Mystery Transformers

I bought two used transformers two days ago from a street peddler. He didn’t know the output voltages. After inspecting them for damages, I bought these two and brought them home.

Mystery Transformer “A”

This is the smaller one. The appearance was very clean for a salvaged transformer from who knows what.

It has this markings rolled on the metal bracket:

EI-57A-9
110/220V
FUSE 115°C

Dimension (core)

  • Width: 28mm.
  • Length: 50mm.
  • Height: 60mm.

Dimension (core, windings, and bracket)

  • Width: 50mm.
  • Length: 75mm.
  • Height: 60mm.

Windings

On one side, it has two 22 AWG yellow wires. These are probably the wires for the primary winding.

On the other side it has three wires: black, blue, and red, all 20 AWG. These are probably the secondary winding wires.

To check the no-load output voltages, I stripped the yellow wires and connected it to mains. The primary wires are very difficult to strip. And as soon as I plugged it to mains, the house circuit breaker tripped.

Either the primary winding is shorted, or the inrush current is so very high. Whatever, I’ll try the other transformer next.

Mystery Transformer “B”

This is the bigger one. It was dirtier compared to Mystery Transformer “A”. It even has a broken tab on the plastic bobbin.

Two stickers are stuck to the metal bracket. The first sticker is on the top of the metal bracket, printed with:

     PNE
AA30-T66-0-4
 EB0 (A)-00

The second sticker is on the side of the metal bracket, printed with:

430F02<scratched>

The second sticker is much more yellowed with age than the first sticker. The second sticker is also scrathed at the end, making the printed marking not completely legible.

Dimension (core)

  • Width: 40mm.
  • Length: 70mm.
  • Height: 61mm.

Dimension (core, windings, and bracket)

  • Width: 66mm.
  • Length: 98mm.
  • Height: 61mm.

Windings

Both the primary and secondary wires are on the same side.

On top, two yellow wires 20 AWG, probably for the primary winding.

At the bottom, three wires: white, blue, and red, all also 20 AWG. probably for the secondary winding.

To check the no-load output voltages, I stripped the yellow wires and connected it to mains. Tripped the house cicuit breaker again.

Oh my. My analysis for this is the same as for Mystery Transformer “A”.

Conclusion

Both Mystery Transformers tripped the house circuit breaker. I think there are two possibilities:

  1. Short circuit in the primary winding. I need to find a way to test short circuit, and if confirmed then I probably have to replace the primary winding. Not looking forward to that.
  2. High inrush current. I need to find a way to measure inrush current, then if confirmed I need to find a suitable AC soft starter design.

That’s all folks.

Update 2021-04-16: I assumed wrong about which wires were for primary and secondary windings. See next post for explanation.