I posted something like this a while back, but I've noticed a good number of newer members forgetting the bypass/decoupling caps in designs, or asking about their purpose in life (the caps, not the members).
The reason for them is "stiffening" of the power supply. The wires on a breadboard, extraneous bus lines on a solderless breadboard, etc all create parasitic resistance and inductance. This resistance and inductance (impedance) of the board resists current flowing from the power supply to the microcontroller or logic IC.
Here is where the bypass capacitor acts like a mini power supply to "hold up" the voltage until the current overcomes the impedance of the power supply lines to feed the IC. I put 3 0.1uF down each edge of the solderless breadboard, spaced evenly, 1 at each edge and one in the middle, for a total of 6. These are a permanent fixture on my solderless breadboards.
As for "Why electrolytic and ceramic in parallel?": Electrolytics have high storage capacity for their size, but this comes at the cost of higher Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). These are the same parasitic elements of the circuit board we are adding the capacitor to correct! Hence, the mica or <insert dielectric here> cap that has nearly zero ESL and extremely low ESR. The smaller cap is able to supply the needed current instantly to "cover the gap" until the Electrolytic can "fill the gap" until the power supply finally "catches up" after a switching operation in a large logic IC, which uses a lot of current, relatively.
The other way of looking at these capacitors is as mathematical models, essentially broadband filters to "bypass" any ripple directly to ground.
I've explained this to a good number of people, and the one I expounded on most (mini-battery concept) is the one people tended to understand where I witnessed new understanding and enlightenment. This is mostly why I didn't cover the filter concept. The other reason is it gets ugly. Most here know capacitors pass AC, but that isn't good enough. In order to fully explain the principles, example values would need to be set up showing the PCB as a transmission line, and it would end up on a Smith Chart before the arguments were started.
I found this website below to be an excellent source of info on why all designs that use pulses (logic, 555 timers, etc) need a bypass cap across the power pins.
Capacitor Design Tips for PCB
It also has information on which type of cap to use and why. It also explains the reasoning behind the common usage of a 22uF Electrolytic in parallel with a 0.1uF Ceramic.
When working on circuits that have external signals in the Khz range, there really isn't much else to add as far as circuit design. When the clocks get higher though, there are entire seminars and rituals on moonless nights for PCB Design, divining layers, what to put on which, where vias can be, where to never put them, etc. So I'm just sticking with the caps on this post, as they solve a lot of problems, and not many people are making more than dual layer boards as a hobby anyway.
Finally, when it comes to any circuit, analog or digital, always remember that performance will ONLY be as good as the Power Supply.
Moderators note: I printed the given page to PDF to preserve the page.
The reason for them is "stiffening" of the power supply. The wires on a breadboard, extraneous bus lines on a solderless breadboard, etc all create parasitic resistance and inductance. This resistance and inductance (impedance) of the board resists current flowing from the power supply to the microcontroller or logic IC.
Here is where the bypass capacitor acts like a mini power supply to "hold up" the voltage until the current overcomes the impedance of the power supply lines to feed the IC. I put 3 0.1uF down each edge of the solderless breadboard, spaced evenly, 1 at each edge and one in the middle, for a total of 6. These are a permanent fixture on my solderless breadboards.
As for "Why electrolytic and ceramic in parallel?": Electrolytics have high storage capacity for their size, but this comes at the cost of higher Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). These are the same parasitic elements of the circuit board we are adding the capacitor to correct! Hence, the mica or <insert dielectric here> cap that has nearly zero ESL and extremely low ESR. The smaller cap is able to supply the needed current instantly to "cover the gap" until the Electrolytic can "fill the gap" until the power supply finally "catches up" after a switching operation in a large logic IC, which uses a lot of current, relatively.
The other way of looking at these capacitors is as mathematical models, essentially broadband filters to "bypass" any ripple directly to ground.
I've explained this to a good number of people, and the one I expounded on most (mini-battery concept) is the one people tended to understand where I witnessed new understanding and enlightenment. This is mostly why I didn't cover the filter concept. The other reason is it gets ugly. Most here know capacitors pass AC, but that isn't good enough. In order to fully explain the principles, example values would need to be set up showing the PCB as a transmission line, and it would end up on a Smith Chart before the arguments were started.
I found this website below to be an excellent source of info on why all designs that use pulses (logic, 555 timers, etc) need a bypass cap across the power pins.
Capacitor Design Tips for PCB
It also has information on which type of cap to use and why. It also explains the reasoning behind the common usage of a 22uF Electrolytic in parallel with a 0.1uF Ceramic.
When working on circuits that have external signals in the Khz range, there really isn't much else to add as far as circuit design. When the clocks get higher though, there are entire seminars and rituals on moonless nights for PCB Design, divining layers, what to put on which, where vias can be, where to never put them, etc. So I'm just sticking with the caps on this post, as they solve a lot of problems, and not many people are making more than dual layer boards as a hobby anyway.
Finally, when it comes to any circuit, analog or digital, always remember that performance will ONLY be as good as the Power Supply.
Moderators note: I printed the given page to PDF to preserve the page.
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