What Role Do Large Capacitors Play in Computer Power Supplies?

[Jimmy87]

Hi PF. This isn't really a homework question so I posted it here. We were supposed to find a list of devices which we found used capacitors for homework but I had some questions about the capacitors I found in an old Del PC. I have attached the picture to this thread. There seems to be a large collection of big capacitors (2200 microfarads) on the left side of the picture which have near them these green circles with coils wrapped aorund them. What are these green circles and what is the role of these specific capacitors in the computer?

I have a basic understanding of capacitors e.g. exponential decay, time constant, dielectrics, touched on displacement current and we have looked at some of the uses of capacitors (smooth lumpy dc voltage, provide quick bursts of energy, block dc component of ac current).

Many thanks for any help offered.

 

[ModusPwnd]

The coils of wire around green circles are not capacitors, they are inductors. They are "chokes", they filter out high frequency noise.

http://en.wikipedia.org/wiki/Choke_(electronics)

 

[Jimmy87]

The coils of wire around green circles are not capacitors, they are inductors. They are "chokes", they filter out high frequency noise.

http://en.wikipedia.org/wiki/Choke_(electronics)

Thanks. Do you know what those large capacitors likely do?

 

[trollcast]

Going by their location beside the cpu I'd say they're probably part of the voltage regulator module which is basically the power supply for the cpu. It takes the higher voltage from the main power supply unit and reduces the voltage down to around 1.5V and obviously makes sure its a very smooth voltage because of the sensitivity of the cpu chip.

http://en.wikipedia.org/wiki/Voltage_regulator_module

 

[Jimmy87]

Going by their location beside the cpu I'd say they're probably part of the voltage regulator module which is basically the power supply for the cpu. It takes the higher voltage from the main power supply unit and reduces the voltage down to around 1.5V and obviously makes sure its a very smooth voltage because of the sensitivity of the cpu chip.

http://en.wikipedia.org/wiki/Voltage_regulator_module

Please could you briefly explain how capacitors are used to reduce an a.c. input (or direct me to a source). Also, why would you use capacitors instead of a transformer to lower the input voltage?

 

[phinds]

Please could you briefly explain how capacitors are used to reduce an a.c. input (or direct me to a source). Also, why would you use capacitors instead of a transformer to lower the input voltage?

Capacitors do not reduce the voltage, they SMOOTH the reduced voltage. Transformers are what reduce the voltage from higher AC voltage to lower AC voltage, then a regulator circuit turns it into DC voltage and capacitors are part of the regulator circuit and help the final output maintain a smooth DC voltage.

 

[Jimmy87]

Capacitors do not reduce the voltage, they SMOOTH the reduced voltage. Transformers are what reduce the voltage from higher AC voltage to lower AC voltage, then a regulator circuit turns it into DC voltage and capacitors are part of the regulator circuit and help the final output maintain a smooth DC voltage.

Where would the transformer to be located on the voltage regulator module in the picture?

 

[phinds]

Where would the transformer to be located on the voltage regulator module in the picture?

I don't see one.

 

[davenn]

Where would the transformer to be located on the voltage regulator module in the picture?

The transformer in inside the Power supply unit a ~ 5 inch square (ish) box with 120/240VAC going in one side and many low voltage DC wires out the other side

Dave

 

[256bits]

Please could you briefly explain how capacitors are used to reduce an a.c. input (or direct me to a source). Also, why would you use capacitors instead of a transformer to lower the input voltage?

If you read this, it explains what those capacitors, inductors and several other components on the motherboard are there for. Look at Figure 10 for something similar to that on your motherboard.
http://en.wikipedia.org/wiki/Buck_converter

 

[enorbet]

It is worthy of note that while technically still sort of "alternating", computer PSUs are of a particular variety called "Switching Power Supplies", or "Switching Mode Power Supplies", usually simply called "SMPS" and behaves much more like a pulsed DC which is why they are also referred to as DC-to-DC power supplies. This is done mainly for efficiency and size reduction of the filtering components as the frequencies are extremely high.

If the common household 50/60 Cycle PSU delivering the typical 300-600 Watts required were used the bank of capacitors alone required for brute force smoothing sufficient to work for a computer would commonly exceed the size of most existing PSUs.

On your motherboard you will find 2 distinctly different looking kinds of capacitors. Those cylinders, often with a shiny, metallic top (also commonly gouged with an "X" on top) are of the brute force variety and easily distinguishable from their brethren, Surface Mount Component type. The latter is used primarily for time constants, isolation, etc. in signal lines as opposed to supply filters/reservoirs. Surface Mount components are commonly very small, where the largest can fit 8-10 of them on your thumbnail.

Here is a PC-specific web page of SMPS - http://www.hardwaresecrets.com/article/Anatomy-of-Switching-Power-Supplies/327