Hi

Can anyone help I replaced a thermostat (ruud) because the power was always on
The fan kept blowing even when the unit have been turn to the off position and the fan was on the auto position
Now nothing comes on I checked the trasformer where it has 120 coming in but no power coming out where it should show 24v
I have a black wire connected from the t-tat to the 120 then checked to see if any power would be present but no luck yet
Can some one tell me what is happening here pleaseeeeeeee
Greatly appreciated

You should not have "black wire connected from the t-stat to the 120". Dangerous.
Most common and basic of Central Air is 240 volts to Air Handler, and 240 Volts to Outside Compressor Unit.
In the Air Handler is Transformer, usually 240 volts to 24 volts.
24 volts goes to Thermostat Red, then if you connect to Green, fan will run. Or turn to cool.
Yellow or blue for cool( puts 24 volts out to compressor contactor), White is sometimes
Heat.
There is in depth info at start of heating and AC(Sticky). Very Possible added info there

It sounds as simple as a bad 24 v transformer. Before replacing check to insure the screws are tight. The tansformer is a common item at home centers, $30.

Did you check the power at the transformer or furnace terminals? Newer furnaces have a fuse on the control board, much cheaper and easier to replace than the transformer. If you don't have a fuse, I would pick up an inline one and wire it in to make sure the new transformer doesn't follow the old.

Note, most furnace-A/C systems the blower and control transformer run off the 120 volt.

If you are testing the transformer, and you have 120 volts going in and nothing on the secondary side, then you have a bad transformer. Do some testing to make sure there are no shorts, also take the above advice, install an inline fuse, it can save you a transformer.

My transformer has 3 wires blue which I don't have it connected to anything
Red not connected as well
Black to the 123v which reads 122v when checked with a meter
On the other side of the t-stat I have 2 yellow wires both of then read 1.9v

I have one going to t-stat and the other to the terminal #1 on the sequencer ithink that is the name is a black thing with numbers from 1-6
One yellow wire from transformer to the 1 terminal which reads 1.9
Terminal 2 reads 1.8v
Terminal 3 reads 122
Terminal 4 reads 122
Terminal 5 and 6 0v
Still no power to my t-stat
Can you please help with my big problem 122v primary 1.9v on secondary

Looks like bad transformer, blue and red unused wires probably different taps for primary.
Black is 120 primary, blue and red, may be 240 volt, or 277 volts.
With no load, measure voltage in and out. Do cap off red and blue.
Is the breaker for your ac 240 volts? Wondering if half of breaker tripped.

Well coming out of the breaker have a black wire and a white both of them read 122v
I'll change the transformer and see if that fixes my problem
Thank you for your help greatly appreciated

The black and white are both on breaker? If you put leads on black AND white, Do you get about 244 volts? If so, you may need a 240 volt to 24 volt transformer.

Do I need to connect the black and white out the breaker to the black on the transformer or just to the black out the breaker
Because I a 120 volts to 24 volts and connected black from transformer to the 120volts out the breaker and I cannot get any reading on the 24 volts side (secondary)
Help please

Very Important! Don't know if I'm missing it. I know you get 120 from black and white.
If you put meter on black, and one lead on white, You may get 240 volts?
You got a 120 volt primary transformer? If so may have harmed transformer.
Use appropriate tap if wrong(240) Not black wire.
You can measure resistance on transformer. If primary or secondary are opened, will
Not work.