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I found a transformer inside a pc power supply but it has only two wires. It says PFC UU32x14.5 YX. Can someone explain what's the use of this thing? (Never seen it before)

enter image description here

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  • \$\begingroup\$ It's a passive PFC choke. \$\endgroup\$ – winny Sep 17 '16 at 20:39
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Bridge caps have a poor power quality effects, on grid power when high volume useage on the same line exists.

It also serves to reduce ripple with a 12dB per nth harmonic(octave) in the DC output vs. 6 dB /octave in an RC filter, where R includes the XFMR and diode ESR. It generates less loss (DC drop and heat rise) for the same ripple to a comparable RC filter.

The pulse charge current is often 10x the discharge current but only 10% duty cycle and this pulse occurs just before peak voltage I.e. shifted by 80 deg or so from zero crossing.

A large (low DCR) laminated steel choke smoothens the current pulse to more sinusoidal to improve power factor effects on neutral wire in grid transformers. It's L value may be towards 1 Henry.

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. The electrical symbol for an inductor. The symbol signifies the winding and, optionally, the iron core. European symbols on the right are not so symbolic but were much easier to draw in the old days.

enter image description here

Figure 2. A simple DC power supply with C-L-C filter to reduce ripple. Source: Talking Electronics.

It's unusual to see that style of inductor in a PC power supply and without more details I can't help further.

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  • \$\begingroup\$ The 1st cap is normally much smaller than the second to filter glitches, and the second is bulk storage for 5-10 cycles. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 17 '16 at 19:47

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