# Diode bridge rectifier convert 13.5v ac to 17v dc

i just make a voltage rectifier as shown below

Four IN4007S diodes

one 15k ohm resistance

one 25v 2200uF capacitor

I have a 220v->13.5v transformer, and transformer output is supposed to convert to a 12v DC signal by the rectifier circuit. everything just went fine, but strange thing happen about output value, it's 17.97v DC!! do you have any idea why output is so hight, while it must be about 12vDC! I remove the capacitor and measure voltage of resistor and it was 12v, but when i put the capacitor, voltage change to about 17v! why is that?

A rectifier with a capacitor will output the peak voltage minus the drop in the diodes, or $$\13.5\ \mathrm{V} · \sqrt2 - 2 · 0.7\ \mathrm{V} = 17.69\ \mathrm{V}\$$.