I bought a 110 v AC transformer to run an item that requires 24.5 v DC. the transformer I bought is 110 v Ac to 30 v DC. can I put a rheostat inline (at the dc end before the unit to be powered) to adjust the voltage output ? and if so, Where do I find one?
Not practical. Sure, you will get a small amount of output, but only as long as you keep moving the rheostat. The efficiency will be in the few percent range. Not counting the mechanical effort to move the rheostat. You have a lot of questions about this. I don't know your goal, but for a practical DC transformer, this will not work. It has zero chance of success. The only practical way is to use a transistor to chop the DC into AC at the proper frequency. This now begins to look like a DC-DC converter. .
Transformers do not rectify and filter the output. It's AC in and AC out. A transformer either steps up the voltage or steps it down, depending upon direction. A transformer is only the first step in converting 110vac to a DC voltage. After the transformer has stepped down the 110v to 30v (AC!), it needs to be rectified into pulsating DC. Using 2 diodes (half wave rectifier), the DC will be 21v (still pulsating an unreliable for your use). If you use 4 diodes (full wave bridge), the resultant pulsating DC will be 42v. Now you need to filter it using capacitors so that it's a steady DC. You COULD use a potentiometer (rheostat) in a voltage divider network to dial down the voltage that you want to use, but fluctuations in the 110v will still be transmitted to the finished DC. So you also want to use a voltage regulator chip. That way, you get a steady, filtered, reliable DC voltage.
