What is this coil inside this AC to DC power converter?

[sepoto]

I have been studying the plans for a few AC to DC converters and I understand that the basic conceptual design is one where the AC power is transformed to lower the voltage and is then passed through a bridge rectifier to convert that power to what I call hopping DC. The power is then smoothed with capacitors and is then regulated to a constant DC output using voltage regulators. In my efforts I took it upon myself to take one of these kinds of power supplies apart to see what is inside of it and I came upon a question that I do not have the answer to. I have attached a picture of the inside of the power supply. Between the square component with the yellow electrical tape and the four small capacitors there is some type of circular device with a helix coil of copper wire around it. I think that this is some type of solenoid however I am inquiring to the purpose of this component and why it would be inside of this circuit? All of my research tells me that solenoids are used to create motion most of the time however I don't see this solenoid attached to anything unless it is somehow being used to power the fan inside the unit. I see these coils inside of every computer power supply I have ever looked inside and I have become exceedingly curious about their intended purpose inside of AC to DC converters like the one I am planning to build out.

Thanks in advance for any comments or answers!

 

[davenn]

Hi sepoto
welcome to PF

OK the PSU you have shown there is called a SMPS = Switch Mode Power Supply
initially the DC voltage is higher than the input RMS AC voltage
for an AC mains input of 230 - 240V as in my country, the smoothed output from the rectifier is ~ 320VDC ( this is because the smoothed DC is more representitive of the peak AC voltage)
This DC voltage is then "chopped" at a high frequency, often ~ 30 - 50kHz, and applied to the primary of the transformer that large component with the yellow insulation tape that you commented on.
Off the secondary of the transformer there will be some power rectifier diodes that produce a lower voltage DC ( there may be several low voltage DC rails depending on the design)
That coil you are referring to is an inductor, not a solenoid, ( a number of turns of wire on a ferrite ring) and is part of the of the smoothing of the DC output along with those 4 electrolytic and other capacitors. That inductor and associated capacitors are used to filter out the high frequency component of the switching circuitry of the primary side that comes through the transformer to the secondary side.
There is also feedback circuitry from the secondary back to the primary that is used for voltage regulation.
The reason for using high frequency switching is so that a smaller transformer can be used making the overall operation of the PSU much more efficient than the older style of a linear PSU using a huge and heavy transformer

Dave

 

[davenn]

OK found a SMPS circuit I used to work on back in the '80's and '90's
from the Atari 520ST and 1040ST computers

Note the dual would inductor coils in the output on the +12VDc and +5VDC power rails
Note the PC101 ( an opto-coupler) and assoc components at the bottom of the diagram
these are the feedback components that are used to control the output voltage regulation.
Basically they control the On/Off switching timing of the switching transistors on the primary side. This is usually done by a method called PWM, Pulse Width Modulation.

do some googling on that :)

cheers
Dave

 

[sepoto]

Thank you very much! I could not have asked for a better answer. Not only did you identify the component for me you also told me what is was for. I've been doing a lot of research on induction on Wikipedia.

 

[davenn]

You are welcome

have an awesome new year, its just 3 and a half hours away for me

Dave