Building and Modifying Mobile Phone Chargers

'and a 3rd winding which is connected to a elec capacitor of large size thru a diode in a loop. I cant make out the purpose of this loop. i also noted taht the capacitor voltage is use to conrol the bias of trasistor usually by shunting the base thru another trasistor. '

You have just answered your own question...It sounds like a feed back loop to provide control of the output voltage!

I also use mobile phone chargers as power supplies, but be very careful, they usually have absolutely no filtering on them, so they are extremely noisy little things...

You should only experiment with one if you have a safety isolated mains supply - they are not only noisy but they have a nasty bite as well...

Personally I wouldn't mess with changing the voltages etc... once you start doing that its best to buy or design what you want...

Those cheap and nasty phone chargers may well be the cause of a fire or a safety hazard due to poor isolation etc...

John.

Get a book on PWM - Pulse Width Modulated power supply design.

Instead of step down then regulate AC to DC with constant output, they supply a rectangular pulse with variable pulse width adjusting to load. Its a lot more complicated then old transformer/regulator based PS. They do depends on the feed back loop to set voltage. It also got a oscillating circuit for the pulse.

Pineapple

HOW TO USE SPELL CHECKER .COM

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Can any friend throw some light and explain the functioning of these?

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The very 1st thing I should mention is the reliability of Switch-Mode Supply [SMS] used in Mobile chargers is that those are most un-reliable due to the rectifier-circuit used.

It a single-diode in series with a resistor & a capacitor; terminated at a zener to get a low-voltage supply to run the [SMS]; using that [discioverd that the trasformer ]with 3 windings.

Mostly I have found the Zener short-circuited.

Otherwise the Switch-Mode Supply [SMS] are quite reliable as in those full AC is rectified &

Switch-Mode Supply [SMS] circuit runs @ nearly 120VDC [in 110VAC Mains] & 250VDC in case of 220VAC Mains. Most reliable MOSFETs areused for reliability. One example is the PSU used in Computers.

Now I come to the your point. Try to trace the circuit of your charger & go to make the circuit on Bread-Board or Experiment-PCBs available in market.

Warning:

It will be safe to try the circuit with 6/9V Dc from cells/battery and after you get it done, try it with the "rectifier-circuit" used in said chargers.

Pray for your success & happiness.

Most of the phone chargers i've picked apart have a diode bridge at the input, some don't have any filtering so raw pulsed dc is fed to the switching transformer and switching device...

This is one reason they are so noisy, there is absolutely no filtering on the input - they must make marvellous rf noise generators...

John.

Yes I there is place for a bridge, but here I got with 1 diode connected & 3 left over.

Filter is not used in Battery chargers as the battery behaves like ∞ [infinite] capacity.

OkK you can use a bridge if you like

Enjoy !!!!!

No, I meant there is no mains input filtering, so the switching transients and other noise is fed back into the mains and re-radiated.