow fuse. since im not using it on anything that i really care if it overpowers and damages, would it be better to use a slow blow? i really just want to avoid blowing out my houses fuse itself, since we only have 1 fuse for the house, and its only rated at 30 amps....and i have blown it a few times recently. damage to the transformer isnt particularly important to me, it can be replaced or repaired easily but obviously something ill avoid if possible, hence the fuse.