That seems like one the least harmful mistakes you could make. Capacitors are sprinkled all over boards in excess’s because it’s probably better than not enough capacitance.

It would probably be an idea to point out here that those are decoupling capacitors used to reduce noise. Capacitance isn't the point with them, but that they pass AC while blocking DC. Not that the capacitance hurts, mind.

But that’s what a capacitor does? The point is to provide a low-inductance path, not really block DC

I have a 3D printer where presumably a smoothing cap just fell off the X axis controller section of the mainboard. Didn't make a lick of difference in anything operationally. Still works great.

Checks out, most boards are made with very conservative amounts of decoupling capacitance because it’s way easier than dealing with random failures due to not enough capacitance

I've understood that capacitors can be used for timing, or smoothing a voltage after a power regulator (I think).

How/what does adding capacitance help with?

Voltage spikes from line inductance, voltage drop-outs from line resistance. Basically you have little reservoirs of charge scattered all around the board (current flow isn't instantaneous in a real circuit).

It helps to always think of current draw in a compete loop, out the "top" of the capacitor, through your IC, and back into the ground side (this isn't necessarily what's happening physically). Shorter loop means less inductance, shorter traces less resistance.

Thanks, that makes no sense to me :)

But I do recall having had inductance issues with high frequency transfers to an atmega.

I have no idea how a capacitor would help. Sounds like it would increase a signal delay (tiny one perhaps).

But definitely interesting, I'll need to learn about this if/when I dive further into electronics.

Decoupling can be used on signal lines, but we’re talking about power lines here. If you have something like an ATMega, it will change its power demands very frequently at high frequencies due to switching signals on and off. Without some additional capacitance on the board, the inductance will dominate at high frequencies. Inductance resists change in current, and you’ll get voltage drops.

Capacitance is in many ways the opposite of inductance. If you place capacitors close to the power sink (e.g. the ATMega), the traces will have lower inductance and be “decoupled” from the power supply.

Smoothing is part of the story: but the important question is what is causing the roughness? Switch mode power supplies have inherent output ripple that can be filtered, but that’s distinct from transient variations in the load. Decoupling capacitors are used to provide a low impedance path at high frequencies i.e. fighting inductance.

It could be there to control emissions. You’d need to analyze the circuit to determine its purpose.

Very possible! I actually have a 100MHz scope and sdrs that tune from 9khz to 2ghz, could be an interesting distraction on the weekend to see if that axis is any noisier than the others.

Way back when a co worker was powering up a fire alarm control panel. Poof, capacitor popped and damaged his eye

Name and shame!

Voltcraft. Can't remember the model number.