Electronics > Projects, Designs, and Technical Stuff
Isolation transformer, wiring question
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cs.dk:
Hi,
Just got my self a 5A isolation transformer for cheap. :-+

However, something looks wrong to me. The blue and yellow are the primary (in), why are they joined like the picture shows?
Can a thermal fuse be potted? The label says it has a 5A thermal fuse on the primary, but i can't see one.
Zero999:
If it has a thermal fuse, it's most likely embedded into the windings. The primary should still be fused anyway, as the thermal fuse is non-replaceable.
cs.dk:
Thanks :)

Any idea why the other wires are soldered together?
ArthurDent:
I would hazard a wild guess that is a used transformer and that the embedded thermal fuse died (it happens) and the previous owner bypassed it. Not a very neat solder job but apparently it worked. A bypassed thermal fuse isn't the worst thing in the world and if that is what was done and you want to restore that function you could get a replacement with an appropriate thermal rating and epoxy it in the donut hole of the transformer. One reason for potting the fuse is so there is good thermal transfer between the transformer and fuse.
Zero999:

--- Quote from: ArthurDent on August 31, 2018, 04:00:44 pm ---I would hazard a wild guess that is a used transformer and that the embedded thermal fuse died (it happens) and the previous owner bypassed it. Not a very neat solder job but apparently it worked. A bypassed thermal fuse isn't the worst thing in the world and if that is what was done and you want to restore that function you could get a replacement with an appropriate thermal rating and epoxy it in the donut hole of the transformer. One reason for potting the fuse is so there is good thermal transfer between the transformer and fuse.

--- End quote ---
I would have thought the fuse would be embedded in the windings, making that unlikely.

I think it's more likely there are two primary or secondary windings for operation at either 120V or 240V. The blue and yellow wires are probably where the two windings are connected in series, for 240V operation. To find out, power it up and measure the voltage between the soldered blue and yellow wires and the blue and yellow wire. If it's 120V, then it's as I thought. If it measures close to 0V to one of the wires and the full mains voltage to the other wire, then it is indeed a fuse connection.
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