# Power Supply Transformer with 3rd Wire on Primary

I'm sitting here with a $600 espresso machine with its power transformer not working (no voltage coming out of secondary, with 110V feeding the primary). I've been scouring the internet for a replacement, but the cheapest I've seen is$249. I noticed on the primary side of the transformer, there are two terminals for live and neutral. I checked the resistance on the terminals and got no readings. But there is an extra wire coming out of the winding that goes to the casing between the 2 terminals, but not finished to be used. I did get a reading on resistance when I check that point with the neutral side. So my question is what is this 3rd winding point coming out of the primary side? Can this point be used to give the transformer a second life? If not, what is the closes specs I can get away with as a replacement.

• You could always use two separate transformers - one 9V and one 15V. Feb 27, 2019 at 0:27
• examine the transformer closely ..... look for a fuse Feb 27, 2019 at 0:48
• It's probably there to support other uses (like 100VAC for Japan). If the primary is open, and you don't want to introduce yourself to the joy of transformer rebuilding, and you can't find a fuse -- it's dead. Feb 27, 2019 at 1:52
• Thermofuse inside the transformer? Feb 27, 2019 at 12:44

## 4 Answers

You could give a try with this transformer: link

The voltages (8V and 16V) are a bit different but remains within almost the 10% range, which the rest should accommodate for, but it might work just as well.

So I took a chance and fed 120V into the unknown leg of the primary, and guess what, it worked. I'm getting 17.4 and 9.7V on my meter on the secondary. Is 17.4V too high for 15.5V use? Nothing is smoking or burning up so far. I'm afraid to run the machine in case the amp draw may burn something up. Never thought there is second chance on these transformers. Is it possible they made this as a backup? Or is it as mentioned made for 100V use. If I feed 120 into 100V input, you think the transformer will fry up again?

• That fits in with the idea of a 100V tapping. If there were a thermal fuse it would be fitted on the common side so it would protect regardless of the tapping being used, so I think that can likely be ruled out. You may be ok using it in this configuration, hopefully the designer of the machine built in a bit of headroom, but whether it’s ok to run your machine on those voltages is hard to say. I’d guess that the 15.5 might be regulated down to 12, so increasing that to 17.4 would mean that the regulator has to dissipate more than 50% more power than normal.
– Frog
Jun 25, 2021 at 5:22

The two black wires that lead from that 3rd (middle) terminal and the right-hand terminal strongly suggest that they go to a thermal high-temperature cutout that is sandwiched between the primary winding and the core. I suspect that thermal sensor has opened up.

You can probably run the transformer as-is but I would suggest that adding your own thermal cutout on top of the primary winding is a REALLY good idea. These are readily available from a large number of online sources, including eBay and Amazon.

I would measure the temperature of the transformer while the device in normal operation, then add 20C or so to that measurement. I, personally, choose to use thermal cutouts rated at 100C for repairing transformers in things like clock radios and such.

Here's a view of the transformer of a 230V 50 Hz machine, that confirms the presence of a thermal fuse.

You may solder a shorting link across the blown thermal fuse.

• I don't think shorting out a blown thermal fuse is a great idea. Maybe it failed from old age, but maybe it failed from something more serious. Short it, and there's no longer any thermal protection. It would be better to install a new thermal fuse there. Jun 25, 2021 at 13:30
• The thermal fuse would be embedded in the winding and cannot be replaced. The ultimate would be to replace the transformer with its identical spare. Jun 25, 2021 at 17:27
• I realize that, but you could replace it with an external thermal fuse. That's better than just shorting it. Jun 25, 2021 at 17:43
• I agree with you. Even a glass fuse would help in case the primary is not otherwise fused. Jun 25, 2021 at 18:10