It says in the datasheet these values in this picture:
But I've also seen it with these values:
So what values should i use or doesn't it matter? And its for when its first starting.
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3\$\begingroup\$ FYI, 0.1 µF is the same as 100 nF. (So the small cap on the right in both figures is the same). The cap on the left in the top figure is equal to 330 nf. \$\endgroup\$– tcrosleyDec 20, 2010 at 23:21
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\$\begingroup\$ Also are one set electrolytic then the other not? \$\endgroup\$– DeanDec 20, 2010 at 23:34
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\$\begingroup\$ Small, general purpose caps, like any nF-value ones, are typically ~5mm^2 yellow ceramic blobs. It's worth getting to know the differences between capacitor dielectrics and plate materials (eg. tantalum vs. aluminum), especially as they relate to polarity and ESR. \$\endgroup\$– tybluDec 20, 2010 at 23:49
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2 Answers
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First Starting?
Build it with either; it will work fine for you. The second option with the large capacitors and the extra small one is "more stable." Just build this, do not worry about why if you are just wanting to start your project. Let us know if something does not work.
100nF
Use the values that are suggested in the datasheet. If you add capacitors a factor of 10 smaller (called decade capacitors) it will help with higher frequency noise (RF, or radio band noise), as an effects of the impedance of a capacitor.
Feel free to add as many decade capacitors as you want, but you will not need them unless you start having FCC testing. They cannot hurt.
100uF
When someone increases this they are allowing their circuit to pull power from the power source for longer. If you have a long power line this will act as a small tank of power. If you are going to have long power lines, or very inductive power lines, add this.
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\$\begingroup\$ High frequency can you explain that term please. \$\endgroup\$– DeanDec 20, 2010 at 23:04
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2\$\begingroup\$ @dean, bluetooth uses 2.4GHz frequency power. If you were powering this circuit you would want decade caps to ensure you stop power from radiating over traces at that frequency. \$\endgroup\$– KortukDec 20, 2010 at 23:09
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2\$\begingroup\$ A little late to join the discussion, but also be aware of the capacitance limits. Linear regulators have a capacitance limits they can interact with before becoming unstable. Usually this is found in the datasheet. \$\endgroup\$ Feb 16, 2016 at 14:29
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1\$\begingroup\$ @Kortuk hey, tnxx, so if the duty of 7905 is for long time, (ie an arduino which is permanently working) 100uF is better? and not matter input voltage is 12 or lower? \$\endgroup\$ Jan 22, 2018 at 10:03
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The Fairchild datasheet says on page 23
2. CI is required if regulator is located an appreciable distance from power supply filter.
3. CO improves stability and transient response.
where CI is the input capacitor and CO is the output capacitor.
So depending on your circuit you have some freedom in picking CI.
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2\$\begingroup\$ "appreciable distance" = more than approx 3 inches \$\endgroup\$– Al KeppJan 20, 2012 at 12:45