If i connect 20v 4A Ac power with bridge rectifier and 6800uf capacitor. what will be the output DC voltage available at capacitor. I mean the formula to calculate this voltage.
Your input voltage is 20Vac. This is an rms value. Your output voltage will be 1.41 x 20V = 28.2V peak. You will also have a voltage drop across your bridge rectifier of 0.65V x 2 (for each diode), so your actual voltage across your cap should be 26.9V