Your circuit there is essentially a half wave rectifier. I would put the rheostat on the output side of the diode for the below reason-
As I recall, your output voltage in this case will be (2 * (24v * sqrt (2) - .7))/pi, or about 21.6vdc, ignoring the ac ripple and assuming 24 VAC rms input.
You will need a voltage divider network on the output side of the diode. I would put the rheostat on the output of the rectifier so you could adjust the output voltage by using it as an adjustable voltage divider.
I would probably try to find a higher value rheostat (more ohm) to limit the current in the circuit as well.
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