6
\$\begingroup\$

I've got an ordinary PC motherboard. So, out of curiosity, I started looking at the power circuits on it and found several large 1000–1500 μF electrolytic caps:

motherboard photo collage

It's easy to see that their cathodes are not connected to any wires on the back side of the board. However, they aren't connected on the front side as well:

motherboard photo

At first I thought that these might be soldered in as reserved components, uhhm... but now I'm inclined to reject that hypothesis. It makes little sense; and what's convincing the most is that other similar caps are wired in the same anode-only fashion.

I guess that this might work, because of electrostatic induction happening inside the caps, which would still allow to store energy in there over 1 wire only (and thereby absorb unwanted ripple current) – but wouldn't that effectively reduce the caps' capacitance?

Also, can the cathodes be wired back to ground, theoretically? Will this enhance power regulation stability?

Finally, I have a suspect that this could be related to the obvious difficulty of routing the ground wire to nearly every component on the board. If I'm correct, is connecting of EL caps by 1 wire only a common practice in the industry? Or did I just notice an extremely dirty awful hack, which I shouldn't ever repeat in my designs?

\$\endgroup\$
  • 8
    \$\begingroup\$ Are you aware a PC motherboard PCB normally has something like 8 layers or more? It's probably connected to an internal ground plane layer that you can't see. \$\endgroup\$ – PeterJ Apr 1 '13 at 0:45
  • \$\begingroup\$ I have to second PeterJ's comment. I have never seen an electrolytic capacitor left unconnected. Those bulk capacitors are used to store enough energy to handle step loads (such as CPU awakening) before the main power supply can possible react and compensate for. \$\endgroup\$ – SunnyBoyNY Apr 1 '13 at 0:52
12
\$\begingroup\$

PC motherboards have more than just the top and bottom copper layers that you can see. The capacitors are connecting to a hidden layer somewhere in the middle. In cross section, it looks like the example labled "1" in this picture:

enter image description here

(image courtesy Wikipedia)

\$\endgroup\$
  • \$\begingroup\$ I didn't know that you can create connections like that. I mean, you can only solder on the top and bottom layer, right? Or are these not soldered? \$\endgroup\$ – angelatlarge Apr 1 '13 at 0:53
  • \$\begingroup\$ @angelatlarge vias are like metal shafts that go through the full PCB stack (unless they stop in the middle somewhere in which case they are called buried vias, or if they start end end in the middle somewhere they rae called blind vias - neither is recommended practice without a very good reason). These metal shafts may connect to copper traces on any layer of the stack up, just like if they do to connect to top to the bottom on a normal two-layer board. \$\endgroup\$ – vicatcu Apr 1 '13 at 1:32
  • \$\begingroup\$ @vicatcu: Yes, I understand that vias are always/often plated through, however, I didn't think that just seating components in plated vias made a reliable electric connection. \$\endgroup\$ – angelatlarge Apr 1 '13 at 3:28
  • \$\begingroup\$ @angelatlarge No, seating components in a plated hole is not reliable. That is why the component is also soldered. But it just has to be soldered to some part of the hole, not everywhere. The hole makes a reliable contact to one or more copper layers, and the component is soldered to the hole's externally visible pad, so ... \$\endgroup\$ – Kaz Apr 1 '13 at 4:36
  • \$\begingroup\$ @Kaz: Ah, yes, that accords with my understanding. I guess I attributed "nothing was soldered" to OP when all OP was saying "I didn't see any traces running from the cap". My bad. \$\endgroup\$ – angelatlarge Apr 1 '13 at 4:43
1
\$\begingroup\$

Ground plane. A good designer will keep split power planes sandwiched between them, and sometimes even more to separate high speed signals from each other.

\$\endgroup\$
1
\$\begingroup\$

A capacitor has to complete a circuit to do anything useful. Like any circuit element, if one of it's terminals is floating that means no current will flow into that node and that component is effectively omitted from the circuit.

An obvious exception to that is an antenna which picks up RF energy and injects it into a circuit, but capacitors are not in that class of circuit elements.

None of that matters in this question though. As @PeterJ commented motherboards are almost always multi-layer constructions. There is a lot more going on, in terms of routing, than what meets the eyes on the top and bottom of the board. More often than not, two or more of the internal layers are unbroken ground planes that ensure impedance controlled transmission lines for high speed signals (like the memory bus and system bus), and also to ensure good EMI/EMC performance. Your mystery capacitors are most likely making a connection to ground on those internal layers.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.