Defective transformer?

[SOTX]

Hello all,

I have a 120Vac to 28Vdc transformer but i'm not getting a DC reading at the output node but instead if i switch my voltmeter to AC mode... i get a reading (30Vac).

is my transformer still functional? do i need to put a real load at the output?

thx

 

[Leftyretro]

Hello all,

I have a 120Vac to 28Vdc transformer but i'm not getting a DC reading at the output node but instead if i switch my voltmeter to AC mode... i get a reading (30Vac).

is my transformer still functional? do i need to put a real load at the output?

thx

Unless I misunderstand you, you should not expect to be able to measure a DC voltage on the output windings of a transformer, only AC voltage. To convert AC to DC voltage the winding must wire to a rectifier and filter capacitor and possibly a voltage regulator circuit or device. Not sure what you mean by output node? So what are you working with, a wall mounted power module? If so are you sure it's a DC module and not a AC module?

Lefty

 

[Sceadwian]

Yep, lefty seems to be perfectly correct there. There is no such thing as an AC to DC transformer, only AC to AC. You need a bridge (or other type) of rectifier to get DC.

 

[SOTX]

ah then my guess was right lol... that is my mistake. i somehow assumed that the transformer by itself will give the dc voltage oops...

yeah i have a voltage regulator that will be connected to this so that with a 7812 (im feeding half of the 28v therefore 14v to it)

so after the 7812 i should be able to get a DC voltage then... coolio

 

[ericgibbs]

ah then my guess was right lol... that is my mistake. i somehow assumed that the transformer by itself will give the dc voltage oops...

yeah i have a voltage regulator that will be connected to this so that with a 7812 (im feeding half of the 28v therefore 14v to it)

so after the 7812 i should be able to get a DC voltage then... coolio

hi,
As SC points out a rectifier is required between the transformer output and the 7812 regulator.
Also capacitors are required after the rectifier to smooth the voltage before its input to the 7812 and on the output of 7812.

Google: for the 7812 datasheet.

 

[Sceadwian]

If you're feeding half the voltage does that mean you're using a single diode as a half wave rectifier directly into the 7812? That's going to provide absolutely no regulation at all. The 7812 datasheet I got from National was useless, no example circuits to go from.

Take a look at this circuit real fast Regulated 12 Volt Supply
Instead of the zener/transistor you would use your regulator at that point. The full bridge rectifier and large electrolytic cap are required, a half wave rectifier will have a LOT of ripple when the regulator is loaded.

 

[SOTX]

sorry i wasn't being specific.

im basically using this circuit but with a 7812 with the diodes bridge and caps

and the transformer that im using has 3 terminals to either give a full 28V or 2x 14V

http://www.circuitstoday.com/wp-content/uploads/2008/03/6v-regulator-circuit-7806_ct.jpg