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| Isolation transformer, wiring question |
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| Zero999:
--- Quote from: cs.dk on September 01, 2018, 04:57:00 am --- --- Quote from: Hero999 on August 31, 2018, 04:07:00 pm --- --- 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. --- End quote --- Now thats weird.. From blue to the soldered wires theres 4.xxV and from yellow the the soldered wires theres 16.xxV - Whats going on there? The output is a little bit higher then the input. 229V in and 239V out, but thats unloaded. I think with a load it will equalize. Arthur, I have attached the label. (Danish translation: Ind = In - Ud = Out) To my surprise when I powered it up, this thing hums relative loud in my opinion. --- End quote --- The output voltage being slightly higher than the input is normal. It will normally have a slightly higher turns ration thant 1:1 to compensate for the voltage dropping under load. 239/229 = 1.04, so the turns ration will be 1:1.04 which seems reasonable for a transformer this size. I don't know about the wires which are soldered together, which is very odd. |
| cs.dk:
--- Quote from: Hero999 on September 01, 2018, 02:04:02 pm ---The output voltage being slightly higher than the input is normal. It will normally have a slightly higher turns ration thant 1:1 to compensate for the voltage dropping under load. 239/229 = 1.04, so the turns ration will be 1:1.04 which seems reasonable for a transformer this size. I don't know about the wires which are soldered together, which is very odd. --- End quote --- Seems resonable that it will drop voltage as the load rises. When measuring on the terminalblock, there's no continuity from blue to yellow, when the soldered pair is open. |
| ArthurDent:
With the added information I'm still going with my initial guess in posts #3 and #5 that the jumpered leads originally went to a blown fuse indicator and when the thermal fuse opened for whatever reason someone cut the leads short and jumpered them to allow the transformer to be used despite the open fuse. The following two quotes from cs.dk support this theory and if you check my drawing, it makes sense. With the wires jumpered the resistance of the primary and secondary at the terminal block should be perhaps several ohms with the primary maybe being ever so slightly lower. It still would be a wise idea to add a replacement thermal fuse in series with the jumpered leads as I described in my previous post. "EDIT: Blue (soldered end) to yellow is open, and yellow (soldered end) to blue is open." "When measuring on the terminalblock, there's no continuity from blue to yellow, when the soldered pair is open." Note: I just took another good look at the soldered connection and I can't be sure not having the transformer right in front of me but it looks under the yellow insulation there appears to be one solid insulated wire just like the winding wire. That yellow wire is soldered to the short blue wire that looks like stranded tinned wire. The other yellow wire that goes to the connection block may also be solid wire. If this is correct then that would also support that the two yellow solid wires are just the primary. |
| cs.dk:
Sorry for the late reply, been busy in the week. I think your right in your theory. The blue is a stranded wire, where all others are solid. It begins to make sense now. I think i'll add a 5A fuse in there, just for good measure. Is a fuse normally on the input or the output of the transformer? |
| SeanB:
Normally on input to protect the transformer, though you can certainly add a separate fuse as well to the output to disconnect in case of a overload on the output side. |
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