4 ms·
There are so many cases of this sort of stuff it's unreal. But it gets even stupider. I found one a few years back when I repaired a linear power supply. This
by ethernot 2y ago
There are so many cases of this sort of stuff it's unreal. But it gets even stupider.
I found one a few years back when I repaired a linear power supply. This required me to reverse engineer it first because there was no service manual. I buzzed the whole thing out and found out that one of the electrolytic capacitors had both legs connected to ground. They must have shipped thousands of power supplies with that error in it and no one even noticed.
- iknowstuff 2y agoName and shame!
- ethernot 2y agoVoltcraft. Can't remember the model number.
- chrisdhoover 2y agoWay back when a co worker was powering up a fire alarm control panel. Poof, capacitor popped and damaged his eye
- jcims 2y agoI 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.
- robomartin 2y agoIt could be there to control emissions. You’d need to analyze the circuit to determine its purpose.
- jcims 2y agoVery 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.
- klysm 2y agoChecks 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
- jopsen 2y agoI'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?
- pokeymcsnatch 2y agoVoltage 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.
- jopsen 2y agoThanks, 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.
- klysm 2y agoDecoupling 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.
- klysm 2y agoThat 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.
- deleted 2y ago[deleted]
- talideon 2y agoIt 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.
- klysm 2y agoBut that’s what a capacitor does? The point is to provide a low-inductance path, not really block DC