Electronics > Beginners

Why is there no voltage on this winding tap?

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lordvader88:
I'm checking an unknown transformer from an old 1970s TV, about a 1kg / 2lb, ~3"x3"x1.5" core

There's 3 windings total, and on the 3rd with 5 wires I measured these resistances

Blk-Blk/Grn   2.16R
Blk-Grn         7.72R
Blk-Grn/Wh   7.76R
Blk-Grn/Red  45.7R

Then with 120V on the primary something seems wrong on the 3rd winding

Blk-Blk/Grn   19.8V
Blk-Grn         17.3V
Blk-Grn/Wh   22.4V
Blk-Grn/Red  only 0.026Vac ?? , no DC at all


It's not getting hot yet, no smoke or smell at all. So whats going on, also I'm curious why the 1st pair is so close to the 2nd/3rd pair. Do they use different wire gauge between the taps ?

I measured inductances too, they seemed normal enough

edit
ok the Blk-Grn/red was only 1.1mH whereas the others were 94mH, 71mH, 121mH

Melt-O-Tronic:
My guess:  Shield winding to block electrostatic coupling between primary/secondary.

Jwillis:
I'm not understanding what your trying to do.Why are your taking measurements of a transformer with a DC setting on your multi meter.Transformers alone CANNOT convert AC to DC or DC to AC; they cannot change the voltage or current of DC; they cannot change the AC supply frequency.  An alternating current (AC) in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings.Do your voltage measurements of a transformer in AC setting only Never in DC .It just leads to confusion .Taking inductance measures is pointless on a transformer.Taking resistance measurements is only relevant to check continuity of the coils.The values are mostly meaningless.Every transformer will have an inherent capacitance..Unless you plan on calculating the total losses of the transformer taking these measurements gives no meaningful values.

If your looking at a Line Output Transformer (LOPT) / Flyback (FBT) here's a typical schematic



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