dc supply with transfomer in the output !!!!

[ahmed osama]

dear sirs


i opened a LED SPOT and i found its driver board takes 220V ac 50hz and supply the led using about 16Vd + 2 rms AC !!
i the board consist of a bridge + 4.7uF 400V Cap in its input and there is some resistors , a diode (may be zener) , transistor , ... then a big surprise a small transformer ( 4 inputs / 2 outputs ) connects somehow indirect with the output !!!

any one know how such circuit work , any diagram , ....etc

thanks

 

[betwixt]

It's a standard SMPS supply !

The 13001 is a high voltage switching transistor and the circled diode is a fast rectifier.

Brian.

 

[goldsmith]

then a big surprise a small transformer

Hi Ahmed
No ! that's not surprising ! because as Brian mentioned above , the circuit is kind of SMPS which will work with high frequency voltage , so transformer will be small .
Best Wishes
Goldsmith

 

[ahmed osama]

Hi Ahmed
No ! that's not surprising ! because as Brian mentioned above , the circuit is kind of SMPS which will work with high frequency voltage , so transformer will be small .
Best Wishes
Goldsmith

transfomer always be in the input stage working with the AC but not in the output with the DC !!!

 

[betwixt]

In a normal linear supply that is true but not in SMPS.

What you have in your board is bridge rectifier directly across the incoming mains AC so it produces about 150V DC if you are in a 110V country or about 300V if you are in a 220V country. Look at the big capacitor and you will see it has a hgh voltage rating. The transistor is part of an oscillator that runs at relatively high frequency, probably around 50KHz and it 'chops' the high voltage to the small transformer. At high frequencies the transformer becomes smaller and lighter because the iron core can be replaced with ferrite and fewer turns are needed on the windings. The transformer still isolates the primary and secondary so the output is (should be!) safe to touch. The secondary of the transformer feeds the small circled diode which produces the DC to the LEDs.

Brian.