Mystery Transformers Okay

I figured the wirings of my Mystery Transformers all wrong.

Mystery Transformer “A”

Previously I suspected either high inrush current or shorted primary windings. So I measured Mystery Transformer “A” using my GM328A component tester, and posted my findings and suspicion to EEVBlog forum.

A regular poster swiftly confirmed that it was shorted, but then three other regulars suspected that the yellow wires were actually for the secondary winding.

As you can see in post #9 in that topic, that made me retest Mystery Transformer “A”, switching my assumption about the wires.

  • Black-Blue-Red now assumed as primary (0-110-220VAC).
  • Yellow-Yellow now assumed as secondary.

I performed two 5-minutes test, connecting Black-Red to mains, with no-load on the secondary:

Measurement 1st Test 2nd Test
1 Mains/Primary Voltage 218VAC 218VAC
2 Primary Current (Min) 26.34mA 25.97mA
3 Primary Current (Max) 28.15mA 28.21mA
4 Secondary Voltage (Min) 10.14VAC 10.07VAC
5 Secondary Voltage (Max) 10.33VAC 10.32VAC
6 Smell None None
7 Temperature Cold Skin warm

Smell wasn’t an actual measurement. I just whiffed the transformer after disconnecting it from mains. If it smelled like burning insulation, we knew something was wrong. As for temperature, I could not measure actual temperature with the multimeter’s temperature probes, because the multimeters were already being used in this test.

Everything were fine: breaker didn’t tripped, transformer didn’t smell and didn’t heat up, and the secondary (Yellow-Yellow) were giving out sensible no-load AC Voltage. Yellow-Yellow really were the secondary, and windings weren’t being shorted.

So I replicated the assumption for Mystery Transformer “B”.

Mystery Transformer “B”

  • White-Blue-Red now assumed as primary (0-110-220VAC).
  • Yellow-Yellow now assumed as secondary.

White-Red to mains, no-load on secondary. Result:

Measurement 1st Test 2nd Test
1 Mains/Primary Voltage 218VAC 218VAC
2 Primary Current (Min) 39.93mA 39.63mA
3 Primary Current (Max) 41.30mA 41.13mA
4 Secondary Voltage (Min) 15.18VAC 15.47VAC
5 Secondary Voltage (Max) 15.13VAC 15.44VAC
6 Smell None None
7 Temperature Cold Skin warm

Everyhing were also fine with this transformer. Just like Mystery Transformer “A”, for “B” Yellow-Yellow were secondary, and nothing was being shorted.

Conclusion

Both transformers are fine. The Yellow-Yellow wires on both transformers are actually for the secondary windings.