What kind of transformer is this and how is it used?

I'm also confused about why there are only 3 wires. I don't see any other pins on it. Although on the opposite side, you use the number 4 on one side and RT on the other.

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    \$\begingroup\$ The Chinese characters refer to the wire colors and are in the same order as the wires (white, orange, black). \$\endgroup\$ Jun 12, 2020 at 19:30
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    \$\begingroup\$ @jonk Yes. It's 橙(色)线 = orange (color) wire chéng (sè) xiàn. Orange (the fruit) is 橙子. Orange color is 橙色. Black and white show up a lot more often like the northern province's poetic name 黑龙江 (black dragon river) or 白酒 baijiu (white "wine", a kind of foul high-proof liquor). \$\endgroup\$ Jun 12, 2020 at 21:23
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    \$\begingroup\$ @jonk Cool. I set up a buying office in a Taipei suburb many years ago. The traditional characters are even harder to learn, but seem more artistic. Eg. 廣州 vs. 广州. Not that I claim to be any good at it. \$\endgroup\$ Jun 12, 2020 at 22:23
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    \$\begingroup\$ The label '0 V 45.5 V 120 V' and its orientation to the wires shows a highly probable connection scheme: white 0 V, orange 45.5 V, black 120 V. The measured DC resistances should be (white to orange) + (orange to black) = (white to black). \$\endgroup\$
    – Uwe
    Jun 13, 2020 at 10:29
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    \$\begingroup\$ Similarly, this transformer model is used in some control boards of cooling systems. This no longer requires the installation of two separate power supplies in the circuit, and with one power supply they power both the electromotor and the circuit elements. (google translation) \$\endgroup\$
    – M-A-karimi
    Jun 13, 2020 at 18:20

2 Answers 2


That's an auto-transformer. It is used to step down from 120 V to 45.5 V but without isolation between the inputs and outputs.


simulate this circuit – Schematic created using CircuitLab

Figure 1. A poor drawing of an auto-transformer. In practice there is one coil with a tap-off point on it.

On a large auto-transformer the lower winding might be thicker than the upper as it will be carrying more current. For one this size the saving in copper cost might not be worth the trouble.

The transformer turns ratio applies to this type of transformer also so the number of turns on the lower half will be 45.5/120 times the total number of turns.

  • \$\begingroup\$ I can't imagine any other kind of transformer using three wires only. The auto-transformer is the only one using three wires. Of course there may be auto-transformers with two taps and four wires altogether, but a simple resistance measurment will show there is no isolated secondary winding. \$\endgroup\$
    – Uwe
    Jun 13, 2020 at 10:16

It seems to be a non-isolated transformer. I suppose it must've been made for a specific application. It's a single coil with three taps. 2 taps at the end and one in the middle. The ratio of that is proportional to the voltage labels: 120v, 45.5v (RMS AC). so if you apply a 120v rms AC to the 120v/0v taps there must be 45v rms AC between 45v/0v taps. You can also check by a multimeter to make sure. If what I said is right, the impedance between 120v/0v taps should be more that double of the impedance between 45v/0v taps.


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