SparkFun Forums

[t1d]

I am working on a PIC18F4550 Demo Board and I am using someone else's circuit design. The circuit calls for a 0.1u, ceramic, decoupling capacitor and a 10u, electrolytic capacitor to defeat interference and both service Pin #1/MCLR. Why can't a single capacitor be used to do both functions?

Also, is there a term for a capacitor whose function is to squelch interference, in a like manner to a capacitor functioning as a "battery" is called a "decoupling" capacitor?

Thank you for your help.

[DanV]

A small cap is useful for filtering higher frequency ripples or noise from the signal while the larger one is better at filtering (or smoothing) lower frequency ripples. Often use both for power supplies to reduce rectified ripple and contain any higher frequency noise.
A super cap is one that can function as a battery.
Decoupling cap is used to separate (decouple) segments of a circuit to reduce the noise from one area affecting another.
I suggest you google the terms you are interested in, concentrating on Wikipedia and / or allaboutcircuits.com to further your education:
http://en.wikipedia.org/wiki/Decoupling_capacitor

[t1d]

A small cap is useful for filtering higher frequency ripples or noise from the signal while the larger one is better at filtering (or smoothing) lower frequency ripples.

That makes sense. Thank you for your help.

Wikipedia and / or allaboutcircuits.com to further your education:
http://en.wikipedia.org/wiki/Decoupling_capacitor

Good suggestion. I believe in doing my homework and climbing the learning curve; I enjoy learning. I thought I was on solid ground with my terminology. So many folks had used the terms as I did in various conversations. I have never heard of a "super cap." I look forward to looking it up.

Here it is for the next "student:"
We group capacitors into three family types and the most basic is the electrostatic capacitor, with a dry separator. This capacitor has a very low capacitance and is used to filter signals and tune radio frequencies. The size ranges from a few pico-farad (pf) to low microfarad (uF). The next member is the electrolytic capacitor, which is used for power filtering, buffering and coupling. Rated in microfarads (uF), this capacitor has several thousand times the storage capacity of the electrostatic capacitor and uses a moist separator. The third type is the supercapacitor, rated in farads, which is again thousands of times higher than the electrolytic capacitor. The supercapacitor is ideal for energy storage that undergoes frequent charge and discharge cycles at high current and short duration. {Per Battery University website}

The circuit design in question used separate 0.1u decoupling caps at the USB and at the MCLR pin. The circuit did not include decoupling caps at the two Vdd pins and I know that Microchip suggests them. With my incorrect thinking, I had added the values of the needed decoupling caps and placed a 0.47u cap in a central location to service all four. Now, I understand better and I will rework my version of the circuit to include separate decoupling caps near the pin they serve.

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