Motherboard query

[rajan1311]

Well this is the hardware Q & A section, so posting this. I had this question in mind for some time now, but never posted it cos it may be very silly, anyways, here it is:


Why do motherboards have so many capacitors ? How are they useful?

 

[pimpom]

Capacitors are used for a variety of purposes in electronics. Most of the capacitors you can see on motherboards are used for filtering the power supply.

The term 'filtering' here means smoothing out any electrical irregularities or spikes. The power supply is supposed to provide steady voltage levels such as 12V, 5V, etc. But in reality, those voltages may have brief moments when they shoot up above the standard values. They may also drop down temporarily due to fluctuations of the mains voltage.

These irregularities can play havoc with the computer's function, and may even damage something.

So, those capacitors, acting alone in some sections, and together with ICs and inductors (coils) in other sections, smooth out those spikes and irregularities. They ensure that only smooth and constant voltages are delivered to the sensitive electronics on a motherboard.

But even these are sometimes not enough and some spikes get through, causing crashes or damage. Also, power supplies can differ greatly in how far they can suppress spikes and fluctuations on the AC power line.

 

[coderunknown]

^^ thanks. i2 not know the exact reason of having so many caps.

 

[pimpom]

^You're welcome.
Capacitors are also one of the factors (but not the only factor) that determine the quality of a PSU (SMPS). They play an important role in determining how much the PSU - and therefore the computer - can tolerate mains fluctuations.

For example, PSUs generally use two capacitors where the mains AC voltage is first converted to DC. Cheap PSUs may use 2x220uF whereas better PSUs may use 2x470, 2x680 or even 2x1000 uF caps. There's a similar situation at the low voltage output side.

 

[rajan1311]

thanks for the reply, but i got another question.

If the caps remove the ripple from the supply, why not add more in the PSU itself? 2 caps and 1 inductor are more than sufficient to get the ripple down to some 0.00002% or something.


Also, all the suplly in the motherboard is DC right? So why do we have motherboards with "12 phase supply" for the CPU? how does phase come into the equation?


Thanks.

 

[pimpom]

thanks for the reply, but i got another question.

If the caps remove the ripple from the supply, why not add more in the PSU itself? 2 caps and 1 inductor are more than sufficient to get the ripple down to some 0.00002% or something.

It's not quite that simple. First, it's not just 100Hz ripple but also multiple-frequency spikes. Secondly, practical capacitors and inductors are not pure components. Inductors have capacitive and resistive components and caps have insuctive and resistive components.

These imperfections cause these components to behave differently from ideal ones. So, no matter how much you increase the caps and inductors on the PSU, some spikes will still get through. Even the connecting wires can pick up some undesireable waveforms on their way to the motherboard. So further filtering and regulation is necessary on the motherboard.

Also, all the suplly in the motherboard is DC right? So why do we have motherboards with "12 phase supply" for the CPU? how does phase come into the equation?

Thanks.

Multi-phase supplies are used for two reasons: easier filtering and sharing of the load. Converting 50Hz AC with a single-phase rectifier produces a very high level of 50Hz ripple that's difficult to filter adequately. The rectifier also has to carry all the load current.

Using a full-wave rectifier produces 100Hz ripple which is easier to filter, and each rectifier has to carry only half of the load current. The same principle applies as we increase the number of phases. (The rectifier may be a diode, a switched transistor or some special device).

A modern CPU needs low voltage at very high currents. This is much more difficult to filter than a higher voltage at a lower current. OTOH, CPUs are more efficient (less waste and heat) when they work at low voltages. The low voltage, high current supply is not taken directly from the PSU output, but regenerated on the motherboard itself using many phases of high-frequency AC.

 

[coderunknown]

rajan, PSU gives current to the board. now the thing is not all boards same. someboards needs more current in some particular places only. & yes one can add more caps on the PSU itself. but what will that do? increase pricing of the PSU. & local PSU (with a few caps) will continue sell for 500 bucks.

phase comes in play cause i think you need divide the power delivered from PSU evenly throughout the system. if you add a very fast proccy (power hungry too) & it ends up eating all the power, what will the ram, chipset, etc feed on? the system will become unstable. so more the phases, more current can be delivered to a particular part of the system. i think you heard about the latest Gigabyte mobo. X58 UD9. i not know how many phase it got, but it can deliver 1200W to the proccy (enough to fry the proccy half a dozen time). but its my idea. i maybe wrong here.

 

[Faun]

that was some serious gyan. Thanks.

 

[coderunknown]

@pimpom, so my idea is wrong. isn't?

 

[rajan1311]

ok, so by having multi phase, it basically splits the current on the board, making it easier to filter, so load is reduced on each rectifier, so it makes operation stable? but then you are risking more harmonics (of leftover AC, before filtering) when you split it right?




@sam: even if a processor taking 75W has about 50A of current, more than enough to char us to ashes
If we have more caps on the PSU itself, the current will have less ripple into the board, easy.

 

[pimpom]

@pimpom, so my idea is wrong. isn't?

I'm afraid so. But you were mature enough to indicate that you were not sure, unlike some people who make brash claims. Many such people, when they are proved wrong, come up with lame excuses and/or try to divert the discussion in another direction.

 

[asingh]

Basically they are used as electronic filters. I did my Bachelors in Physics (Electronics) around 10 years ago, so will not embarrass my self. This seems a good, basic explanation.

 

[rajan1311]

Basically they are used as electronic filters. I did my Bachelors in Physics (Electronics) around 10 years ago, so will not embarrass my self. This seems a good, basic explanation.

wow did not know you were that old.

Yes I know they were used to filter, but could never understand why so. pimpom gave a good explination, thanks buddy, so how do i rep you?

 

[coderunknown]

I'm afraid so. But you were mature enough to indicate that you were not sure, unlike some people who make brash claims. Many such people, when they are proved wrong, come up with lame excuses and/or try to divert the discussion in another direction.

i still a student. so can't gut i "Guarantee" seal on my explanation. furthur it was just my general idea. i not read about it anywhere so didn't took the risk

Basically they are used as electronic filters. I did my Bachelors in Physics (Electronics) around 10 years ago, so will not embarrass my self. This seems a good, basic explanation.

and i currently doing mine (Computer Science).

wow did not know you were that old.

old saying, "Old is Gold". carries perfect meaning here

 

[pimpom]

i still a student. so can't gut i "Guarantee" seal on my explanation. furthur it was just my general idea. i not read about it anywhere so didn't took the risk

That's the right attitude - to admit to yourself when you don't know something. No one knows everything, including me.

I've been in electronics and computers for a long time. Now I hold classes for +2 students, college students and engineers from time to time (no time to do it full time). But there's still a lot I don't know and I keep on learning.

and i currently doing mine (Computer Science).

Keep it up. Getting a degree and landing a job are important, but remember that it's not everything. Continuing to acquire the skills to be good at what you do is important, not only to get a good salary, but also to lead a fulfilling life.

 

[coderunknown]

That's the right attitude - to admit to yourself when you don't know something. No one knows everything, including me.

thanks

I've been in electronics and computers for a long time. Now I hold classes for +2 students, college students and engineers from time to time (no time to do it full time). But there's still a lot I don't know and I keep on learning.

WOW. so you got a lot of experience in this field.

Keep it up. Getting a degree and landing a job are important, but remember that it's not everything. Continuing to acquire the skills to be good at what you do is important, not only to get a good salary, but also to lead a fulfilling life.

yes job is important (for me & everyone) but i come here to learn more & new things thats evolving in tech world all the time. i not know if it'll be helpfull to me in future in any ways but just its my hobby.