Using bypass/decoupling capacitors 

 July 12, 2018

By  Peter

In the circuit above, the two capacitors highlighted with the red background perform decoupling and noise-reduction functions.

In DC electronics, capacitors can be used as a source of energy, and as a filter.

In the first instance, a capacitor can be used as a reservoir of energy that can substitute a battery of a power supply in situations where the circuit needs more current than what the main sources can provide. There is a particular class of capacitors, “super-capacitors” that are used in the place of batteries for devices that work in environments where a battery may not do too well (like in very hot places).

In many DC applications, you can add a capacitor across the Vcc the Vcc and GND lines to smooth out noise from the power supply. This is not so important if your circuit is powered by a battery (unless your circuit contains fast-switching components, like a microcontroller), but it is important if your circuit is powered from an AC/DC power supply. This kind of capacitor is called “decoupling”. You typically find very small-capacity capacitors here, around 0.1uF. They are not meant to store energy, but to filter out this external power noise.

But noise can also come from within your circuit. When your circuit contains fast-switching components, like an Atmega328p, these components can draw current from the power supply that changes rapidly. These tiny changes in the current draw can cause tiny fluctuations in the voltage supplied. A capacitor connected to the microcontroller’s GND and Vcc pins, as close as possible to those pins, will help smooth out those fluctuations.

So, in practical terms, when would you use a bypass/decoupling capacitor?

If your circuit contains a microcontroller or something similarly fast switching, then always include a small ceramic decoupling capacitor connected very close to that fast component’s Vcc and GND pins. This will take care of the noise.

If your circuit contains a component that can, occasionally, draw a lot of current like a motor or a transmitter, then also add a larger bypass capacitor.

This Atmel guide recommends that you include decoupling capacitors to all Vcc-GND pairs and also provides the appropriate values and locations (on the PCB) for these capacitors. The schematic above is from page 6.

Peter


Peter Dalmaris is an educator, electrical engineer, electronics hobbyist, and Maker. Creator of online video courses on DIY electronics and author of three technical books, and has recently released his book Maker Education Revolution.   As a Chief Tech Explorer since 2013 at Tech Explorations, the company he founded in Sydney, Australia, Peter’s mission is to explore technology and help educate the world.  Tech Explorations offers educational courses and Bootcamps for electronics hobbyists, STEM students and STEM teachers. A life-long learner, Peter’s core skill is in explaining difficult concepts through video and text. With over 15 years of tertiary teaching experience, Peter has developed a simple yet comprehensive style in teaching that students from all around the world appreciate.  His passion for technology and in particular for the world of DIY open source hardware has been a dominant driver that has guided his personal development and his work through Tech Explorations. Peter’s current online courses have helped over 60,000 people from around the world to be better Makers. 

Peter Dalmaris

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