Quote:
Originally Posted by andy
I ran into a situation repairing a computer power supply that might be relevant. The power supply had the usual load of bad caps, plus an overheated inductor. It was a simple toroidal inductor with 9 turns of heavy gauge wire, so I rewound it with the same gauge wire. Within minutes of testing the power supply, the coil had gone up in smoke and got hot enough to unsolder itself, so I replaced it with a new coil and all was well.
In trying to figure out what happened, I discovered that the ferrite core had become highly conductive (as in less than 1 ohm across). This basically turned the coil into a little inductive heater. I'm not sure whether the ferrite becoming conductive caused the original failure, or if the original failure (caused by bad caps) caused the core to break down from heat. Either way, it seems like the same thing could happen to a TV flyback core, and would make rewinding impossible. It's definitely something to check.
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Toroids used in SWMPs are usually made by winding a strip of a silicon steel alloy. You would measure a low resistance from one side to the other, but since the ends of the strips are open it would not act as a closed loop or turn. Yours may have shorted between one of the layers of the strip and would behave as you described.
I don't know what the core is for the RCA John Folsom is rewinding, but if it's ferrite, MO=Fe2O3 or the ilk, the ferrite has a high electrical resistivity and shouldn't suffer that type of failure.
John