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Can you please explain this circuit? Answered

I am planning to create a step down switch mode power supply using only discrete components.
I opened my charger and found a step down switch mode circuit without IC.
I wrote the board's schematic by looking at each component one-by-one and found this...
I does this thing works? I cannot understand the part of the circuit that is exposed into the mains.



Best Answer 6 years ago

First this only oscillates during the positive half of the ac line cycle
because the 1N4007 (1000 PRV 1Amp ) diode blocking the reverse
AC current flow.

The NPN transistor is launched into oscillation by the megohm resistor
connected to the base emitter junction.
The base coil winding is sized to drive the NPN through the 33 ohm
resistor and series capacitor on the positive High Fq transformer cycle.

During the negative excursion of the base coil the zener and 3.3K
resistor in parallel with the capacitor limits base to -3 volts of NPN
guaranteed shut down.

The frequency of the oscillations will increase as the power builds
on the slow line cycle to a peak and slows as it recedes back to

The oscillation is controlled by the transformer core saturation.

A current through the primary coil causing a sustained voltage at
the base coil feeding the NPN until the magnetic core saturates and base voltage falls shutting the NPN down.


Am i getting it right? the primary coil is the upper coil? and the base coil is the lower coil?

"During the negative excursion of the base coil the zener and 3.3K
resistor in parallel with the capacitor limits base to -3 volts of NPN
guaranteed shut down."

What is the purpose of 4148 diode? will it not short out the base coil at the negative excursion?

What kind of oscillator is this? (name)

Yes the upper coil is the Primary.

Yes the zener diode limits the negative voltage to 3 volts.

The 1N4148 diode allows a current to charge the -3v capacitor in
parallel to the zener diode and 3.3k resistor. during the negative
ie non-conduction cycle of the base drive coil.

The 1N4148 main reason for being is it prevents any current flow
through the zener group during the positive base drive excursion.

Oscillation based on BH curve plot of a saturable iron or powder core.
Sorry I  can't remember an inventors name.


A switched mode PSU works by switching the AC ON and OFF very quickly. Because there is little time when the power components are in the transition phase there is very little power wasted as heat.

They also avoid the need for large step down transformers and so save cost and weight as well as bulk.

The disadvantage is that the fast switching can produce electrical noise which must be filtered out.

In your case the transformer will be a high frequency transformer designed to isolate the input and output - It is relatively small and by the looks one tapping is providing feedback to the switching transistor base to control the switching action..

How does this circuit work?
At what frequency is it working?

You can't get DC OUT of a transformer. What happens on the secondary ?

Owww sorry i forgot to write everything about the secondary. Of course, there is a rectifier, filter and a regulator.