Need circuit for 12 volt 2 Amp step down power supply from 18 volt 1.5 Amp Input
12V AC reaches a peak voltage of about 17 volts, then via the rectifier it might produce about 15V DC across a suitable capacitor (rated at 25 volts). Maybe you had the capacitors wired back to front or maybe you misapplied the bridge rectifier. Maybe the bridge was broken in a previous experiment.

I'd still look for a buck regulator like this: -



Found here on ebay costing $2.86. It's a switching buck regulator that has an adjuster to set the output to be anywhere from 3V to 40V: -

Type / name:LM2596HVS DC-DC step-down module

Input Voltage:4.5V ~ 53V

Output Voltage:3V ~ 40V

Output Current:3A (max)

Conversion efficiency:92% (the highest)

Output Ripple:&lt;30mV

Switching frequency:150KHz

Operating Temperature:-45 ℃ ~ +85 ℃

Size:43mm * 21mm * 14mm (L * W * H)