Wallwarts - SMPS & Transfomers

[enjunear]

TY
I've tried measuring the loaded and unloaded voltage of a wall wart (maybe its a transformer type), and there was huge diff when measured unloaded. I don't know the theoretical reason, but practically i know it's because the wall warts are designed for that ampere rating. Please tell me theoretically why it happens. Do you mean there will be no problem in SMPS type(due to opto couplers-feedback).

Transformers have losses, like every real part (so do caps, inductors, diodes, etc). The following is an equivalent circuit model of a real transformer. The part inside the dashed lines is the ideal transformer (no losses, perfect at every frequency).
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You can see that there are several resistive loss mechanisms inside a transformer. The R1 and R2 are primarily composed of the winding resistance (resistance of the wire used to create the primary and secondary coils). Rc is a parasitic resistance that can be thought of as the equivalent resistance of the magnetic material (you don't get perfect conversion from electric field to magnetic field, then back to electric field).

If you think about the unloaded and loaded cases, you can quickly see that as you draw current through R1 and R2, the output voltage will drop. This is basically the effect you are seeing.

 

[ElectroEnthusiast]

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If you think about the unloaded and loaded cases, you can quickly see that as you draw current through R1 and R2, the output voltage will drop. This is basically the effect you are seeing.

Whoa! I understood this. Thank You Again.
Left Side = Primary; Right Side = Secondary.
Case i: Unloaded ==> Output Voltage read is More.(EX: 9V)
Case ii: Loaded ==> Output Voltage read is Less. (EX: 5V)

Rc is a parasitic resistance that can be thought of as the equivalent resistance of the magnetic material (you don't get perfect conversion from electric field to magnetic field, then back to electric field).

But, why does that appear in the primary winding of the transformer?

 

[enjunear]

re Rc: But, why does that appear in the primary winding of the transformer?

One of the best things about transformers is that they..... transform things :wink:.

You can take any of the parasitic elements and represent them on the opposite side by scaling them using the turns ratio. The "magnetic parasitic elements" are shown on the primary side out of convention (most people show it that way). You could just as easily put their mathematical equivalents on the secondary side and be technically correct in your representation. The side you put it on is often determined by what you are solving for, and what location makes the math easier to deal with.

 

[ElectroEnthusiast]

One of the best things about transformers is that they..... transform things :wink:.

You can take any of the parasitic elements and represent them on the opposite side by scaling them using the turns ratio. The "magnetic parasitic elements" are shown on the primary side out of convention (most people show it that way). You could just as easily put their mathematical equivalents on the secondary side and be technically correct in your representation. The side you put it on is often determined by what you are solving for, and what location makes the math easier to deal with.

0H 0k!!!!!!!!!!

 

[ElectroEnthusiast]

Switching Regulator Wall wart that i used is faulty now. It's giving reverse of the mains voltage and it toasted my connected devices. How can this happen? So Transformer adaptors are much reliable than these.