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

janeliza · Mar 4, 2015

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 · Mar 4, 2015

hi
welcome to the forums

janeliza said:

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.
Click to expand...

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

janeliza said:

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

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 · Mar 6, 2015

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.

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

janeliza

davenn Moderator

Colin Mitchell

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