Hi,
For my next project, I plan to take some distance from Arduino by building my own circuit entirely. First, I'd like to be sure that I understand what/how a microcontroller power supply is/works.
- On the Arduino Uno schematics, there are electrolytic capacitors PC1 and PC2 around the voltage regulator named U1. Is their purpose only to let the high-frequency noise and other spikes go to ground while keeping the DC flowing ?
- Why are these present on both sides ? Once PC1 has done its job, isn't the current nice and clean ? why the need for another ground connection via PC2 ?
- Is C2 for the same purpose? And why isn't this one a polarized cap ?
- On the standalone Arduino setup explanation, the power supply is very similar (with a different but same kind of regulator). But why is there no C2 here ?
Thanks for your time.
Edit: added links for convenience
The capacitors filter-out noise and prevent the regulator from potentially oscillating. The [u]datasheet[/u] for the regulator (which I have not studied) should give you some recommended application schematics and maybe a bit more information.
Is C2 for the same purpose? And why isn't this one a polarized cap ?
Polarized capacitors (electrolytic and tantalum) tend to not "act like" capacitors at very-high frequencies. Since oscillation and noise can occur at very high frequencies, a non-polarized (typically ceramic) is often used in parallel.
But why is there no C2 here ?
I don't know... Many times, a circuit will work just fine without the capacitors... You can probably remove several bypass caps on your computer motherboard with no ill-effect. But to make sure every-board works every-time, it's good-practice to use them wherever recommended by the chip manufacturer.
I believe, the circuit was designed to accommodate a multitude of Wall Warts, batteries etc.
Since the platform is used in an experimenters environment the designers go a little bit farther to ensure things will work.
Generally lets look at a LM7805 regulator.
If the supply voltage to the regulator is sufficiently regulated then only a small, .1uF to .3uF, capacitor is needed at the input. This is used to prevent high frequency oscillation internal to the regulator. Some designers add a larger capacitor to the I/P to supply short term current regulation since Wall Warts O/Ps have long wires.
An O/P capacitor is added to help to make up for instantaneous current demands from the load. Also for decoupling, a small capacitor is add to suppress high frequencies.
I see. Many thanks to both of you.
From datasheet:
Frequency compensation for the regulator is provided by capacitor Cout and its use is mandatory to ensure output stability. A minimum capacitance value of 4.7uF with an equivalent series resistance (ESR) that is within the limits of 0.25 to 2.2 ohms is required. The capacitor type can be ceramic, tantalum, or aluminum electrolytic as long as it meets the minimum capacitance value and ESR limits over the circuit’s entire operating temperature range. Higher values of output capacitance can be used to enhance loop stability and transient response with the additional benefit of reducing output noise.
It's typical to use an electrolytic or tantalum cap as they have the ESR that meets the requirements; if the ESR is too low then the voltage regulation will be unstable. If you use a ceramic capacitor then you should doublecheck the ESR rating (it will typically be too low). You can add an additional resistor in series with the ceramic cap to make up for it.
