Then I'm measuring the DC voltage on both sides.
Well there's your problem right there. Transformers only work for AC. It's inductive coupling. As the primary's voltage goes up and down, a magnetic field expands and collapses. This magnetic field's velocity induces voltage in the winds of the secondary, giving your your secondary voltage. When it's only DC, the magnetic field is constant, and so there's no change in the magnetic field to create movement in the electrons.
Any ideas? The small transformer got warm, which at least indicates
that it's doing something, but the large one stays cool...
Well, the warmth is created by the resistance of the wire. It's usually pretty thin gauge wire, which has been dipped in varnish and baked to keep winds from shorting. I make large inductors at work similar to this, but they weigh 300 pounds. Sometimes we 'DC Bake' them. The heat from DC current will cook off any wet varnish solvent inside after we bake them in a large oven. Mmmm Xylene eating my brain cells.
So, if you want to evaluate a transformer, you're going to need a sine wave generator. Any AC will do, but a sine wave is a good place to start. Plus, you get to turn the knob and make space sounds. Outer space sounds like someone turning a knob on an oscillator, by the way.
Then you use your multimeter in AC mode and measure primary and secondary voltage.
The one with five wires is probably center tapped, which means you get full secondary voltage across the two outer wires, but half of that from the center wire to either outer one. Center tapped transformers are usually used in non-bridge full wave rectifier circuits for DC power supplies and such.
-Tavys
