question about Capacitors

[khold]

I am wondering what purpose capacitors serve on a motherboard? I learned from my college electricity class how capacitors block DC voltage, and since motherboards have DC voltage I don't understand how the capacitor works on a motherboard.

Thanks.

 

[Barnaby W. Füi]

Electronics newbie here but I believe they're mostly used to smooth out the signal/voltage/etc.

 

[khold]

Yeah I've vaguely heard about smoothing out voltages, but I don't get why the capacitors just don't stop the flow of electricity completely. When a capacitor charges up fully it stops the flow of electricity in the direction that it was charged in. Electrolyte capacitors that computers use are polarized so they only charge one way.

 

[ArmenK]

The signal on your motherboard is not pure DC, it was converted to DC by your power supply. Thus, it is not a perfect DC signal and the capacitors help to smooth out the signal.

 

[CTho9305]

Basically, capacitors like to keep the voltage across them constant. If you put them in parallel with a circuit, once they are charged, they will absorb small spikes in the voltage (the voltage across them is higher, so they charge up more), and fill small drops by discharging some (the voltage across the pins is lower, so the capacitor supplies current and discharges).

When you have a device like a CPU that doesn't draw current constantly (it draws most current when the clock switches), it is very difficult to keep the supplied voltage steady (if you don't do a good job, it will drop when the clock ticks, or spike afterwards, causing problems). Adding capacitors in parallel helps because they can supply the extra current needed when the clock ticks, and reduce any spikes when the power consumption is less.