i don't get it?

[findhelporbust]

???

trying to do a simple circuit and it's not turning on.
i have an ac line going to a bridge rectifier. does this work without a transformer?
because my led and fan do not turn on. actually nothing does.

can anybody help?!

 

[audioguru2]

Hi Bust,
Welcome to our forum. ;D
You can't operate low voltage devices from 120VAC efficiently. I don't see the 2nd AC wire in your circuit so I corrected it, but with 22k resistors then the fan and LED get only about 5mA so the fan probably won't run and the LED won't be very bright.
The 22k resistors will dissipate about 0.64W so better be rated for about 2W so they won't get too hot.

View attachment 37782

 

[findhelporbust]

hmm, i have another question, see if youv'e come across this. the 22 k in the drwing is like 20 resistors in series, but number wise they add up to 22k but when i measure with a multimeter it says like 40k?!?
I don't get it, do i have to use smaller value resistors to get 22k?

(what i mean is.... paper value is 22k reality value is closer to 40k? why is this... you think i took to long to soder them? because they are surface mount resistors.)

 

[audioguru2]

You used 20 tiny resistors in series instead of a single big one?
I think you cooked a few. Measure each one with your ohm-meter to find them.

Did you ever get the LEDs to light dimly?
Does the fan run?

 

[findhelporbust]

not yet but i'll find out later today

 

[findhelporbust]

i don't under stand, all of them are much higher values than they indicate... did i fry them all?

is there a special way to soder the surface mount resistors? ???

 

[audioguru2]

I've soldered surface-mount parts without any problems, but I didn't use the 150W soldering gun like you.
My soldering iron is temperature controlled and I have a small tip for it.

 

[findhelporbust]

it says it's a 12w gun... hummm...

you can just splice into an ac line right, you don't need a transformer?

another thing is i tried to measure my resistance from the positive to the negative... what kind of shocked me is that it measured 10k ohms? short in the bridge?

 

[audioguru2]

you don't need a transformer?

Most low voltage electronic circuits use a transformer. Even my soldering iron has one.

i tried to measure my resistance from the positive to the negative... what kind of shocked me is that it measured 10k ohms? short in the bridge?

If your switch is turned off, then you are measuring the resistance of the "resistor series" on your schematic, whatever that is.

 

[findhelporbust]

well i have the resistor series disconected.

I guess to be effiecient it needs a transformer, but don't have the space so i can do without right?

 

[audioguru2]

I have some fairly small and very lightweight switching power supplies that have an output of 5V/2A. Cell phones also use a small charger. They use electronic switching tricks instead of a transformer. 

 

[Ldanielrosa1]

Um, mind if I ask what this is for that a transformer won't fit?  And assuming it's not a terribly hazardous application, can you be more specific about the power requirements of the fan?

 

[Alun]

I have some fairly small and very lightweight switching power supplies that have an output of 5V/2A. Cell phones also use a small charger. They use electronic switching tricks instead of a transformer.

 

[audioguru2]

Switching power supplies do use transformers, all switching regulators that have an isolated output use a transformer, the only differance is it's a smaller ferrite transformer that operates at a higher frequency typically 50kHz to 500kHz.

Correct! :-[
The transformer is so small I could hardly see it. It has tiny surface-mount Mosfets driving it.

 

[findhelporbust]

lookes like i have to do more research on these transformers -
:'(
the reason i could not fit an iron core transformer (the normal size ones) is that i have a very restrictive area, can not  fit much more than wires, the fan and led have cutouts in the frame, would not look nice or professional if it had a transformer hanging out....

 

[audioguru2]

It would look very professional to have a jack for the plug and wire from a DC output wall-wart AC adapter.