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I am making my own arduino. I am in the process of hooking up power to the Atmega328.

In both these articles they use the L7805 voltage regulator with two 10uF capacitors.

https://www.arduino.cc/en/Main/Standalone

http://www.instructables.com/id/How-to-Build-an-Arduino-Uno-on-a-BreadBoard/?ALLSTEPS

I am looking at the datasheet for the regulator at http://pdf.datasheetcatalog.com/datasheet_pdf/sgs-thomson-microelectronics/L7805CD2T_to_L7885CV.pdf

The data sheet provides a number of figures for how to correctly hook up the L7805 in different situations (pages 4, 16-23). I think Figure 13 represents our situation (Fixed Output Regulator). In the figure they use 0.33uF and 0.1uF capacitors. Could someone explain why the examples use 10uF and the data sheet specifies 0.33uF and 0.1uF?

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    on a fixed voltage regulator, it's typically not critical. adjustable ones might want a certain rage to avoid oscillation, to speed feedback, or to allow faster output voltage adjustments on high-impedance circuits. if something uses a constant rate, it needs less output buffering. if input voltage is already smooth, it needs less input smoothing. fwiw, the 7805 usually seems ok w/ none. – dandavis Apr 14 '17 at 19:02
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The answers are in the notes:

  1. Although no output capacitor is need for stability, it does improve transient response.
  2. Required if regulator is locate an appreciable distance from power supply filter.

So the values aren't critical to the operation, but you should have them there. Bigger capacitors give more energy to the regulator and more energy to your circuit (input and output capacitors respectively) but at the cost of lower frequency response. The actual values you select depend on your circuit, not the regulator. 10µF is a good "ballpark" figure to use in most circumstances.

The input capacitor reduces the impedance of the power source by providing a local source of power. The output capacitor is like the "reservoir" capacitor you should have at the power entry to all boards anyway, and reduces the impedance to the rest of your circuit.

For the higher frequency responses you should have smaller value decoupling capacitors by all your active components anyway.

You can also add more smaller capacitors to the output of the regulator to improve high frequency responsiveness if needed - you're not just limited to the one capacitor.

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  • Why are specific values specified in the data sheet if they are not dependent on the regulator? In general how\where would one go to learn about choosing an appropriate value for their circuit? – denver Apr 13 '17 at 18:04
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    I have no idea why they have specified values. Maybe they are the minimum to use if you do use them (which you should). Unless you have specific requirements 10µF is a good enough value. Maybe add a 100nF on the output as well. – Majenko♦ Apr 13 '17 at 18:06
  • When you say maybe a 100nF on the output as well, are you saying put both capacitors between the Vout and ground? – denver Apr 13 '17 at 18:08
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    Yes, in parallel. – Majenko♦ Apr 13 '17 at 18:08
  • Neat answer! I've had problems with an GSM transceiver, that only worked on "new" rechargable batteries. Near the end of the project, it wouldn't work anymore. It seemed that the internal impedance of the battery had risen and couldn't supply the sudden burst of power required. (The designer actually said: "well it did work and the battery can deliver 2A so it should be OK, you should check your code again"). Spend quite some time to figure this out as I was more into embedded software as in hardware. Moral of the story, better be safe than sorry ;D – Paul Apr 13 '17 at 19:05
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Could someone explain why the examples use 10uF and the data sheet specifies 0.33uF and 0.1uF?

One obvious reason is that those two examples are non compliant with the datasheet.

But generally speaking the ordinary 78xx regulators use emitter output and are quite tolerant to the output capacitance. 10if is not a bad number.

However, there are also variants of 78xxx that take their output from the collector. They are more sensitive to output capacitance, as most ldos are.

Generally it is a good idea to stay within the datasheet but that doesn't mean you shouldn't step out from time to time.

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  • I wonder how such regulator would work well with a lot of distributed capacitance on a PCB - a TTL graveyard comes to mind (though probably less relevant these days). 100 nF are easily exceeded. – Ghanima Apr 13 '17 at 23:07

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