How many capacity does it need everywhere ? - Page 1
Forum section: electronics / beginners

author: Jan Audio
author_rank: Frequent Contributor
timestamp: November 12, 2019, 04:28:13 pm
content: Hi, i have this project, it runs on a 15volt wall-adapter, and takes 100mA.The 15 volt can be from switching or linear power-supply.The 15 volt goes into a 12volt regulator, so it dont matters what adapter is used to keep all in tune.The 12 volt goes into 2 5volt regulators, one for digital one for analog.The 12 volt also goes into a 6volt regulator.The 12 volt goes into more then 10 circuits each having theyr own 10u or 22u capacity on board.Same for the 5 and 6volt all having 10u on almost all circuits, exept where no space only has 100n each IC.Every regulator has 100n and 330n ceramic where needed on in and outputs.So how many capacity do i need where ?Where the 15volt power goes in i need a big one ?How about directly on the regulator outputs ?thanks in advance.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 12, 2019, 04:40:24 pm
content: Sounds like you have enough already.The overall current your project uses is small, a 100n cap by each digital IC is good, 10u on each in and out on the regulators would be fine.As for the analog, you might need a selection of small caps, it depends what kind of frequencies you are expecting to see. If it's in the audio range, you might use, say, a 1u + 100n in parallel.If you have a 10-22u reservoir cap on each circuit, I see no reason to add another larger one on the 15V stage, those linear regulators are pretty good at supply noise rejection, but if you're really intent on belt and braces, add a 100-220u cap there too.Remember to post pics of your finished project!

author: floobydust
author_rank: Super Contributor
timestamp: November 12, 2019, 05:00:39 pm
content: The input capacitor (to 12V reg) is important, usually 10-1,000uF depending if the 15V wall adapter an SMPS or 50/60Hz transformer, to deal with the ripple that is there. 100mA is a small load. If you have a 15V SMPS wall-adapter running at 66kHz, then a linear voltage reg usually does not cope with the harmonics, LDO ICs are slow.Huge capacitors are used for lowest ripple and noise, even >1,000uF on linear regulator outputs for audio DACs and pre-amps. It's a bit crazy in some designs.A danger is during power-down or power-up, that one rail goes up/collapses first but the other is behind/stays up. It can cause IC's to latch up, so you need extra protection diodes.It's better to smartly use LC filters, ferrite beads to control HF noise from a SMPS wall-adapter, instead of using huge capacitors or having a million small ones everywhere.I would put an LC filter at the 12V reg's input, a few uH and 100-470uF and the other capacitors seem OK.Check with a scope the 5V digital noise is not polluting the 12V rail, you might use a ferrite bead there. I don't know what all this is powering.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: November 12, 2019, 05:46:58 pm
content: Quote from: floobydust on November 12, 2019, 05:00:39 pmYes i have a linear 7812, and possible a switching wall adapter.So the big cap is needed for sure ?, all good now ?, ok.Adding 1500u from 15 to 12v regulator.Quote from: floobydust on November 12, 2019, 05:00:39 pmI dont have dual rail, just a 6v virtual ground.All opamps have cap between negative input and output.Quote from: floobydust on November 12, 2019, 05:00:39 pmI dont have uH coils in house, i dont use them normally.Do i need it ?For the 5V noise, should i take it from the 15 volt maybe ?, or is it bad for a linear regulator to take away at least 10volt ? ( non switching unstabilized can even be 18 volt or more )Can use heatsink.I do have a switching 5v regulator, if it is better for the MCUs and diplay.I dont know about ferrite beds at all.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: November 12, 2019, 05:49:37 pm
content: Quote from: ThickPhilM on November 12, 2019, 04:40:24 pmI am ashamed in how much work i put in a simple sound, totally in DIP with wires and all.

author: mariush
author_rank: Super Contributor
timestamp: November 12, 2019, 11:55:37 pm
content: Linear regulators don't require a lot of capacitance on the input.You do need something to smooth out the ripple of the power adapter, and also because the long cable of the adapter can act as inductor, when the circuit's power consumption varies by some significant amount.A formula approximates how much capacitance you would need when using classic transformers, not wallwart/switching power supplies.Capacitance (Farads) = Current / [ 2 x Mains Frequency x ( Vdc peak - Vdc minimum desired) ]In your case, you're producing 12v using a 7812 linear regulator. Such a regulator wants an input voltage 2v above the output voltage, so you want at least 14v.Using the above formula, you'd use C = 0.1 A / 2x60x(15 - 14) = 0.1 / 120 = 0.000833 Farads or 833uFso technically even with a classic transformer, you'd only need around 833 uF to always have at least 14v.As you have a switching power supply, you'd probably be fine with a much lower value, like 220uF..470uF, whatever standard value you can find.On the output of linear regulators, it depends on the regulator. Some linear regulators don't require capacitors.Some say in datasheet that some amount is recommended, and some linear regulators also require that capacitor to have certain characteristics. For example, some 1117 linear regulators require a capacitor with ESR between 0.1 ohm and 1 ohm, which means those linear regulators will not work well with ceramic or polymer capacitors, as these have too low ESR.In the case of these 7812 , it doesn't really need capacitance on the output, but it won't hurt to have something on the output. An electrolytic with value between let's say 10uF and 100uF will be enough.If the digital part is not that picky, you could have saved a regulator by getting 5v from 6v output. A simple 1n400x diode would cause a voltage drop of around 0.7v..0.8v, so you end up with around 5.2v, and anything that works with 5v should tolerate 5.2v just fine.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: November 13, 2019, 02:45:54 pm
content: Ok thanks mariush.Quote from: floobydust on November 12, 2019, 05:00:39 pmHow many uH should i order ?is this the correct schematic ?