why we use capacitors in pcb

[anissk45]

hi,

i am a real fan of this blog, i was wondring what is the use of capacitors and inductors in pcbs?.

[Psi]

Short answer:  Storing energy.

[anissk45]

well yeah i know all of that is there any long answer?

[GeoffS]

If you give an example i.e. circuit diagram, I'm sure someone will explain the functions of any capacitors and inductors.

[anissk45]

here is the schematic

[Alana]

Most capacitors here are used for decoupling, removing noise from power supply.
http://en.wikipedia.org/wiki/Decoupling_capacitor

Those near quartz are part of oscillator - without them quartz would not start.

Inductor - with capacitors next to it it forms LC filter that should prevent noise from digital part of the circuit to get to the analog part - here its the ADC build in into atmega.

There is also a timing capacitor connected to reset pin of one of the atmegas - at power on it holds reset pin low for some time allowing atmega to boot up properly.

[c4757p]

Just a minor clarification / potential correction - the cap on the reset pin isn't meant to hold the pin low on startup, unless the design was a bit misguided, as the brown-out detection on that chip should handle that just fine. The chip will hold itself in reset while the power comes up. It's more of a filter - prevents noise on that pin from accidentally triggering resets.

(Unless that design really does depend on the left-hand micro coming up around 1 ms later than the right-hand one. In that case... WTF? And it won't really work properly anyway.)


As an aside, I wonder if this style of schematic is the new norm? I guess the natural progression would then be to this, followed soon by this as a response to "but you have no wires!"... How hard would it be to put those blocks in some logical order so they can be properly connected? And the little diagonal sections swerving around other components are bizarre.

[Psi]

As an aside, I wonder if this style of schematic is the new norm? I guess the natural progression would then be to this, followed soon by this as a response to "but you have no wires!"... How hard would it be to put those blocks in some logical order so they can be properly connected? And the little diagonal sections swerving around other components are bizarre.

It all depends on the size of the circuit. Sometimes it's just messy to connect everything up in one big diagram.
Sure, you could spend many hours rewiring everything to find the best circuit layout that allows you to connect everything together in one diagram but it's not a very good use of time for something that's abstract (unlike the pcb).

Breaking it up into sections makes a lot of sense most of the time.
It's easier to develop the circuit that way, and you can change netlabels between sections without having to rerun lots of rewiring.
There's also advantages for circuit reuse, you can copy/paste the regulator section from another project very easily etc..

But as you say, it can be overdone.
Sometimes a huge A2 sheet of the entire circuit can be very useful. Troubleshooting or fault finding an unfamiliar design for example, where you don't want 10 pages of circuits you have to keep switching between or multiple sections with no clear indication of how they interact.

[anissk45]

so they are decoupling capacitors.. i think i got to read a bit more about capacitors

[c4757p]

Of course, the schematic still has to be drawn smartly. I like having things connected in general, but just barfing them onto the page and then hamfistedly scribbling wires between them is much worse even than this. (Hence my second example.) I am a huge fan of KiCad's hierarchical schematics (I'm not aware of any other affordable tool that does this, but then again, I haven't put much effort into any others - for all I know they all have it...). I try to have everything on one page connected "properly", but separated into subcircuits. For example, on the electronic load I'm working on, the entire analog "load" section is monolithic, but there are hierarchical labels for control points that run out to the master sheet. That way one entire circuit is represented as one entire circuit, but the huge project that really contains many circuits doesn't become a pile of wire spaghetti.

It looks like this schematic is meant to be divided into "subcircuits", but they're bizarre. I'm not sure why four of the decoupling capacitors are in their own block, when all the rest are just shoved up against whatever they decouple. There's an entire section just for an LED, the naming is terrible (one section is "Microcontroller", but there is another section also with a microcontroller yet unlabeled), and as far as things like the IRQ button and debug link, it looks like the decision on what goes in a subcircuit and what does not was completely arbitrary. There is also no text of any sort on the schematic - I couldn't tell you to save my life what it does, seeing as it's just a generic lump of microcontrollers and memory.

Anyway, I should probably avoid derailing any more threads for a while, so... goodbye