I am trying to identify this component removed from a Mean Well PB-120N-27C charger. MW TF-605-R5 side

In this picture I see two pins (bottom left) connected to two small wires. They are shorted. Same thing for the other side. Could it be a broken internal fuse?

Do you have any suggestion to what I should read from the other pins?

MW TF-605-R5 Isometric view

  • 4
    \$\begingroup\$ What evidence do you have that the transformer has failed? Showing a short is normal for many types of transformer; measure the inductance, not resistance. \$\endgroup\$
    – Hearth
    Commented Jun 20, 2022 at 14:27
  • \$\begingroup\$ I measured 320V on the primary side and nothing on the secondary. \$\endgroup\$
    – stenio
    Commented Jun 20, 2022 at 14:47
  • 2
    \$\begingroup\$ That should be in the question; as it is, the only information led me to think that you had misidentified the problem. If you measured nothing directly across the secondary, then it's likely the transformer is indeed bad, though replacing it is unlikely to be economical compared to getting a new PSU. Yes, it's not a super cheap PSU, but the amount of time and frustration it would take to re-wind this, not to mention invalidating any safety ratings, is probably not worth the money you'd save over buying a new one. \$\endgroup\$
    – Hearth
    Commented Jun 20, 2022 at 14:59
  • 1
    \$\begingroup\$ You measured 320V DC or AC? Do you have 230VAC mains? Do you expect that to be a 50Hz mains transformer? \$\endgroup\$
    – Justme
    Commented Jun 20, 2022 at 15:03
  • \$\begingroup\$ I have 230 VAC @ 50 Hz in my country and I measured in AC and DC mode, reading something only in DC mode. I also identified a SIP bridge rectifier and measured 230 V AC @50 Hz between pins 2 and 3 and 320 V DC between pins 1 and 4. \$\endgroup\$
    – stenio
    Commented Jun 20, 2022 at 15:16

2 Answers 2


There is nothing to identify. It is a custom transformer for the charger and likely it is not even broken.

The charger has a switching mode power supply and this is the main switching transformer. It does not work at 50 Hz but at maybe tens of kilohertz. The 230VAC mains input is rectified to 325 VDC raw supply which is then driven to the transformer by a swiching mode controller chip.

There are several typical modes of failure. Since there is 320VDC preset on board, it has not blown a fuse, so it is not completely burnt up. One of the simplest failure modes is just a broken bootstrap resistor, so the switch mode chip won't get initial power supply and stays off.

As you already removed the transformer, it may have been damaged in the process, so it may not be worth to put it back and fix what is actually broken.

  • \$\begingroup\$ I don’t think I’ve damaged it. Maybe without it I can find the real cause of the problem and, hopefully, is something easier to replace, who knows. \$\endgroup\$
    – stenio
    Commented Jun 20, 2022 at 20:52
  • \$\begingroup\$ I think you are right: there is a TL3845 PWM controller chip which has an internal under voltage lock out circuit. From the data sheet I understand that at least 8.4 V are required for the chip to start working, but I measured 7.9 V on the Vcc pin. Furthermore I tried to inject 9 V on the pin and the power supply started. \$\endgroup\$
    – stenio
    Commented Sep 21, 2022 at 6:34

It’s a custom transformer. If it is indeed burnt up, then the simplest if tedious way to fix it would be to measure wire diameter and number of turns for each winding while you unwind. Then rewind it using new magnet wire.

But you should only suspect the transformer if the windings will measure out of the likely range for the application. You could look up application notes for the controller chip they use, and see what the operating frequency is, and the range of inductances you’d expect for each winding.

At first sight, the transformer doesn’t look overheated, so if you’re thinking of replacing it – there may be no need to.

If you’re thinking of copying the supply, then the application notes/datasheets for controller chips you can actually buy will be a good starting point.


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