Alternating current generally refers to its effective value, and its peak value is twice the root sign of the effective value. The value converted into DC depends on what kind of circuit you are, and whether you want a peak value or an average value. The output peak value of half-wave, full-wave and bridge rectifier is the same as that of alternating current,15 *1.414 = 21.21v. The average output value of half-wave rectifier is 0.45* 15=6.75V, and that of full-wave rectifier and bridge rectifier is 0.9* 15= 13.5V (actually slightly smaller than these values). If there is capacitor filtering in the rectified circuit, the average output value is the voltage on the capacitor, and its value is the sum of the capacitor sizes. If the capacitance is large and the load is small (for example, there is no load), the output will be close to the peak value of 21.21v. With the decrease of capacitance or the increase of load, the output voltage will drop, and the specific situation should be calculated.