why it become 20vdc after bridge rectifier while supplied 18vac

[janeliza]

hello. i have a question. i built a power supply whose desired to have an output of regulated 12vdc, the problem is when i've done the assembly, when i test it the output becomes 20vdc right after bridge rectifier, while i've supplied it with 18vac. what shoul i do to make its output 18vdc or lil bit lower.? also when it passes thru regulator, it doesnt change the output voltage. i really hope for your help.

 

[davenn]

hi
welcome to the forums

i have a question. i built a power supply whose desired to have an output of regulated 12vdc, the problem is when i've done the assembly, when i test it the output becomes 20vdc right after bridge rectifier, while i've supplied it with 18vac.

doesn't sound correct, it should be higher ... if working correctly you should be seeing ~ 25V - 28V

also when it passes thru regulator, it doesnt change the output voltage. i really hope for your help.

the regulator obviously isn't wired correctly

This may be the cause of the first problem


show us your circuit and be prepared to also show a pic or 2 of your actual construction
so that we can possibly see a wiring error

cheers
Dave

 

[Colin Mitchell]

The regulator is either connected around the wrong way or the "common"terminal is not connected to 0v or the regulator is damaged.
The 12v regulator can deal with 15v to 25v.